Intel 8254 Timer HMOS

8/6/2019 Intel 8254 Timer HMOS

1/21

September 1993 Order Number 231164-005

8254PROGRAMMABLE INTERVAL TIMER

Y Compatible with All Intel and MostOther Microprocessors

Y Handles Inputs from DC to 10 MHz 8 MHz 8254 10 MHz 8254-2

Y Status Read-Back Command

Y Six Programmable Counter Modes

Y Three Independent 16-Bit Counters

Y Binary or BCD CountingY Single a5V Supply

Y Available in EXPRESS Standard Temperature Range

The Intel 8254 is a countertimer device designed to solve the common timing control problems in microcom-puter system design It provides three independent 16-bit counters each capable of handling clock inputs upto 10 MHz All modes are software programmable The 8254 is a superset of the 8253

The 8254 uses HMOS technology and comes in a 24-pin plastic or CERDIP package

2311641

Figure 1 8254 Block Diagram

2311642

Figure 2 Pin Configuration


2/21

8254

Table 1 Pin Description

SymbolPin

Type Name and FunctionNo

D7 D0 1 8 IO DATA Bi-directional three state data bus lines connected to systemdata bus

CLK 0 9 I CLOCK 0 Clock input of Counter 0OUT 0 10 O OUTPUT 0 Output of Counter 0

GATE 0 11 I GATE 0 Gate input of Counter 0

GND 12 GROUND Power supply connection

VCC 24 POWER a5V power supply connection

WR 23 I WRITE CONTROL This input is low during CPU write operations

RD 22 I READ CONTROL This input is low during CPU read operations

CS 21 I CHIP SELECT A low on this input enables the 8254 to respond toRD and WR signals RD and WR are ignored otherwise

A1 A0 20 19 I ADDRESS Used to select one of the three Counters or the ControlWord Register for read or write operations Normally connected tothe system address bus

A1 A0 Selects

0 0 Counter 00 1 Counter 11 0 Counter 21 1 Control Word Register

CLK 2 18 I CLOCK 2 Clock input of Counter 2

OUT 2 17 O OUT 2 Output of Counter 2


CLK 1 15 I CLOCK 1 Clock input of Counter 1


OUT 1 13 O OUT 1 Output of Counter 1

FUNCTIONAL DESCRIPTION

General

The 8254 is a programmable interval timercounterdesigned for use with Intel microcomputer systemsIt is a general purpose multi-timing element that canbe treated as an array of IO ports in the systemsoftware

The 8254 solves one of the most common problemsin any microcomputer system the generation of ac-curate time delays under software control Instead ofsetting up timing loops in software the programmerconfigures the 8254 to match his requirements andprograms one of the counters for the desired delayAfter the desired delay the 8254 will interrupt theCPU Software overhead is minimal and variablelength delays can easily be accommodated

Some of the other countertimer functions commonto microcomputers which can be implemented withthe 8254 are

Real time clock

Event-counter

Digital one-shot

Programmable rate generator

Square wave generator

Binary rate multiplier

Complex waveform generator

Complex motor controller

Block Diagram

DATA BUS BUFFER

This 3-state bi-directional 8-bit buffer is used to in-

terface the 8254 to the system bus (see Figure 3)

2


3/21

8254

2311643

Figure 3 Block Diagram Showing Data Bus Buffer and ReadWrite Logic Functions

READWRITE LOGIC

The ReadWrite Logic accepts inputs from the sys-tem bus and generates control signals for the otherfunctional blocks of the 8254 A1 and A0 select oneof the three counters or the Control Word Registerto be read fromwritten into A low on the RD in-put tells the 8254 that the CPU is reading one of thecounters A low on the WR input tells the 8254that the CPU is writing either a Control Word or aninitial count Both RD and WR are qualified by CSRD and WR are ignored unless the 8254 has beenselected by holding CS low

CONTROL WORD REGISTER

The Control Word Register (see Figure 4) is selectedby the ReadWrite Logic when A1A0 e 11 If theCPU then does a write operation to the 8254 thedata is stored in the Control Word Register and isinterpreted as a Control Word used to define theoperation of the Counters

