Файл: PICmicro MCU C - An itroduction to programming The Microchip PIC in CCS C (N.Gardner, 2002).pdf

shitf_right(&z,3,0);

rotate_left and rotate_right work like the shift functions above except the bit shifted out of one side gets shifted in the other side. For example:

int x[3] = {0b10010001, 0b00011100, 0b10000001};

//x msb first is: 10000001, 00011100, 10010001 rotate_left(x,sizeof(x));

//x msb first is: 00000010, 00111001, 00100011

The swap function swaps the upper 4 bits and lower 4 bits of a byte. For example:

int x;

x = 0b10010110

swap(x); //x is now 01101001

9.4Timers

All PICmicro®’s have an 8-bit timer and some PIC’s have two more advanced timers. The capabilities are as follows:

rtcc (timer0) = 8Bit.

May increment on the instruction clock or by an external source.

Applying a pre-scaler may slow increment.

When timer0 overflows from 255 to 0, an interrupt can be generated (not 16C5X series)

timer1 = 16Bit.

May increment on the instruction clock or by an external source.

Applying a pre-scalar may slow increment.

When timer1 overflows from 65535 to 0, an interrupt can be generated.

In capture mode, the timer1 count may be saved in another register when a pin changes. An interrupt may also be generated.

In compare mode, a pin can be changed when the count reaches a preset value, and an interrupt may also be generated.

This timer is used as part of the PWM.

114

timer2 = 8Bit.

May increment on the instruction clock or by an external source.

Applying a pre-scalar may slow increment.

When timer2 overflows from 255 to 0, an interrupt can be generated.

The interrupt can be slowed by applying a post-scaler, so it requires a certain number of overflows before the interrupt occurs.

This timer is used as part of the PWM.

The following is a simple example using the rtcc to time how long a pulse is high:

#include <16c74.h> #fuses HS,NOWDT

#use delay(clock=1024000)

#use rs232(baud=9600,xmit=PIN_C6,rcv=PIN_C7)

main() {

int time;

setup_counters(rtcc_internal, rtcc_div_256); //increments 1024000/4*256 times per second //or every millisecond

while(!input(PIN_B0)); //wait for high set_rtcc(0);

while(!input(PIN_B0)); //wait for low time = get_rtcc();

printf(“High time = %u ms.”,time);

}

The following is an example using the timer1 capture feature to time how long it takes for pin C2 to go high after pin B0 is driven high:

#include <16c74.h> #fuses HS,NOWDT

#use delay(clock=8000000)

#use rs232(baud=9600,xmit=PIN_C6,rcv=PIN_C7) #bit capture_1 = 0x0c.2 //pir1 register

//bit 2 = capture has taken place

main()

{

long time;

setup_timer1(t1_internal | t1_div_by_2); //Increments every 1 us

setup_ccp1(ccp_capture_re);

//configure CCP1 to capture rise

capture_1=0;

115

set_timer1(0); output_high(PIN_B0); while(!capture_1);

time = ccp_1;

printf(“Reaction time = %1u us.”,time);

}

ADCON1

ANALOG/

DIGITAL

CONTROL

9.5A/D Conversion

The A/D in the 16C7x and 12C67x devices has a resolution of 8 bits. This means that the voltage being measured can be resolved to one of 255 values. If a 5 volt supply is used, then the measured accuracy is 5/255 = 19.6mV over a 0 to 5 volt range. However, if the reference voltage is reduced to 2.55 volts, the resolution becomes 10mV but the working range falls to 0 to 2.55 volts.

Other Microchip parts have 10, 11, 12 and 16 bits resolution.

NOTE:

The default for ports having both analog and digital capability is ANALOG.

It is important to note which combination of I/O lines can be used for analog and digital. The following tables are extracted from the data sheets.

116

In C, the setup and operation of the A/D is simplified by ready made library routines.

set_adc_channel(0-7)

select the channel for a/d conversion

setup_adc(mode)

sets up the analog to digital converter The modes are as follows:

adc_off, adc_clock_div_2, adc_clock_div_8, adc_clock_div_32,adc_clock_internal

setup_adc_ports(mix)

will setup the ADC pins to be analog, digital or combination. The allowed combinations for mix vary depending on the chip.

The constants all_analog and no_analog are valid for all chips. Some other example constants:

ra0_ra1_ra2_ra3_analog/a0_ra1_analog_ra3_ref

read_adc()

will read the digital value fro the analog to digital converter. Calls to setup_adc and set_adc_channel should be made sometime before this function is called. This function returns an 8-bit value 00h – FFh on parts with an 8 bits A/D converter. On parts with greater than 8 bits A/D the value returned is always a long with the range 000h – FFFFh.

The range may be fixed regardless of the part to aid in compatibility across parts by adding on of the following directives:

#device ADC=8

#device ADC=16

Example

117

printf(“A/D value = %2x\n\r”, value);//prints value

9.6Data Communications/RS232

RS232 communications between PCs, modems etc. form part of an engineer’s life. The problem seems to arise when self built products need to be interfaced to the outside world. The permutations of 9 or 25 pins on a D connector and the software controlling communications are endless. A minimum interface can be 3 wires – Ground, Transmit, and Receive – but what to do with the remaining pins? The voltage levels are between ±3 and ±15 volts allowing

plenty of leeway for both drivers and receivers. When connecting equipment with RS232 interfaces, it is important to know which is classified as the Data Controlling Equipment (DCE) and which is Data Terminal Equipment (DTE).

The remaining pins have other functions not normally used for basic interconnection, and are documented in the EIA-232-D or CCTT V24/28 specification.

118

Receiver samples in middle of each data element

Parity

Parity checking requires the addition of an extra bit to the data byte. The parity system may be either ‘odd’ or ‘even’ and both systems give the same level of error detection.

In an odd parity system, the overall count of ‘1’s in the combined data byte, plus parity bit, is odd. Thus, with an 8 bits data byte of ‘10101100’ the parity bit would be set to ‘1’.

In an even parity system, the overall count of ‘1’s in the combined data byte, plus parity bit, is even. Thus, with an 8 bits data byte of ‘10101100’ the parity bit would be set to ‘0’.

If corruption of either data bytes or of the parity bit itself takes place, when the receiver carries out the parity check, the corruption will be recognized. In the event of more than one bit being corrupted, it is possible that the receiver will not recognize the problem, provided that the parity appears correct. So, parity checking is not a cast iron method of checking for transmission errors, but in

119

practice, it provides a reasonable level of security in most systems. The parity system does not correct errors in itself; it only indicates that an error has occurred and it is up to the system software to react to the error state; in most systems this would result in a request for re-transmission of the data.

The PICmicro®MCU does not have on-chip parity testing or generation, so the function needs to be generated in software. This adds an overhead to the code generated which could have a knock on effect on execution times.

Bit Rate Time Calculation

As BAUD is bits per second, each data bit has a time of 1/(baud rate)

This works out as 1200 baud = 833uS, 2400 baud = 416uS, 9600 baud = 104uS

ASCII Conversion Table

Control

120