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Mike Walne
Joined: 19 Feb 2004
Posts: 1785
Location: Boston Spa UK
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| Interrupts to regulate ADC sampling interval |
 Posted: Sun Mar 11, 2012 10:42 am |
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Yes ADC=8 sets 8 bit ADC conversions.
The CCS manual gives several examples of how to do interrupts.
In your case, you want to sample the ADC at 2ms intervals.
Basically you set up a 2ms interrupt in your initialisation routine, and write an interrupt routine.
So the whole thing COULD be of the form:-
| Code: |
int8 data_store[2,x];
initialise()
{
set_your_timer_of_choice_to_overflow_at_say_~2ms_intervals();
set_your_timer_of_choice_interrupt_on;
set_global_interrupts_on;
}
interrupt_routine_ISR()
{
read_adc;
start_next_conversion;
save_adc_data_to_data_store;
}
main()
{
initialise();
while (true)
{
taska();
taskb();
.....
}
}
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Where taska, taskb, ...... are your other functions.
Because you are sending data via USB at the same time as collecting fresh data you may get a conflict.
The way round would be to have a two dimensional array, data_store[2,x], and work in two phases.
In phase_0; data goes to data_store[0,_], and data_store[1,_] is sent to USB, until empty.
When data_store[0,_] is full, you go to phase_1.
In phase_1; data goes to data_store[1,_], and data_store[0,_] is sent to USB, until empty.
When data_store[1,_] is full, you go back to phase_0.
Mike |
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Ttelmah
Joined: 11 Mar 2010
Posts: 19826
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| Posted: Sun Mar 11, 2012 11:54 am |
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If you look at the thread I already pointed to:
<http://www.ccsinfo.com/forum/viewtopic.php?t=47494>
This shows 'how' to do ADC sampling at a timed interval.
You only need to change this line:
CCP_2=399;
to sample at a lower frequency (It is actually an instruction count-1, so at 48MHz - 12MIPS, to sample at 500Hz, a value of 23999, will work).
Then just expand the number of samples - as shown it takes 256, increase the array to 1024, and the count the same amount, and send these how you want.
The existing code uses CDC, and sends the data in hex, but will still send 1000 records of data in under 1/50th of a second, if your program at the other end accepts it this fast....
Seriously look at your PC code carefully, if things are taking too long.
Best Wishes |
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Mike Walne
Joined: 19 Feb 2004
Posts: 1785
Location: Boston Spa UK
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| Posted: Sun Mar 11, 2012 12:07 pm |
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| Quote: |
In phase_0; data goes to data_store[0,_], and data_store[1,_] is sent to USB, until empty.
When data_store[0,_] is full, you go to phase_1.
In phase_1; data goes to data_store[1,_], and data_store[0,_] is sent to USB, until empty.
When data_store[1,_] is full, you go back to phase_0.
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Forgot to say, I'm assuming the rest of your system allows the USB to empty one data store in LESS time than the PIC fills the other. If this does not happen, you're in trouble; you then need to sort out the USB etc.
Mike |
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hops-cc
Joined: 06 Mar 2012
Posts: 6
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| Posted: Wed Mar 14, 2012 3:58 pm |
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Hi Everyone;
Now i take 64 samples at each time and send them to matlab. I write a delay (delay_ms(2)) in order to obtain a sampling frequency of 500Hz = 1/2ms. (I Know, This calculation is theorically and it's not exactly 500 Hz because of TAd and ADC settling times.)
In matlab i can see the fourier spektrum of the signal, BUT i saw it a little bit shifted in frequemcy domain when i wrote the sampling frequency=500 Hz. That's because matlab requires the exact sampling frequency of the signal and i dont know my PIC's sampling interrval exactly. How can i know the exact value of the sampling interval ?