The Control Word Register can only be written tostatus information is available with the Read-Back

Command

COUNTER 0 COUNTER 1 COUNTER 2

These three functional blocks are identical in opera-tion so only a single Counter will be described Theinternal block diagram of a single counter is shownin Figure 5

The Counters are fully independent Each Countermay operate in a different Mode

The Control Word Register is shown in the figure itis not part of the Counter itself but its contents de-termine how the Counter operates

The status register shown in Figure 5 whenlatched contains the current contents of the ControlWord Register and status of the output and nullcount flag (See detailed explanation of the Read-Back command)

The actual counter is labelled CE (for Counting Ele-ment) It is a 16-bit presettable synchronous downcounter

OLM and OLL are two 8-bit latches OL stands forOutput Latch the subscripts M and L stand forMost significant byte and Least significant byte

3


4/21

8254

2311644

Figure 4 Block Diagram Showing Control Word Register and Counter Functions

2311645

Figure 5 Internal Block Diagram of a Counter

4


5/21

8254

respectively Both are normally referred to as oneunit and called just OL These latches normally fol-low the CE but if a suitable Counter Latch Com-mand is sent to the 8254 the latches latch thepresent count until read by the CPU and then returnto following the CE One latch at a time is enabled

by the counters Control Logic to drive the internalbus This is how the 16-bit Counter communicatesover the 8-bit internal bus Note that the CE itselfcannot be read whenever you read the count it isthe OL that is being read

Similarly there are two 8-bit registers called CRMand CRL (for Count Register) Both are normallyreferred to as one unit and called just CR When anew count is written to the Counter the count isstored in the CR and later transferred to the CE TheControl Logic allows one register at a time to beloaded from the internal bus Both bytes are trans-ferred to the CE simultaneously CRM and CRL arecleared when the Counter is programmed In thisway if the Counter has been programmed for onebyte counts (either most significant byte only or least

significant byte only) the other byte will be zeroNote that the CE cannot be written into whenever acount is written it is written into the CR

The Control Logic is also shown in the diagramCLK n GATE n and OUT n are all connected to theoutside world through the Control Logic

8254 SYSTEM INTERFACE

The 8254 is a component of the Intel MicrocomputerSystems and interfaces in the same manner as all

other peripherals of the family It is treated by thesystems software as an array of peripheral IOports three are counters and the fourth is a controlregister for MODE programming

Basically the select inputs A0A1 connect to the A0

A1 address bus signals of the CPU The CS can bederived directly from the address bus using a linearselect method Or it can be connected to the outputof a decoder such as an Intel 8205 for larger sys-tems

OPERATIONAL DESCRIPTION

General

After power-up the state of the 8254 is undefinedThe Mode count value and output of all Countersare undefined

How each Counter operates is determined when it is

programmed Each Counter must be programmedbefore it can be used Unused counters need not beprogrammed

Programming the 8254

Counters are programmed by writing a Control Wordand then an initial count

The Control Words are written into the Control WordRegister which is selected when A1A0 e 11 TheControl Word itself specifies which Counter is beingprogrammed

2311646

Figure 6 8254 System Interface

5


6/21

8254

Control Word FormatA1A0 e 11 CS e 0 RD e 1 WR e 0

D7 D6 D5 D4 D3 D2 D1 D0

SC1 SC0 RW1 RW0 M2 M1 M0 BCD

SCSelect Counter

SC1 SC0

0 0 Select Counter 0



1 1 Read-Back Command

(see Read Operations)

RWReadWrite

RW1 RW0

0 0 Counter Latch Command (see ReadOperations)

0 1 ReadWrite least significant byte only

1 0 ReadWrite most significant byte only

1 1 ReadWrite least significant byte first

then most significant byte

MMode

M2 M1 M0

0 0 0 Mode 0

0 0 1 Mode 1

X 1 0 Mode 2

X 1 1 Mode 3

1 0 0 Mode 4

1 0 1 Mode 5

BCD

0 Binary Counter 16-bits

1 Binary Coded Decimal (BCD) Counter

(4 Decades)

