Using Ready-for-PIC and SDR Libraries

Using the Ready-for-PIC Board with “SDR”

Libraries

Corrado Santoro

ARSLAB - Autonomous and Robotic Systems LaboratoryDipartimento di Matematica e Informatica - Universita di Catania, Italy

[email protected]

S.D.R. Course

Corrado Santoro Using the ReadyForPIC

Ready-for-PIC + SDR Libraries

The “Ready-for-PIC” is an evaluation board for

microcontrollers of the PIC18F Microchip Family.

It is equipped with the PIC18F25K22, running at 64 MHz.

It has been “patched” in the DMI Labs with some LEDs,

pushbuttons and trimmers.

It includes:

Six LEDs, connected to some PIC digital outputs;

Three Pushbuttons, connected to some PIC digital inputs;Two trimmers;

A UART-to-USB bridge to interact with a PC.

The “SDR Library” is a set of functions, developed at DMI

Labs, to easily use the peripherals of the PIC without

having a deep knowledge of the PIC special function

registers.


Part I

Handling Digital I/O Lines


A “LED Flashing” example

✞#define _XTAL_FREQ 64000000

// Tell to our ‘‘delay’’ functions which is our clock frequency

#include <xc.h>

#include "sdr_lib.h"

void main(void)

{

sdr_digital_init();

// initialize digital lines

for (;;) { // loop forever

sdr_set_led_0(LED_ON); // turn on LED 0

sdr_delay_ms(200); // wait 200ms

sdr_set_led_0(LED_OFF);// turn off LED 0

sdr_delay_ms(200); // wait 200ms

}

}

✡✝ ✆


A “LED + pushbutton” example


#include <xc.h>


void main(void)

{

sdr_digital_init();



if (sdr_get_pushbutton_0() == PUSHBUTTON_PRESSED)

sdr_set_led_1(LED_ON);

else

sdr_set_led_1(LED_OFF);

}

}

✡✝ ✆


Another “LED + pushbutton” example


#include <xc.h>


void main(void)

{

int led_status = LED_OFF;

sdr_digital_init();



if (sdr_get_pushbutton_0() == PUSHBUTTON_PRESSED) {

led_status = !led_status;

sdr_set_led_1(led_status);

// now wait for pushbutton release

while (sdr_get_pushbutton_0() == PUSHBUTTON_PRESSED)

{}

}

}

}

✡✝ ✆


SDR Library functions

sdr digital init(): initializes digital I/O lines;

sdr set led 0(status), ...

sdr set led 5(status), sets or clears the relevant LED

on the basis of status, which can be LED OFF or LED ON;

sdr set led(led number,status), sets or clears the

led number on the basis of status, which can be LED OFF

or LED ON;

sdr get pushbutton 0(), ...

sdr get pushbutton 2(), returns the status of the

relevant pushbutton; returns PUSHBUTTON PRESSED if the

pushbutton is pressed.

sdr delay ms(timems), waits for given number of

milliseconds (it needs the constant XTAL FREQ to be set).


Part II

Using Timers


Timers

A timer is a circuit to let a software have the “knowledge of

flow of time”

It is a composed of:

A clock source; usually the system clock or an externalsignal;

A programmable frequency divisor, called prescaler, to

divide clock source frequency, if needed;Some SFRs which hold a 8-, 16- or 32-bit value that is

incremented in hardware using the clock source.Some SFRs which give some state information,

e.g overflow (zero crossing).

PIC18F family has 7 timers, called TIMER0, TIMER1, ...,

TIMER5, TIMER6

Each timer has different characteristics and may be used

together with other peripherals.


The TIMER0 of PIC18

TIMER0 is a 8/16 bit timer/counter (figure shows the 16bit mode);

It has a 8/16 bit register TMR0 which increments on the basis of a

certain clock source, which can be:

System clock divided by 4 (FOSC/4);

Signal from external pin T0CKI


The TIMER0 of PIC18

Clock source may be divided by a programmable prescaler to perform

a division by 1, 2, 4, 8, 32, ..., 256.

Since the TMR0 register increments, when it overflows (transition from

0xFFFF to 0x0000), an overflow flag is activated by the hardware and

can

be polled by the software

generate an hardware interrupt

The flag must be reset by software.


A case-study: a timer to flash a LED at 500ms

We want to use the system clock;

We have FOSC = 64MHz, therefore the basic frequency is

FOSC/4 = 16MHz, the P = 62.5ns;

Let’s use the prescaler and divide the frequency by 256, so the timer

will increments using a period P = 62.5ns ∗ 256 = 16µs.

So 500ms/16µs = 31250 counts.

The timer always counts up, so to recognize the overflow we must

start from 65536 − 31250 = 34286.

Indeed we can also use the 2’s complement value, i.e. −31250.