Here's my codes. Can you help me with obtaining desired sampling frequency 500Hz ?
| Code: |
#include <18F2550.h>
#device ADC=8
#fuses HSPLL,USBDIV,PLL5,PUT,CPUDIV1,VREGEN,NOWDT,NOPROTECT,NOLVP,NODEBUG,NOMCLR
#use delay(clock=20000000)
#define USB_HID_DEVICE FALSE
#define USB_EP1_TX_ENABLE USB_ENABLE_BULK //Uçnokta1'de Yığın transferi aktif
#define USB_EP1_RX_ENABLE USB_ENABLE_BULK
#define USB_EP1_TX_SIZE 64 //Uçnokta1 için maksimum alınacak ve gonderilecek
#define USB_EP1_RX_SIZE 64 //veri boyutu (64 byte)
#include <pic18_usb.h>
#include <USB_Konfigurasyon.h> //USB konfigurasyon bilgileri bu dosyadadır.
#include <usb.c>
#define UcNokta1 1
#define Komut gelen_paket[0]
//Yazılım (Firmware) Sürüm no
#define surum_no1 0x00 //surum_no2.surum_no1 ornek: 1.0
#define surum_no2 0x01
//Komutlar
#define surum_oku 0x03
#define olcum_al 0x02
static unsigned int16 ADC_deger1 = 0;
static unsigned int16 ADC_deger2 = 0;
unsigned int8 i ;
void user_init(void)
{
set_tris_a(0x01); //RA0,RA1 giriş
set_tris_b(0xFF);
setup_adc_ports(AN0);
setup_adc(ADC_CLOCK_INTERNAL);
}
unsigned int16 ADC_Oku(byte kanal)
{
unsigned int16 olcum=0;
set_adc_channel(kanal);
delay_us(20);
olcum = read_adc();
return olcum;
}
void main(void)
{
byte sayac=0;
byte gelen_paket[64]; //gelen paket
byte gond_paket[64]; //gönderilecek paket
byte surum1=surum_no1;
byte surum2=surum_no2;
user_init();
usb_init();
usb_task();
usb_wait_for_enumeration(); //Cihaz, hazır olana kadar bekle
if(usb_enumerated())
output_high(Pin_A2); //USB bağlantısı kurulduysa LED'i yak.
while(usb_enumerated())
{
for (i=1;i<=64;++i)
{
delay_ms(2);
ADC_deger1=ADC_Oku(0);
gond_paket[i] = (byte) ADC_deger1;
}
sayac = 0x40;
if(sayac!=0)
{
usb_put_packet(UcNokta1, gond_paket, sayac, USB_DTS_TOGGLE);
sayac = 0;
Komut = 0;
}
}
}
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Another problem is that; when i take 64 samples each time. The first byte of char, always equals to 75
(gond_paket[0]=75) why that fault occurs ?
The third thing it that; I want to make an isochronous transfer but i couldn't made it yet. There's not enough information on the web as far as i searched. |
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Mike Walne
Joined: 19 Feb 2004
Posts: 1785
Location: Boston Spa UK
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| ADC timings etc. |
| Posted: Wed Mar 14, 2012 5:37 pm |
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From your code
| Code: |
for (i=1;i<=64;++i)
{
delay_ms(2);
ADC_deger1=ADC_Oku(0);
gond_paket[i] = (byte) ADC_deger1;
}
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The inter-sample interval is just over 2ms (assuming that you actually have a 20MHz crystal).
Ttelmah and I have shown you two ways of getting well defined sample intervals, both take care of the set up times etc.
You are NOT taking a new sample gond_paket[0].
The 75 value you're complaining about is whatever default value happens to be in RAM.
Look at your code; the START value for i is 1, rather than the zero you seem to expect.
replace your code
| Code: |
for (i=1;i<=64;++i)
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with
| Code: |
for (i=0;i<=63;++i)
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Mike |
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hops-cc
Joined: 06 Mar 2012
Posts: 6
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| Posted: Thu Mar 15, 2012 9:40 am |
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| Thanks to everypost. You guys really helped me out. I've just seen all the replies. I will try all of them and write a feedback here. Thanks. |
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