NOTEDont care bits (X) should be 0 to insure compatibility with future Intel products

Figure 7 Control Word Format

By contrast initial counts are written into the Coun-ters not the Control Word Register The A1A0 in-puts are used to select the Counter to be writteninto The format of the initial count is determined bythe Control Word used

Write Operations

The programming procedure for the 8254 is veryflexible Only two conventions need to be remem-bered

1) For each Counter the Control Word must be writ-ten before the initial count is written

2) The initial count must follow the count formatspecified in the Control Word (least significantbyte only most significant byte only or least sig-nificant byte and then most significant byte)

Since the Control Word Register and the threeCounters have separate addresses (selected by theA1A0 inputs) and each Control Word specifies theCounter it applies to (SC0SC1 bits) no special in-struction sequence is required Any programmingsequence that follows the conventions in Figure 7 isacceptable

A new initial count may be written to a Counter atany time without affecting the Counters pro-grammed Mode in any way Counting will be affectedas described in the Mode definitions The new countmust follow the programmed count format

If a Counter is programmed to readwrite two-bytecounts the following precaution applies A programmust not transfer control between writing the firstand second byte to another routine which also writesinto that same Counter Otherwise the Counter willbe loaded with an incorrect count

6


7/21

8254

A1 A0Control WordCounter 0 1 1

LSB of countCounter 0 0 0

MSB of countCounter 0 0 0

Control WordCounter 1 1 1LSB of countCounter 1 0 1


Control WordCounter 2 1 1















LSB of countCounter 2 1 0MSB of countCounter 2 1 0














NOTEIn all four examples all Counters are programmed to readwrite two-byte counts These are only four of many possibleprogramming sequences

Figure 8 A Few Possible Programming Sequences

Read Operations

It is often desirable to read the value of a Counterwithout disturbing the count in progress This is easi-ly done in the 8254

There are three possible methods for reading thecounters a simple read operation the CounterLatch Command and the Read-Back Command

Each is explained below The first method is to per-form a simple read operation To read the Counterwhich is selected with the A1 A0 inputs the CLKinput of the selected Counter must be inhibited byusing either the GATE input or external logic Other-wise the count may be in the process of changingwhen it is read giving an undefined result

COUNTER LATCH COMMAND

The second method uses the Counter Latch Com-mand Like a Control Word this command is writtento the Control Word Register which is selectedwhen A1A0 e 11 Also like a Control Word theSC0 SC1 bits select one of the three Counters buttwo other bits D5 and D4 distinguish this commandfrom a Control Word

A1A0 e 11 CS e 0 RD e 1 WR e 0

D7 D6 D5 D4 D3 D2 D1 D0

SC1 SC0 0 0 X X X X

SC1SC0specify counter to be latched

SC1 SC0 Counter

0 0 0

0 1 1

1 0 2

1 1 Read-Back Command

D5D400 designates Counter Latch Command

Xdont care

NOTEDont care bits (X) should be 0 to insure compatibilitywith future Intel products

Figure 9 Counter Latching Command Format

7


8/21

8254

The selected Counters output latch (OL) latches thecount at the time the Counter Latch Command isreceived This count is held in the latch until it is readby the CPU (or until the Counter is reprogrammed)The count is then unlatched automatically and theOL returns to following the counting element (CE)

This allows reading the contents of the Counterson the fly without affecting counting in progressMultiple Counter Latch Commands may be used tolatch more than one Counter Each latched Coun-ters OL holds its count until it is read Counter LatchCommands do not affect the programmed Mode ofthe Counter in any way

If a Counter is latched and then some time laterlatched again before the count is read the secondCounter Latch Command is ignored The count readwill be the count at the time the first Counter LatchCommand was issued

With either method the count must be read accord-ing to the programmed format specifically if theCounter is programmed for two byte counts two

bytes must be read The two bytes do not have to beread one right after the other read or write or pro-gramming operations of other Counters may be in-serted between them