The “Timer flashing LED” example


#include <xc.h>


void main(void)

{

int led = LED_OFF;

sdr_digital_init();

// initialize timer 0

sdr_timer0_init(TIMER0_CLOCK_INT,TIMER0_PS256,TIMER0_INT_OFF);

// set TMR0 value

sdr_timer0_set(-31250);

// start the timer

sdr_timer0_start();

for (;;) {

sdr_set_led_0(led);

// wait for overflow

while (!sdr_timer0_overflow()) {}

// reload timer value

sdr_timer0_set(-31250);

// clear overflow flag

sdr_timer0_rearm();

led = !led;

}

return;

}

✡✝ ✆


SDR Library functions for timers

sdr timer0 init(clock,ps,int): initializes timer 0:clock, clock source set-up

TIMER0 CLOCK INT, use system clock

TIMER0 CLOCK EXT, use external source

ps, prescaler set-up

TIMER0 PS1, no presaler

TIMER0 PS2, divide by 2


...


int, hardware interrupt generation at overflow

TIMER0 INTERRUPT OFF, do not generate interrupt

TIMER0 INTERRUPT ON, generate interrupt


SDR Library functions for timers

sdr timer0 start(), starts the timer;

sdr timer0 stop(), stops the timer;

sdr timer0 set(val), sets the timer to val;

sdr timer0 overflow(), returns true if the overflow

event occurred;

sdr timer0 rearm(), clears the overflow event.


Part III

Analog-to-Digital Converter


What is an ADC?

An ADC (Analog-to-Digital-Converter) is a circuit which gets an

analog voltage signal and provides (to software) a variable

proportional to the input signal.

An ADC is characterised by:

The range (in Volts) of the input signal (typical [0,5V ] or

[0,3.3V ]).

The resolution (in bits) of the converter.

Example:

Range = [0,5V ]

Resolution = 10 bits

results in range [0,210− 1] = [0,1023]

0V → 0 2.5V → 512 5V → 1023


ADC: Basic working scheme

1 Sample: the signal is sampled by closing the switch and

charging the capacitor.

2 Conversion: the switch is open and the sampled signal is

converted; the result is stored in the 16-bit variable.

3 End-of-conversion: a proper bit is set to signal that the

operation has been done.


The ADC Circuit of PIC18F25K22


SDR Library functions for ADC

sdr adc init(...), performs ADC setup;

sdr adc set channel(chan), determines the channel

to convert;

sdr adc start(), starts the conversion for the selected

channel;

sdr adc conversion done(), returns true if the

conversion is completed;

sdr adc value(), returns the value of the conversion.


ADC Initialization

sdr adc init(clock,acq time,fmt,int): initializes ADC circuit:

clock, ADC clock source set-upADC CLOCK FOSC2, use system clock / 2ADC CLOCK FOSC4, use system clock / 4...ADC CLOCK FOSC64, use system clock / 64ADC CLOCK FRC, use internal RC oscillator (600 KHz)

acq time, sample time (in multuples of ADC clock)ADC ACQ TIME CLOCK 2, 2 ADC ClockADC ACQ TIME CLOCK 4, 4 ADC ClockADC ACQ TIME CLOCK 6, 6 ADC ClockADC ACQ TIME CLOCK 8, 8 ADC ClockADC ACQ TIME CLOCK 12, 12 ADC ClockADC ACQ TIME CLOCK 16, 16 ADC ClockADC ACQ TIME CLOCK 20, 20 ADC Clock

fmt, data formatADC FORMAT LEFT, format is left-justifiedADC FORMAT RIGHT, format is right-justified

int, hardware interrupt generation at end-of-conversionADC INTERRUPT OFF, do not generate interruptADC INTERRUPT ON, generate interrupt


ADC Data Format


Part IV

Interrupt Handling


Interrupts in MCUs

The core of MCUs manages interrupts, as in normal CPUs

In a MCU, each peripheral event can generate aninterrupt, e.g.:

The overflow of a timer;

The reception/trasmission of a byte through the UART(serial port);

The end of conversion of the ADC;

...

The peripheral is called interrupt source

When an interrupt is generated, the normal program flow is

interrupted, a specific function is invoked, called ISR -

Interrupt Service Routine; at the end, the normal

program flow is resumed.


Handling Interrupts in Microchip MCUs with SDR Lib

Enable the interrupt generation in peripheral initialization;

Define a C function marked as interrupt, in which;

Check which peripheral has generated the interrupt;

Serve the peripheral interrupt;

Reset the peripheral interrupt flag.

Handling TMR0 interrupt

✞void interrupt isr()

{

if (sdr_timer0_interrupt()) {

// ... handle the TMR interrupt

sdr_timer0_clear_interrupt();

}

}

...

main()

{

...

sdr_timer0_init(TIMER0_CLOCK_INT,TIMER0_PS8,TIMER0_INTERRUPT_ON);

}

✡✝ ✆


Using the Ready-for-PIC Board with “SDR”

Libraries

Corrado Santoro

ARSLAB - Autonomous and Robotic Systems LaboratoryDipartimento di Matematica e Informatica - Universita di Catania, Italy

[email protected]

S.D.R. Course


Documents

Using Ready-for-PIC and SDR Libraries