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maurolarrat
Joined: 05 May 2009
Posts: 5
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| About the VREF+ and VREF- |
 Posted: Fri Jun 19, 2009 3:16 pm |
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hi all,
I have a problem with CCS, when I need specify a reference voltage to the ADC in a PIC16F877A, for a value different of 0V and 5V (vref- and vref+, respectivelly).
If you could show me a schema, I thanks a lot.
Translation:
| Quote: |
Hello to all,
I have a problem with CCS to specify the A/D voltage reference.
There exists the method of setup_adc_ports() where the parameter
passed to it is AN0_VREF_VREF.
In the case of setup_adc_ports(AN0_VREF_VREF):
In accordance with the header file of the PIC, it says to me that with
AN0_VREF_VREF, the reading will be in pin RA0, with RA2 being VREF-
and RA3 being VREF+, but this is not what happens. I feed RA2 and RA3
with 0.7 and 2.7 Volts respectively, but the reference voltage continues
being 0V and 5V. If somebody has already worked with a voltage
reference for the PIC and will be able to give me a tip, I will be thankful
to you. There follows an attachment of my test circuit .
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Olá a todos,
Estou com um probleminha no CCS quando preciso especificar uma tesão de referência para o AD diferente de 0V a 5V .
existe um método setup_adc_ports( );
onde o parâmetro passado para ele é AN0_VREF_VREF, no caso
setup_adc_ports(AN0_VREF_VREF );
De acordo com o arquivo cabeçalho do PIC, AN0_VREF_VREF me diz que a leitura será no pino RA0, com RA2 sendo VREF- e RA3 sendo VREF+, mas não é isso que acontece. Eu alimento RA2 e RA3 com 0.7 e 2.7 Volts respectivamente, mas a tensão de referência continua sendo 0V e 5V.
Se alguém já estiver trabalhado com uma tensão de referência diferente da do PIC e puder me dar uma dica agradeço.
segue anexado o esquema do meu circuito de teste. |
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gilavar
Joined: 03 Mar 2009
Posts: 24
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| Re: About the VREF+ and VREF- |
| Posted: Fri Jun 19, 2009 3:27 pm |
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This what i used in my application for analog inputs 5 and 6.
| Code: |
setup_adc_ports(sAN5 | sAN6 | sAN11, VREF_VREF);
setup_adc(ADC_CLOCK_DIV_4); |
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PCM programmer
Joined: 06 Sep 2003
Posts: 21708
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| Posted: Fri Jun 19, 2009 3:30 pm |
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| He's using a 16F877A. It doesn't use the "sANx" style of constants. |
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gilavar
Joined: 03 Mar 2009
Posts: 24
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| Posted: Fri Jun 19, 2009 3:52 pm |
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| PCM programmer wrote: | | He's using a 16F877A. It doesn't use the "sANx" style of constants. |
Ooops,
(AN0_VREF_VREF) Does not look right, isn't it?  |
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PCM programmer
Joined: 06 Sep 2003
Posts: 21708
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| Posted: Fri Jun 19, 2009 5:04 pm |
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I created the Vref voltages with a voltage divider made from 3 resistors,
and I connected it to the PIC as shown below.
| Code: |
2.2K 2.0K 820 ohms
+5v ---/\/\/\/---o---/\/\/\/---o---/\/\/\/--- Ground
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2.8v 0.8v
To pin RA3 To pin RA2
Vref+ Vref-
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I have a 5K trimpot connected to pin A0 to provide the input analog
voltage. If I set the trimpot to 2.82v, I get this output from the program
below. (The compiler is doing the A/D conversions in 8-bit mode).
| Quote: | 254
254
254
254
254 |
If I set the voltage on pin A0 to 0.83 volts, I get the following output:
Those readings are correct, for the given Vref circuit. This was tested
with vs. 4.093 of the compiler, on a PicDem2-Plus board.
Here is the test program:
| Code: |
#include <16F877A.H>
#fuses XT, NOWDT, NOPROTECT, BROWNOUT, PUT, NOLVP
#use delay(clock=4000000)
#use rs232(baud=9600, xmit=PIN_C6, rcv=PIN_C7, ERRORS)
//=============================
void main()
{
int8 result;
setup_adc_ports(AN0_VREF_VREF);
setup_adc(ADC_CLOCK_DIV_8);
set_adc_channel(0);
delay_us(20);
while(1)
{
result = read_adc();
printf("%u \n\r", result);
delay_ms(500);
}
} |
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Guest
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| about the solution |
| Posted: Sat Jun 20, 2009 8:06 am |
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thanks, I'm using the Proteus 7.4 to simulate the circuit. Then, I print the voltage values on LCD 16X2. I'll try to use your example and come back to show the results.
Thanks. |
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