Another feature of the 8254 is that reads and writesof the same Counter may be interleaved for exam-ple if the Counter is programmed for two bytecounts the following sequence is valid

1) Read least significant byte

2) Write new least significant byte

3) Read most significant byte

4) Write new most significant byte

If a Counter is programmed to readwrite two-bytecounts the following precaution applies A programmust not transfer control between reading the firstand second byte to another routine which also readsfrom that same Counter Otherwise an incorrectcount will be read

READ-BACK COMMAND

The third method uses the Read-Back CommandThis command allows the user to check the countvalue programmed Mode and current states of theOUT pin and Null Count flag of the selected coun-ter(s)

The command is written into the Control Word Reg-ister and has the format shown in Figure 10 Thecommand applies to the counters selected by set-

ting their corresponding bits D3 D2 D1 e 1

A0 A1 e 11 CS e 0 RD e 1 WR e 0

D7 D6 D5 D4 D3 D2 D1 D0

1 1 C OUNT S TAT US CNT 2 CNT 1 CNT 0 0

D5

0 e Latch count of selected counter(s)D4 0 e Latch status of selected counters(s)D3 1 e Select Counter 2D2 1 e Select Counter 1D1 1 e Select Counter 0D0 Reserved for future expansion Must be 0

Figure 10 Read-Back Command Format

The read-back command may be used to latch multi-ple counter output latches (OL) by setting theCOUNT bit D5 e 0 and selecting the desired coun-ter(s) This single command is functionally equiva-lent to several counter latch commands one foreach counter latched Each counters latched countis held until it is read (or the counter is repro-grammed) The counter is automatically unlatchedwhen read but other counters remain latched until

they are read If multiple count read-back commandsare issued to the same counter without reading thecount all but the first are ignored ie the countwhich will be read is the count at the time the firstread-back command was issued

The read-back command may also be used to latchstatus information of selected counter(s) by settingSTATUS bit D4 e 0 Status must be latched to beread status of a counter is accessed by a read fromthat counter

The counter status format is shown in Figure 11 BitsD5 through D0 contain the counters programmedMode exactly as written in the last Mode ControlWord OUTPUT bit D7 contains the current state ofthe OUT pin This allows the user to monitor the

counters output via software possibly eliminatingsome hardware from a system

D7 D6 D5 D4 D3 D2 D1 D0

OutputNull

RW1 RW0 M2 M1 M0 BCDCount

D7 1 e OUT Pin is 10 e OUT Pin is 0

D6 1 e Null Count0 e Count available for reading

D5 D0 Counter programmed mode (see Figure7)

Figure 11 Status Byte

8


9/21

8254

NULL COUNT bit D6 indicates when the last countwritten to the counter register (CR) has been loadedinto the counting element (CE) The exact time thishappens depends on the Mode of the counter and isdescribed in the Mode Definitions but until the countis loaded into the counting element (CE) it cant be

read from the counter If the count is latched or readbefore this time the count value will not reflect thenew count just written The operation of Null Countis shown in Figure 12

This Action Causes

A Write to the control word register (1) Null Count e 1

B Write to the count register (CR) (2) Null Count e 1

C New Count is loaded into Null Count e 0

CE (CRxCE)

NOTE1 Only the counter specified by the control word willhave its Null Count set to 1 Null count bits of othercounters are unaffected2 If the counter is programmed for two-byte counts(least significant byte then most significant byte) Null

Count goes to 1 when the second byte is written

Figure 12 Null Count Operation

If multiple status latch operations of the counter(s)are performed without reading the status all but thefirst are ignored ie the status that will be read isthe status of the counter at the time the first statusread-back command was issued

Both count and status of the selected counter(s)may be latched simultaneously by setting both

COUNT and STATUS bits D5D4 e 0 This is func-tionally the same as issuing two separate read-backcommands at once and the above discussions ap-ply here also Specifically if multiple count andorstatus read-back commands are issued to the samecounter(s) without any intervening reads all but the

first are ignored This is illustrated in Figure 13

If both count and status of a counter are latched thefirst read operation of that counter will return latchedstatus regardless of which was latched first Thenext one or two reads (depending on whether thecounter is programmed for one or two type counts)return latched count Subsequent reads return un-latched count

