MicrocontrollersLab 7

GENERATING SQUARE WAVES USING 8051 TIMES

Objectives of the LabLearning about 8051 timers

Registers governing timers

Generating delays or certain frequencies using timers

Deciding Pins or Ports to use

Use withcaution

If EA high*We onlyneed one pinto generate sqwave

Timers The 8051 has two timers/counters they can be used

either as Timers to generate delay. Event counters to count events happening outside the

microcontroller. Both Timer0 and Timer1 are 16 bits wide

8051 has an 8-bit architecture, each 16-bit timer is accessed as two separate registers of low byte and high byte.

Accessed as low byte and high byte The low byte register is called TL0/TL1. The high byte register is called TH0/TH1. Accessing: MOV TL0,#4FH; MOV R5,TH0

/ /

TMOD RegisterBoth timers0 and 1 use the same register, called TMOD

(timer mode), to set the various timer operation modes.

Starting and Stopping TimersTimers of 8051 do starting and stopping by either

software or hardware control. In using software to start and stop the timer where GATE=0.The start and stop of the timer are controlled by way of

software by the TR (timer run) bits TR0 and TR1.TR0=1, timer 0 starts and TR0=0 timer 0 stops.Above instructions work if GATE=0 in TMOD register.

The hardware way of starting and stopping the timer by an external source is achieved by making GATE=1 in the TMOD register.

TCON RegistersIn TCON register, the upper four bits are used to store the

TF and TR bits of both timer 0 and timer 1.The lower 4 bits are for controlling the interrupt bits

mainly used for counting external events.We have discussed the TR bit but what about TF (Timer

flag) bit?TF bit for the respective timers is set when the timer

registers TL and/or TH overflows i.e. go from the highest value to 0.

Timer in Mode1 and Mode2Mode1 is of 16 bit and after the timer flag is set, we

manually have to load the value again to use.

Mode2 is of 8 bit and after the timer flag is set, TL is automatically loaded with the value of TH.

Pseudo-Code for Generating Delay using Mode1

1. First convert the ‘delay in seconds’ in machine cycles. e.g. 100μs/1.085μs = 92 m.c.

2. As Mode1 is 16-bit, calculate 65536-92=65444 => FFA4. This is known as initial count value. Let xx=FF and yy=A4.

3. First, select the timer and mode by loading value in TMOD register, e.g. mov tmod,#00010000b

4. Load TH and TL with xx and yy respectively.5. Now, set TR bit to start the timer.6. Wait till TF bit gets set. jnb TF1,$7. Now, to repeat this, first clear TF1, clear TR1, then reload

TH1 and TL1, then again set TR1 and watch for TF1.

Generating Delay using Mode1 example:mov tmod,#0010000bagain:mov th1,#xx; mov th1,#high(-92)mov tl1,#yy; mov tl1,#low(-92) setb tr1rep: jnb tf1,repclr tf1clr tr1jmp again

Pseudo-Code for Generating Delay using Mode2

1. First convert the ‘delay in seconds’ in machine cycles. e.g. 100μs/1.085μs = 92 m.c.

2. As Mode1 is 8-bit, calculate 256-92=164 => A4. This is the initial count value.

3. First, select the timer and mode by loading value in TMOD register, e.g. mov tmod,#00100000b

4. Load TH and TL with xx and yy respectively. (for first cycle only)

5. Now, set TR bit to start the timer.6. Wait till TF bit gets set. jnb TF1,$7. Now, to repeat this, only clear TF1 and again watch for TF1.

Remember that in mode-2 TL is automatically reloaded with TH.

Generating Delay using Mode2 example:mov tmod,#0010000bagain:mov th1,#yy; mov th1,#low(-92) or mov th1,#-92mov tl1,#yy; mov tl1,#low(-92) or mov tl1,#-92 setb tr1rep: jnb tf1,repclr tf1jmp rep

When to use Mode1 and Mode2?

Frequency GenerationWe will want to generate square wave on a specific pin of the

microcontroller, all we have to do is to complement the pin after a certain delay and that delay is T/2. Choose the mode an load the initial count corresponding T/2 to generate the required frequency.

For example, f=10kHz, T=1/10k=100μs, T/2=50μs, 50/1.085=46 m.c. => Mode 2 and count=low(-46)

Why to use timers and not delay subroutine for frequency?

PWM GenerationThis is a square wave with fixed frequency but variable high

and low time.We have to make two different delays for when the pin is high

and the pin is low.

External Event CountingGive initial value to TMOD by setting the C/T bit to 1 e.g.

mov tmod,#01010000b

Because, Gate bit =1, the timer is still run or started using TR bit. But the value updating in registers TH and TL is governed by an external signal falling upon pins P3.4 and P3.5.

External Event Counting (Contd.)

Hardware control to start timersFollowing figure explains that when GATE = 0, the start

of the timer is done by hardware. TR bit still needs to be set but if the external input on pins P3.2 and P3.3 is low, the timer never starts.

Proteus Devices needed in this Lab1. AT89c51

2. Oscilloscope

Lab TasksGenerate frequency of Workstation number in kHz.

Quiz Next Week of LCD.

Last Week Schedule: Project Checking, Vivas from Project and Lab, Lab Manual Submission (in lab experiment groups)