Here is a test program btw:
/*
A program for measuring musical frequencies. Adapted from johnt's CCP test
program found at http://www.mikroe.com/forum/viewtopic.php?f=88&t=26489
The program measures a pulse train on the CCP1 input pin (PORTC.B2) and
calculates the input frequency through an average of 16 samples.
The result is accurate down to +/- 0.1 Hz, which is good enough for audio
frequencies as the human ear cannot distinguish between changes less than 3 Hz.
HARDWARE:
The code is written for PIC18F458 at 8MHz.
T1 a 16 bit counter in free running mode is used to capture pulse widths
PORTC Pin 2 is used for input
PORTB are used an LCD display, connected to the default pins of the
Mikroelektronika EasyPIC3 development board.
Written using MicroC PRO for PIC v 4.60
If changing the operating speed, you must take care so that T1 does not over-
flow.
The current implementation seems to be accurate for frequencies in the range
150Hz to 1.25kHz.
To interface the circuit with an audio signal, an external circuit is
necessary. Ex: To convert a +/- 15V sine or similar signal into a 0-5v input
pulse, the following circuit can be used (though it requires a triple-
value power supply):
LM311 comparator /--------------> output to PORTC.B2
------ |
GND <-------------|1 8|---> +15V |
Input <-------------|2 7|-------------o-[ 1kOhm pull up resistor ]--> +5V
GND <-------------|3 6|---\ (connect pin 5 to pin 6) -15V <-------------|4 5|---/ ------ Joakim Tysseng, March 2011 */ #include "built_in.h" // to get useful built in commands highest() delay_ms() etc //PROTOTYPES void interrupt(void); void main(void); void hardwareInit(void); // setup hardware start conditions void storeInput(void); // copies a captured value into the array of periods. void startCapture(); // performs a capture of the selected number of pulses. unsigned int getAverageCapture(); unsigned int calculateFrequency(unsigned int timerValue); void Lcd_Out_Timer(unsigned short row, unsigned short col, unsigned int number); void Lcd_Out_Freq(unsigned short row, unsigned short col, unsigned int number); // LCD module connections sbit LCD_RS at RB2_bit; sbit LCD_EN at RB3_bit; sbit LCD_D4 at RB4_bit; sbit LCD_D5 at RB5_bit; sbit LCD_D6 at RB6_bit; sbit LCD_D7 at RB7_bit; sbit LCD_RS_Direction at TRISB2_bit; sbit LCD_EN_Direction at TRISB3_bit; sbit LCD_D4_Direction at TRISB4_bit; sbit LCD_D5_Direction at TRISB5_bit; sbit LCD_D6_Direction at TRISB6_bit; sbit LCD_D7_Direction at TRISB7_bit; // End LCD module connections //GLOBAL CONSTANTS const unsigned short pulsesToCapture = 16; //PREDEFINED TEXTS char txt1[] = "Timer....: "; char txt2[] = "Frequency: "; //GLOBAL VARIABLES unsigned int capturedData = 0 ; // value captured by the CCP module unsigned short captures[2*pulsesToCapture] ; // cyclic storage for 16 captures (ints) unsigned short pulsesCaptured = 0 ; //GLOBAL POINTERS unsigned short *pCaptures ; // pointer to the captures array void main(){ unsigned int frequency; unsigned int average; hardwareInit() ; Lcd_Init(); Lcd_Cmd(_LCD_CLEAR); Lcd_Cmd(_LCD_CURSOR_OFF); Lcd_Out(1,1,txt1); Lcd_Out(2,1,txt2); startCapture(); // first capture do { //perform another capture when we are done treating the previous one. if(pulsesCaptured > pulsesToCapture){
average = getAverageCapture();
frequency = calculateFrequency(average);
Lcd_Out_Timer(1, 12, average);
Lcd_Out_Freq(2, 11, frequency);
startCapture();
}
} while (1);
}
// Start capture process, enabling the CCP module and resetting necessary
// variables.
void startCapture(){
pCaptures = &captures ; // resetting overwrites previous captures
pulsesCaptured = 0;
CCP1CON = 0b00000101 ; // Start capture, capture on every rising edge
}
//returns the frequency x 10, the last digit is the first decimal place.
unsigned int calculateFrequency(unsigned int timerValue){
unsigned int clockcycles;
// We have to add 17 timer ticks to the supplied timer value.