CS RD WR A1 A0

0 1 0 0 0 Write into Counter 0



0 1 0 1 1 Write Control Word0 0 1 0 0 Read from Counter 0

0 0 1 0 1 Read from Counter 1

0 0 1 1 0 Read from Counter 2

0 0 1 1 1 No-Operation (3-State)

1 X X X X No-Operation (3-State)

0 1 1 X X No-Operation (3-State)

Figure 14 ReadWrite Operations Summary

CommandDescription Result

D7 D6 D5 D4 D3 D2 D1 D0

1 1 0 0 0 0 1 0 Read back count and status of Count and status latched

Counter 0 for Counter 0

1 1 1 0 0 1 0 0 Read back status of Counter 1 Status latched for Counter 1

1 1 1 0 1 1 0 0 Read back status of Counters 2 1 Status latched for Counter

2 but not Counter 1

1 1 0 1 1 0 0 0 Read back count of Counter 2 Count latched for Counter 2

1 1 0 0 0 1 0 0 Read back count and status of Count latched for Counter 1

Counter 1 but not status

1 1 1 0 0 0 1 0 Read back status of Counter 1 Command ignored status

already latched for Counter 1

Figure 13 Read-Back Command Example

9


10/21

8254

Mode Definitions

The following are defined for use in describing theoperation of the 8254

CLK Pulse a rising edge then a falling edge in

that order of a Counters CLK in-put

Trigger a rising edge of a Counters GATEinput

Counter loading the transfer of a count from the CRto the CE (refer to the FunctionalDescription)

MODE 0 INTERRUPT ON TERMINAL COUNT

Mode 0 is typically used for event counting After theControl Word is written OUT is initially low and willremain low until the Counter reaches zero OUT thengoes high and remains high until a new count or anew Mode 0 Control Word is written into the Coun-ter

GATE e 1 enables counting GATE e 0 disablescounting GATE has no effect on OUT

After the Control Word and initial count are written toa Counter the initial count will be loaded on the nextCLK pulse This CLK pulse does not decrement thecount so for an initial count of N OUT does not gohigh until N a 1 CLK pulses after the initial count iswritten

If a new count is written to the Counter it will beloaded on the next CLK pulse and counting will con-tinue from the new count If a two-byte count is writ-ten the following happens

1) Writing the first byte disables counting OUT is setlow immediately (no clock pulse required)

2) Writing the second byte allows the new count tobe loaded on the next CLK pulse

This allows the counting sequence to be synchroniz-ed by software Again OUT does not go high untilNa1 CLK pulses after the new count of N is written

If an initial count is written while GATE e 0 it willstill be loaded on the next CLK pulse When GATEgoes high OUT will go high N CLK pulses later noCLK pulse is needed to load the Counter as this hasalready been done

MODE 1 HARDWARE RETRIGGERABLEONE-SHOT

OUT will be initially high OUT will go low on the CLKpulse following a trigger to begin the one-shot pulseand will remain low until the Counter reaches zero

OUT will then go high and remain high until the CLKpulse after the next trigger

After writing the Control Word and initial count theCounter is armed A trigger results in loading theCounter and setting OUT low on the next CLK pulse

thus starting the one-shot pulse An initial count of Nwill result in a one-shot pulse N CLK cycles in dura-tion The one-shot is retriggerable hence OUT willremain low for N CLK pulses after any trigger Theone-shot pulse can be repeated without rewriting thesame count into the counter GATE has no effect onOUT

If a new count is written to the Counter during a one-shot pulse the current one-shot is not affected un-less the counter is retriggered In that case theCounter is loaded with the new count and the one-shot pulse continues until the new count expires

MODE 2 RATE GENERATOR

This Mode functions like a divide-by-N counter It istypically used to generate a Real Time Clock inter-rupt OUT will initially be high When the initial counthas decremented to 1 OUT goes low for one CLKpulse OUT then goes high again the Counter re-loads the initial count and the process is repeatedMode 2 is periodic the same sequence is repeatedindefinitely For an initial count of N the sequencerepeats every N CLK cycles

