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pilar
Joined: 30 Jan 2008
Posts: 197
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| How compute the Sampling frequency of Pic18F4550 |
 Posted: Wed Apr 28, 2010 7:52 pm |
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Someone can tell me how to compute the frequency of sampling in this code considerate all the delays, I'm using a Pic18f4550 with 20Mhz of frequency
| Code: | #include <18F4550.h>
#DEVICE ADC=8
//Configurado para trabajar con 20MHz
#fuses HSPLL,NOWDT,NOPROTECT,NOLVP,NODEBUG,USBDIV,PLL5,CPUDIV2,VREGEN
#use delay(clock=48MHZ)
#use rs232(baud=9600, xmit=PIN_C6, rcv=PIN_C7, ERRORS)// RS232 Estándar
void main() {
int32 Buffer_ADC0;
float Valor_ADC0;
setup_adc_ports(AN0);
setup_adc(ADC_CLOCK_DIV_32);
set_adc_channel(0);
delay_us(10);
while (TRUE){
set_adc_channel(0);
delay_us(4);
Buffer_ADC0 = read_adc();
delay_us(10);
Valor_ADC0 = (float)Buffer_ADC0 *(0.0529);
printf("%4.2f \r\n", Valor_ADC0);
}
} |
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PCM programmer
Joined: 06 Sep 2003
Posts: 21708
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| Posted: Wed Apr 28, 2010 7:59 pm |
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Put a pulse in the loop and look at it on an oscilloscope or logic analyzer.
I put a 1us delay in it, to make sure the pulse would be long enough for
you to see on the scope:
| Quote: |
while (TRUE){
output_high(PIN_B0);
delay_us(1);
output_low(PIN_B0);
set_adc_channel(0);
delay_us(4);
Buffer_ADC0 = read_adc();
delay_us(10);
Valor_ADC0 = (float)Buffer_ADC0 *(0.0529);
printf("%4.2f \r\n", Valor_ADC0);
}
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Or, toggle a pin. Measure the frequency and multiply it by 2 to get
the actual loop frequency.
| Quote: |
while (TRUE){
output_toggle(PIN_B0);
set_adc_channel(0);
delay_us(4);
Buffer_ADC0 = read_adc();
delay_us(10);
Valor_ADC0 = (float)Buffer_ADC0 *(0.0529);
printf("%4.2f \r\n", Valor_ADC0);
} |
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pilar
Joined: 30 Jan 2008
Posts: 197
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| Posted: Wed Apr 28, 2010 9:10 pm |
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 Thank.. |
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Ttelmah
Joined: 11 Mar 2010
Posts: 19328
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| Posted: Thu Apr 29, 2010 2:16 am |
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In fact though, the main 'timing' element, will be the serial transmission. Not really the PIC's sampling speed. 4 characters minimum for the float, plus a space, plus the line feed, and carriage return. 7 characters. Default 8bits, no parity, one stop, so 70 bit times. About 7.3mSec. The last two characters will be 'in transmission', when the code loops, so the ADC conversion time, delays etc., will all occur while these are sending. The multiplication, and the divisions needed for the printf, will add up to no more than perhaps 1mSec, so will all be 'done' before the second character completes transmission, so the timing should be within a very few uSec, of this time. So about 135 bytes/sec.
Now, if this is being sent to another device, who's speed can be controlled, you could basically double this by going to 19200bps. Going 'beyond' this, then requires saving time in the loop. For instance, choosing to keep the number as an integer, and multiply by '53', then dividing by ten and sending using %4.2LW, might be acceptable, and will be slightly faster, allowing the baud rate to be increased yet further and still get full performance. Reducing the string length (do you really 'need' the trailing space, and both a LF, and a CR?) - if not just send the number, and CR, and the speed will increase by over 25%...
Go to 57600bps, switch to the integer arithmetic, and reduce the transmitted string to just five characters, and you could push something over 1000 samples/second.
Best Wishes |
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