Capture Mode of Pic18F252

Transcription

1 Capture Mode of Pic18F252 PIC18F253 has two Capture/Compare/Pulse Width Modulation modules. Some devices such as ADCs, Sensors (position, velocity, accelearstion, temperature [MAX6577 converts the ambient temperature into a square wave with a frequency proportional to absolute temperature] and etc), generate pulses which their frequency or pulse width is the value of the sensor measurement. In this section we look at how we can measure the frequency and and Pulse Width of the in coming signal using a Pic18F252 microcontroller. The Block diagram below shows the Capture mode of CCP module in the PIC18F252. When the Capture is made, the interrupt flag bit CCPxIF (x is 1 or 2) is set, which if the interrupt is enabled, the program would jump to the Interrupt Service Routine (ISR) to deal with the event. This flag MUST be cleared in software. In capture mode the CCPx pin must be configured as input. The timer (timer1 or timer3) used must be running in the timer mode (i.e. not in counter mode). There are three prescalars (1, 4 and 16) with rising edge and one (just prescalar 1) with falling edge. T3CCPx bit of T3Con register determines which timer is to be used. ENG531 Microcontrollers and Programmable Logic : Hassan Parchizadeh Page 1

2 CCP1CON REGISTER/CCP2CON REGISTER To set up the CCP in Capture mode, CCPxCON bits, CCPxM3 -> CCPxM0 should configuired as one of the 4 modes shown below :- ENG531 Microcontrollers and Programmable Logic : Hassan Parchizadeh Page 2

3 Eample 1: A sensor produces a signal which it s period/frequency is dependent on the measured value. A PIC18F252 with a crystal oscillator of 24 MHz is used to capture the signal and measure the period of the siganal. The longest period of the sensor signal is shorter than cycles. Write a C program to achieve this. #include <p18f252.h> void main (void){ unsigned short Period //Configuration of T1 to be used as the base clock for CCP //****************************************************************** TRISCbits.TRISC2 = 1 // CCP1 pin as set as input T3CON = 0x80 // timer1 as base for CCP1 T1CON = 0x81 // enable 16-bit T1,Prescalrt 1 // clear CCP1 flag TMR1L= TMR1H = 0 // clear TMR1 CCP1CON = 0x05 // capture on every rising edge Period = CCPR1 CCP1CON = 0 Period = CCPR1 - Period while(1) // wait for the 1st rising edge // save the 1st rising edge event // clear CCP1 flag // wait for the 2nd rising edge // disable CCP1 capture // calculate the Period in M/Cs // do nothing Eample 2 : Repeat the Example 1, using CCP1 interrupt service routine. ENG531 Microcontrollers and Programmable Logic : Hassan Parchizadeh Page 3

4 #include <p18f252.h> unsigned short Period=0,edge=0 void ccp_isr (void) // edge=0, set up for the first edge // declare interrupt function before definition #pragma code high_vector=0x08 void high_interrupt (void){ _asm GOTO ccp_isr _endasm #pragma code #pragma interrupt ccp_isr void ccp_isr(void) { if(edge==0){ // check for first rising edge Period = CCPR1 // save the 1st rising edge event edge = 1 // set up for the second edge else{ // is the second rising edge Period = CCPR1 - Period // calculate the Period in M/Cs edge = 0 // set up for the first edge TMR1L= TMR1H = 0 // clear TMR1 // clear int flag PORTBbits.RB0=~PORTBbits.RB0// just a test of int. being called void main (void) { //Configuration of T1 to be used as the base clock for CCP //******************************************************************* TRISB = 0 // PORTB as outputs T3CON = 0x80 // timer1 as base for CCP1, T3 OFF T1CON = 0x81 // enable T1, 16-bit,Prescalr 1 TMR1L= TMR1H = 0 // TMR1 = 0 CCPR1 = 0 // clear Capture register //CCP interrupt configuration //******************************************************************* TRISCbits.TRISC2 = 1 // make RC2/CCP1 pin input CCP1CON = 0b // capture mode, event=rising edge // make sure Timer 1 interrupt flag is cleared // General interrupt configuration //******************************************************************* RCONbits.IPEN = 1 // enable interrupt priority scheme PIE1bits.CCP1IE = 1 // enable Capture interrupt INTCONbits.GIEH = 1 // enable global interrupts while(1){ /*do other thing here*/ // wait for interrupts in infinite loop ENG531 Microcontrollers and Programmable Logic : Hassan Parchizadeh Page 4

5 Eample 3: A sensor produces a signal which it s pulse width is dependent on the measured value. A PIC18F252 with a crystal oscillator of 24 MHz is used to capture the signal and measure the period of the siganal. The longest pulse width of the sensor signal is shorter than cycles. Write a C program to achieve this. #include <p18f252.h> void main (void){ /* CCP registers configuration * Fosc = Hz,use Timer 1, Prescaler is 1 * Ensure that your PWM pin is configured as digital input */ unsigned short Period TRISCbits.TRISC2 = 1 // CCP1 pin as set as input T3CON = 0x81 // timer1 as base for CCP1 T1CON = 0x81 // enable 16-bit T1,Prescalrt 1 // clear CCP1 flag TMR1L= TMR1H = 0 // clear TMR1 CCP1CON = 0x05 // capture on every rising edge Period = CCPR1 CCP1CON = 0x04 // wait for the 1st rising edge // save the 1st rising edge event // capture on every falling edge // clear CCP1 flag CCP1CON = 0 Period = CCPR1 - Period while(1) // wait for the 2nd rising edge // disable CCP1 capture // calculate the Period in M/Cs // do nothing Eample 4 : Repeat the Example 3, using CCP1 interrupt service routine. ENG531 Microcontrollers and Programmable Logic : Hassan Parchizadeh Page 5

6 #include <p18f252.h> unsigned int Period=0,edge=0 void ccp_isr (void) // edge=0, set up for the first edge #pragma code high_vector=0x08 void high_interrupt (void){ _asm GOTO ccp_isr _endasm #pragma code #pragma interrupt ccp_isr void ccp_isr(void){ if(edge==0){ Period = CCPR1 edge = 1 CCP1CON = 0b else{ Period = CCPR1 - Period edge = 0 TMR1L= TMR1H = 0 CCP1CON = 0b // check for first rising edge // save the 1st rising edge event // set up for the second edge // capture mode, event=rising edge // is the second rising edge // calculate the Period in M/Cs // set up for the first edge // clear TMR1 // capture mode, event=rising edge // clear int flag PORTBbits.RB0=~PORTBbits.RB0// just a test of int. being called void main (void){ TRISB = 0 T3CON = 0x80 // timer1 as base for CCP1, T3 OFF T1CON = 0x81 // enable T1, 16-bit,Prescalr 1 TMR1L= TMR1H = 0 // TMR1 = 0 CCPR1 = 0 // clear Capture register //CCP interrupt configuration //************************************************************************* TRISCbits.TRISC2 = 1 // make RC2/CCP1 pin input CCP1CON = 0b // capture mode, event=rising edge // make sure Timer 1 interrupt flag is cleared // General interrupt configuration //************************************************************************* RCONbits.IPEN = 1 // enable interrupt priority scheme PIE1bits.CCP1IE = 1 // enable Capture interrupt INTCONbits.GIEH = 1 // enable global interrupts while(1){ // wait for interrupts in infinite loop ENG531 Microcontrollers and Programmable Logic : Hassan Parchizadeh Page 6