GATE e 1 enables counting GATE e 0 disablescounting If GATE goes low during an output pulseOUT is set high immediately A trigger reloads theCounter with the initial count on the next CLK pulseOUT goes low N CLK pulses after the trigger Thusthe GATE input can be used to synchronize theCounter

After writing a Control Word and initial count theCounter will be loaded on the next CLK pulse OUTgoes low N CLK Pulses after the initial count is writ-ten This allows the Counter to be synchronized bysoftware also

Writing a new count while counting does not affectthe current counting sequence If a trigger is re-ceived after writing a new count but before the endof the current period the Counter will be loaded withthe new count on the next CLK pulse and countingwill continue from the new count Otherwise thenew count will be loaded at the end of the currentcounting cycle In mode 2 a COUNT of 1 is illegal

MODE 3 SQUARE WAVE MODE

Mode 3 is typically used for Baud rate generationMode 3 is similar to Mode 2 except for the duty cycleof OUT OUT will initially be high When half the

10


11/21

8254

2311647

NOTEThe following conventions apply to all mode timing diagrams1 Counters are programmed for binary (not BCD) counting and for readingwriting least significant byte (LSB) only2 The counter is always selected (CS always low)3 CW stands for Control Word CW e 10 means a control word of 10 HEX is written to the counter4 LSB stands for Least Significant Byte of count5 Numbers below diagrams are count values The lower number is the least significant byte The upper number is themost significant byte Since the counter is programmed to readwrite LSB only the most significant byte cannot be read

N stands for an undefined countVertical lines show transitions between count values

Figure 15 Mode 0

11


12/21


13/21

8254

2311649

NOTEA GATE transition should not occur one clock prior to terminal count

Figure 17 Mode 2

new count Otherwise the new count will be loadedat the end of the current half-cycle

Mode 3 is implemented as follows

Even counts OUT is initially high The initial count isloaded on one CLK pulse and then is decrementedby two on succeeding CLK pulses When the countexpires OUT changes value and the Counter is re-loaded with the initial count The above process isrepeated indefinitely

Odd counts OUT is initially high The initial countminus one (an even number) is loaded on one CLKpulse and then is decremented by two on succeed-ing CLK pulses One CLK pulse after the count ex-pires OUT goes low and the Counter is reloadedwith the initial count minus one Succeeding CLKpulses decrement the count by two When the countexpires OUT goes high again and the Counter isreloaded with the initial count minus one The aboveprocess is repeated indefinitely So for odd countsOUT will be high for (N a 1)2 counts and low for(N b 1)2 counts

13


14/21

8254

23116410

NOTEA GATE transition should not occur one clock prior to terminal count

Figure 18 Mode 3

14


15/21

8254

MODE 4 SOFTWARE TRIGGERED STROBE

OUT will be initially high When the initial count ex-pires OUT will go low for one CLK pulse and thengo high again The counting sequence is triggeredby writing the initial count

GATE e 1 enables counting GATE e 0 disablescounting GATE has no effect on OUT

After writing a Control Word and initial count theCounter will be loaded on the next CLK pulse ThisCLK pulse does not decrement the count so for an

initial count of N OUT does not strobe low until N a

1 CLK pulses after the initial count is written

If a new count is written during counting it will beloaded on the next CLK pulse and counting will con-tinue from the new count If a two-byte count is writ-

ten the following happens1) Writing the first byte has no effect on counting

2) Writing the second byte allows the new count tobe loaded on the next CLK pulse

This allows the sequence to be retriggered bysoftware OUT strobes low N a 1 CLK pulses afterthe new count of N is written

23116411

Figure 19 Mode 4

15


16/21

8254

MODE 5 HARDWARE TRIGGERED STROBE(RETRIGGERABLE)