// This is because we lose some time between when the interrupt is triggered
// and the timer is restarted. If we do not compensate for this, the measured
// frequency will be too high. The error will be larger for higher
// frequencies, as the lost time accounts for a larger proportion of the total
// period.
clockcycles = (timerValue + 17) * 4;
return (Clock_kHz() * 10000 ) / clockcycles;
}
unsigned int getAverageCapture(){
unsigned short i;
unsigned long sum = 0;
for(i=0; i<2*pulsestocapture;>> 4;
}
void hardwareInit(void) { // basic hardware control
ADCON1 = 0x07;
PORTA = 0x00 ;
TRISA = 0X00 ;
PORTC = 0b00000100 ; // normally high - depending on hardware
TRISC = 0b00000100 ; // portc.b1 as input
T1CON.TMR1ON = 0 ; // ensure it's off
T1CON.T1OSCEN = 0 ; // ensure OSC is OFF
T1CON.TMR1CS = 0; // Timer1 counts pulses from internal oscillator
T1CON.T1CKPS1 = 0; // prescale the input pulses
T1CON.T1CKPS0 = 0; // Assigned T1 prescaler rate is 1:1 so T1 is Fosc/4 with 20Mhz clock = 0.2uS per tick
INTCON.PEIE = 1 ; // enable peripheral interrupts
PIE1.CCP1IE = 1 ; // enable CCP1 capture interrupt
INTCON.GIE = 1 ; // enable global interrupts
}
// How to accurately measure the delay between when an interrupt occurs and
// the time T1 is restarted (PIC18):
// - Every interrupt has a latency of three instruction cycles.
// - Each instruction takes two cycles, but the instruction pipeline handles
// two instructions at the time, meaning instructions only take one cycle in
// practice.
// - If a branch/goto is triggered, the pipeline is reset and the first
// instruction takes two cycles.
// In this code:
// 3 instructions interrupt latency
// 2 instructions in C, none of which triggers a branch.
// 11 instructions in assembly
// = 16 cycles.
// NB: For some reason the correct number to add to the timer when
// calculating the correct frequency is always one more than this.
// I am not sure where the last cycle goes, could it be interrupt jitter?
//INTERRUPT HANDLING
void interrupt(void) {
// Capture CPP interrupt ie. rising edge on PORTC pin 2 has occured
if (PIR1.CCP1IF){
// The following asm code is compiled from the C code below, and is
// kept in ASM to keep the instruction count the same for all versions
// of the compiler (necessary to achieve correct timing).
asm{
MOVF CCPR1H+0, 0
MOVWF _capturedData+1
MOVF CCPR1L+0, 0
MOVWF _capturedData+0
BCF T1CON+0, 0
CLRF TMR1H+0
CLRF TMR1L+0
BCF T1CON+0, 5
BCF T1CON+0, 4
BCF PIR1+0, 0
BSF T1CON+0, 0
}
// C code for the ASM above - if you want to recompile the code.
/*
hi(capturedData) = CCPR1H ; // capture data
lo(capturedData) = CCPR1L ;
// Restart Timer1
T1CON.TMR1ON = 0 ; //++ Turning off the timer also resets T1 prescaler
TMR1H = 0x00 ; // Set initial value for the timer TMR1 for maximum counts available
TMR1L = 0x00 ;
T1CON.T1CKPS1 = 0 ; // Assigned T1 prescaler rate is 1:1
T1CON.T1CKPS0 = 0 ;
PIR1.TMR1IF = 0 ; // clear T1 interrupt flag
T1CON.TMR1ON = 1 ; // turn T1 back on
*/
storeInput() ; // copy the captured data to the array of samples.
PIR1.CCP1IF = 0 ; // clear capture flag after grabbing data
}
}
void storeInput(void) {
if (pulsesCaptured == 0) { // ignore startup delay - start storing between first + second edge
pulsesCaptured = 1 ;
return ;
}
pulsesCaptured++;
if(pulsesCaptured > pulsesToCapture){
CCP1CON = 0 ; // Stop capture when the desired number of pulses have been captured.
}
*(pCaptures++) = hi(capturedData) ; // store captured hibyte:lobyte in array
*(pCaptures++) = lo(capturedData) ;
}
// Utility function that writes the five last digits of an integer to the LCD
// display
void Lcd_Out_Timer(unsigned short row, unsigned short col, unsigned int number){
unsigned char digits[5];
digits[0] = number % 10;
digits[1] = (number / 10) % 10;
digits[2] = (number / 100) % 10;
digits[3] = (number / 1000) % 10;
digits[4] = (number / 10000) % 10;
Lcd_Chr(row, col+4, 48 + digits[0]);
Lcd_Chr(row, col+3, 48 + digits[1]);
Lcd_Chr(row, col+2, 48 + digits[2]);
Lcd_Chr(row, col+1, 48 + digits[3]);
Lcd_Chr(row, col, 48 + digits[4]);
}
void Lcd_Out_Freq(unsigned short row, unsigned short col, unsigned int number){
unsigned char digits[5];
digits[0] = number % 10;
digits[1] = (number / 10) % 10;
digits[2] = (number / 100) % 10;
digits[3] = (number / 1000) % 10;
digits[4] = (number / 10000) % 10;
Lcd_Chr(row, col+5, 48 + digits[0]);
Lcd_Chr(row, col+4, 46);
Lcd_Chr(row, col+3, 48 + digits[1]);
Lcd_Chr(row, col+2, 48 + digits[2]);
Lcd_Chr(row, col+1, 48 + digits[3]);
if(digits[4] == 0){
Lcd_Chr(row, col, 32);
} else {
Lcd_Chr(row, col, 48 + digits[4]);
}
}
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