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arunb
Joined: 08 Sep 2003 Posts: 492 Location: India
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Problem setting up ADC in 16F690 mcu |
Posted: Wed Feb 27, 2008 5:31 am |
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MCU: 16F690
Compiler: PCM 3.228
Hi,
I use the code below to setup the ADC of the 16F690 mcu.
Code: |
#include <16F690.h>
#fuses INTRC_IO,NOWDT,PUT,NOPROTECT,NOCPD,BROWNOUT,NOIESO,NOMCLR,NOFCMEN
#use delay(clock=8000000)
#byte WPUA=0x95 //Weak Pull ups on portA
#bit WPUA0=WPUA.0
void initialise()
{
setup_comparator(NC_NC_NC_NC);
set_tris_a(0b11111111);
set_tris_b(0b01101111);
set_tris_c(0b10110111);
WPUA0=0; //turn off pullup on RA0
setup_adc_ports(sAN0|VSS_VDD);
setup_adc(ADC_CLOCK_INTERNAL);
}
//this reads channel 0 of the ADC
float read_intensity()
{
float ft;
set_adc_channel(CHN_LED_INTENSITY); //Read the ADC channel
delay_us(10);
fTmp=((read_adc()*5.0)/1024.0)*1000; //calculation of voltage on ADC channel
return fTmp;
}
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The above code compiles well, and I am able to get the MCU going, but I find that when powering up the mcu, the pin RA0 generates a voltage of 5.4V (whereas VDD is only 5 VDC), also the pin is not connected to any other device, so there should not be any voltage
Isn't RA0 supposed to have approx. 0 V ??, I have also managed to disable pull-ups on RA0, by clearing bit 0 of WPUA (see code above).
Kindly explain what is wrong with the code ??
thanks
arunb
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ECACE
Joined: 24 Jul 2006 Posts: 94
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Posted: Wed Feb 27, 2008 8:44 am |
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Couple of things:
1) You have not defined CHN_LED_INTENSITY
2) You have not defined fTmp
3) In your hardware, do you have floating pins? If so, you should tie any floating pins to ground via say 2.2K. Floating pins have a way of reacting erratic. _________________ A HW Engineer 'trying' to do SW !!! Run!!! |
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arunb
Joined: 08 Sep 2003 Posts: 492 Location: India
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RE: |
Posted: Wed Feb 27, 2008 11:34 pm |
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I have extracted the relevant parts from the main program code,
CHN_LED_INTENSITY is 0 this defines the channel number of the ADC.
#define CHN_LED_INTENSITY 0
fTemp is a float value.
I believe the ADC is not setup properly, it seems I am missing out on some thing.
Kindly explain what you mean by floating pin, isn't the ADC pin supposed to be floating, but in my case the pins shows a steady voltage of 5.4 V DC !!
I wonder why ??
thanks
arunb |
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Ttelmah Guest
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Posted: Thu Feb 28, 2008 3:51 am |
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Think about it for a moment.
The PIC _cannot_ 'generate' an output above the voltages present on it's pins (the only internal 'voltage generator', is the programming voltage generator for the flash memory, and this is itself 'slaved' to the voltage on Vpp). If you are seeing '5.4v', it implies _either_, that a voltage above 5v, _is_ being presented to the pin (most likely - triple check for a short somewhere...), _or_ that another pin is being pulled to a higher voltage, and with the internal protection diodes, is _raising_ the supply the PIC is internally using. Both scenarios, are external wiring faults, not faults with the code.
Though there may be faults that will stop the ADC working, _none_ can generate voltages above the supply rail. You need to find what is generating the 5.4v signal, bfore proceeding...
Best Wishes |
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