OUT will initially be high Counting is triggered by arising edge of GATE When the initial count has ex-pired OUT will go low for one CLK pulse and then

go high again

After writing the Control Word and initial count thecounter will not be loaded until the CLK pulse after atrigger This CLK pulse does not decrement thecount so for an initial count of N OUT does notstrobe low until N a 1 CLK pulses after a trigger

A trigger results in the Counter being loaded with theinitial count on the next CLK pulse The countingsequence is retriggerable OUT will not strobe lowfor N a 1 CLK pulses after any trigger GATE hasno effect on OUT

If a new count is written during counting the currentcounting sequence will not be affected If a triggeroccurs after the new count is written but before thecurrent count expires the Counter will be loadedwith the new count on the next CLK pulse andcounting will continue from there

23116412

Figure 20 Mode 5

16


17/21

8254

Signal Low

Status Or Going Rising High

Modes Low

0 Disables Enables

Counting Counting

1 1) Initiates Counting

2) Resets Output

after Next

Clock

2 1) Disables

Counting Initiates Enables

2) Sets Output Counting Counting

Immediately

High

3 1) Disables

Counting Initiates Enables

2) Sets Output Counting Counting

Immediately

High

4 Disables Enables

Counting Counting

5 Initiates

Counting

Figure 21 Gate Pin Operations Summary

ModeMin Max

Count Count

0 1 0

1 1 0

2 2 0

3 2 0

4 1 0

5 1 0

NOTE0 is equivalent to 216 for binary counting and 104 forBCD counting

Figure 22 Minimum and Maximum Initial Counts

Operation Common to All Modes

PROGRAMMING

When a Control Word is written to a Counter all

Control Logic is immediately reset and OUT goes toa known initial state no CLK pulses are required forthis

GATE

The GATE input is always sampled on the risingedge of CLK In Modes 0 2 3 and 4 the GATE inputis level sensitive and the logic level is sampled onthe rising edge of CLK In Modes 1 2 3 and 5 theGATE input is rising-edge sensitive In these Modesa rising edge of GATE (trigger) sets an edge-sensi-tive flip-flop in the Counter This flip-flop is then sam-pled on the next rising edge of CLK the flip-flop isreset immediately after it is sampled In this way atrigger will be detected no matter when it occursahigh logic level does not have to be maintained until

the next rising edge of CLK Note that in Modes 2and 3 the GATE input is both edge- and level-sensi-tive In Modes 2 and 3 if a CLK source other thanthe system clock is used GATE should be pulsedimmediately following WR of a new count value

COUNTER

New counts are loaded and Counters are decre-mented on the falling edge of CLK

The largest possible initial count is 0 this is equiva-lent to 216 for binary counting and 104 for BCDcounting

The Counter does not stop when it reaches zero InModes 0 1 4 and 5 the Counter wraps around tothe highest count either FFFF hex for binary count-ing or 9999 for BCD counting and continues count-ing Modes 2 and 3 are periodic the Counter reloadsitself with the initial count and continues countingfrom there

17


18/21

8254

ABSOLUTE MAXIMUM RATINGS

Ambient Temperature Under Bias 0C to 70C

Storage Temperature b65C to a150C

Voltage on Any Pin with

Respect to Groundb05V to a7VPower Dissipation 1W

NOTICE This is a production data sheet The specifi-cations are subject to change without notice

WARNING Stressing the device beyond the AbsoluteMaximum Ratings may cause permanent damageThese are stress ratings only Operation beyond the

Operating Conditions is not recommended and ex-tended exposure beyond the Operating Conditionsmay affect device reliability

DC CHARACTERISTICS TA e 0C to 70C VCC e 5V g10%

Symbol Parameter Min Max Units Test Conditions

VIL Input Low Voltage b05 08 V

VIH Input High Voltage 20 VCC a05V V

VOL Output Low Voltage 045 V IOL e 20 mA

VOH Output High Voltage 24 V IOH e b400 mA

IIL Input Load Current g10 mA VIN e VCC to 0V

IOFL Output Float Leakage g10 mA VOUT e VCC to 045V

ICC VCC Supply Current 170 mA

CIN Input Capacitance 10 pF fc e 1 MHz

CI0 IO Capacitance 20 pF Unmeasured pins

returned to VSS(4)

