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Sid2286
Joined: 12 Aug 2010
Posts: 119
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| MCP4921 not working |
 Posted: Wed Dec 08, 2010 12:22 am |
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Hi,
I made this code wherein I get ADC input from a pot and display it on a LCD. This part works fine. However, I'm not able to get a DAC output. I have used mcp4921.c which is available.
My code is as follows:-
| Code: |
#include <18f2520.h>
#device ICD=TRUE
#device ADC=10
#fuses HS,NOWDT,NOPROTECT,NOLVP
#use delay(clock=4000000)
#include <flex_lcd.c>
#include <stdlib.h>
#include <mcp4921.c>
void main()
{
int16 a;
lcd_init();
init_dac();
setup_adc_ports(AN0);
setup_adc(ADC_CLOCK_DIV_64);
set_adc_channel(0);
delay_ms(100);
while(1)
{
a=read_adc();
delay_ms(1);
printf(lcd_putc,"\f %ld",a);
write_dac(a);
delay_ms(100);
}
}
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thanks,
Sid |
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PCM programmer
Joined: 06 Sep 2003
Posts: 21708
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| Posted: Wed Dec 08, 2010 12:13 pm |
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Post a list of all connections between the PIC and the mcp4921.
Also, are you testing this in real hardware, or in Proteus ? |
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Sid2286
Joined: 12 Aug 2010
Posts: 119
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| Connection details |
| Posted: Wed Dec 08, 2010 7:13 pm |
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C4-> CLK
C5-> CS
C6->DIN
C7->LDAC
Vref to vcc
yes, i'm testing it it on proteus.
Thanks,
sid |
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PCM programmer
Joined: 06 Sep 2003
Posts: 21708
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| Posted: Wed Dec 08, 2010 7:49 pm |
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Post the list of #define statements for the pins in mcp4921.c
This is the list in the CCS mcp4921.c driver:
| Code: | #define DAC_CS PIN_B0
#define DAC_CLK PIN_B1
#define DAC_DI PIN_B2
#define DAC_LDAC PIN_B4 |
You have changed the pins to use Port C pins.
Post the #define statements that you now have in mcp4921.c. |
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Sid2286
Joined: 12 Aug 2010
Posts: 119
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| Connection details |
| Posted: Wed Dec 08, 2010 9:05 pm |
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I made changes on mcp4921.c file
| Code: |
#define DAC_CS PIN_C5
#define DAC_CLK PIN_C4
#define DAC_DI PIN_C6
#define DAC_LDAC PIN_C7
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these are my #define on mcp4921 file |
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Sid2286
Joined: 12 Aug 2010
Posts: 119
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| Posted: Thu Dec 09, 2010 1:31 am |
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Ok! Finally i got it....
However I have a doubt... when I connect a ammeter in series with the pot and analog input of PIC. I don't get any reading.
But when i connect a voltmeter i get a reading which is 2.5V and a display as 512.
Ideally voltmeter shouldn't be working and ammeter should be working.
Is there anything wrong in my connections??
regards,
Sid |
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Ttelmah
Joined: 11 Mar 2010
Posts: 19509
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| Posted: Thu Dec 09, 2010 3:42 am |
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Not sure why you would expect detectable current to flow.....
The input impedance of the PIC, is extremely high. Current flow into the ADC, will be in the nA area _except_ when the voltage is changing fast. The PIC input, is seen as a couple of resistors, connecting into a small capacitor (hence current flow when the signal is changing), and then a few nA of leakage.
Best Wishes |
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Sid2286
Joined: 12 Aug 2010
Posts: 119
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| Posted: Thu Dec 09, 2010 4:51 am |
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Ok, i might have a confusion here.
Like in my program I want to see the input as well as the output from the PIC through the MCP4921.
So I was thinking that the analog input will be equal to the output. I guess its not the case.
I would like to know if I would have to add external circuit to calibrate with the input, like add another opamp.
Regards,
Siddhesh |
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temtronic
Joined: 01 Jul 2010
Posts: 9226
Location: Greensville,Ontario
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| Posted: Thu Dec 09, 2010 7:07 am |
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Well, at least we found out early this is yet another simulation !
The output of the DAC will never always be equal to the input to the PIC.
And there's the simulated voltmeter reading the simulated voltage on the simulated circuit problems...
Even in the real world, you must account for noise, ripple, adc ref. errors, impedance mismatches, etc. Every ADC and DAC is always 'off' +-1 bit, regardless of the number of bits in the device.
You can of course minimize the 'error' by filtering (hard or soft).
I figured I was doing really good with +-3 bit errors using 16 bit ADC within 2 feet of the ARL Spectrometers (20KV, 300ma, spark gap 1/4"). |
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