AC CHARACTERISTICS TA e 0C to 70C VCC e 5V g10% GND e 0V

Bus Parameters(1)

READ CYCLE

Symbol Parameter8254 8254-2

UnitMin Max Min Max

tAR Address Stable Before RDv 45 30 ns

tSR CS Stable Before RDv 0 0 ns

tRA Address Hold Time After RDu 0 0 ns

tRR RD Pulse Width 150 95 ns

tRD Data Delay from RDv 120 85 ns

tAD Data Delay from Address 220 185 ns

tDF RDu to Data Floating 5 90 5 65 ns

tRV Command Recovery Time 200 165 ns

NOTE1 AC timings measured at VOH e 20V VOL e 08V

18


19/21

8254

AC CHARACTERISTICS TA e 0C to 70C VCC e 5V g10% GND e 0V (Continued)

WRITE CYCLE


Unit

Min Max Min Max

tAW Address Stable Before WRv 0 0 ns

tSW CS Stable Before WRv 0 0 ns

tWA Address Hold Time After WRv 0 0 ns

tWW WR Pulse Width 150 95 ns

tDW Data Setup Time Before WRu 120 95 ns

tWD Data Hold Time After WRu 0 0 ns

tRV Command Recovery Time 200 165 ns

CLOCK AND GATE


Unit

Min Max Min Max

tCLK Clock Period 125 DC 100 DC ns

tPWH High Pulse Width 60(3) 30(3) ns

tPWL Low Pulse Width 60(3) 50(3) ns

tR Clock Rise Time 25 25 ns

tF Clock Fall Time 25 25 ns

tGW Gate Width High 50 50 ns

tGL Gate Width Low 50 50 ns

tGS Gate Setup Time to CLKu 50 40 ns

tGH Gate Setup Time After CLKu 50(2) 50(2) ns

tOD Output Delay from CLKv 150 100 ns

tODG Output Delay from Gatev 120 100 ns

tWC CLK Delay for Loadingv 0 55 0 55 ns

tWG Gate Delay for Sampling b5 50 b5 40 ns

tWO OUT Delay from Mode Write 260 240 ns

tCL CLK Set Up for Count Latch b40 45 b40 40 ns

NOTES2 In Modes 1 and 5 triggers are sampled on each rising clock edge A second trigger within 120 ns (70 ns for the 8254-2) ofthe rising clock edge may not be detected3 Low-going glitches that violate tPWH tPWL may cause errors requiring counter reprogramming4 Sampled not 100% tested TA e 25C5 If CLK present at TWC min then Count equals Na2 CLK pulses TWC max equals Count Na1 CLK pulse TWC min toTWC max count will be either Na1 or Na2 CLK pulses6 In Modes 1 and 5 if GATE is present when writing a new Count value at TWG min Counter will not be triggered at TWGmax Counter will be triggered7 If CLK present when writing a Counter Latch or ReadBack Command at TCL min CLK will be reflected in count value

latched at TCL max CLK will not be reflected in the count value latched

19


20/21

8254

WAVEFORMS

WRITE

23116413

READ

23116414

20


21/21

8254

WAVEFORMS (Continued)

RECOVERY

23116415

CLOCK AND GATE

23116416

Last byte of count being written

AC TESTING INPUT OUTPUT WAVEFORM

23116417AC Testing Inputs are driven at 24V for a Logic 1 and 045Vfor a Logic 0 Timing measurements are made at 20V for aLogic 1 and 08V for a Logic 0

AC TESTING LOAD CIRCUIT

23116418CL e 150 pFCL Includes Jig Capacitance

REVISION SUMMARY

The following list represents the key differences be-tween Rev 004 and Rev 005 of the 8254 DataSheet

1 References to and specifications for the 5 MHz8254-5 are removed Only the 8 MHz 8254 andthe 10 MHz 8254-2 remain in production

21

Documents

Intel 8254 Timer HMOS