how to config only port AN4 for A/D input (16F876A)

sorasit46 · Joined: 07 May 2007 Posts: 69

Please help! 16F876A.
How to direct config AN4 for A/D. I try 3 ways.
1. setup_adc_ports(RA5_Analog); compile error .
2. setup_adc_ports(AN4); compile error
3. setup_adc_ports(AN4_Analog); compile error
Can I use only AN4(RA5) for A/D and leave other pin for digital I/O?
Thanks
regards

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

Look in the 16F876A.H file, in the section on Setup_ADC_Ports.
It shows all the options that you can use to configure the A/D pins.
You can't select AN4 as an analog pin by itself. This is a limitation
of the PIC hardware design in the 16F876A. Some later PICs allow
individual selection of A/D pins, but not the 16F876A.

Ttelmah · Guest

However, it is worth 'remembering', that (with an increase in the 'noise' present on AD readings), you can use bits defined as 'ADC' inputs, as logic outputs. So (for instance), you setup:

sorasit46 · Joined: 07 May 2007 Posts: 69

In fact only Pin AN4 use A/D.Another port use normal I/O port,If I set :setup_adc_ports(All_ANALOG);I don't sure .It may be have problem on another ports.
regards

Ttelmah · Guest

PCM programmer has already answered this. It is _not_ possible on this chip, to only select AN4 as analog. Read the _data sheet_. However it _is_ possible to select all the pins (AN0 to AN4) as analog, and then still use the pins for digital output. When a pin is selected as 'analog', it turns off the logic input buffer, but the output drivers are still available, if enabled by clearing the TRIS bit. There are only four selections that enable AN4 as analog, the other three, use one of more of the Vref pins, and _all_the selections alowing analog input on AN4, also use all the lower pins as analog.
This is a real example, that you _must_ use the chip's data sheet, to make decisions about what to connect to pins, and even what chip to use.

Best Wishes

sorasit46 · Joined: 07 May 2007 Posts: 69

Now I have big problem. Because I'm ready have pcb.If I can not config only pin_an4 for A/D input and leave other pin for digital I/O.Can you advice other chip which place all pin like 16F876A ( include portA,portB,portC).Except only pin_an4 (pin 7)for direct config A/D input.
regards

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

Look at the 16F886.
http://www.microchip.com/stellent/idcplgidcplg?IdcService=SS_GET_PAGE&nodeId=1335&dDocName=en026562

Also check the errata sheet on that page. The i2c modes have got
problems.

sorasit46 · Joined: 07 May 2007 Posts: 69

In 16F876A I'm solve by open A/D a moment. When finish read A/D,
I cancel A/D back to normal I/O port.
.
.
set_tris_a(0b00101100);
.
.
setup_adc_ports(All_ANALOG); // All pin(AN0-AN4) for ADC
setup_adc(ADC_CLOCK_INTERNAL); // Clock RC
set_adc_channel(4); // Read Analog input RA5/AN4 (channel=4)
delay_us(10); // must let afew time for read analog input
Vread = Read_ADC();
setup_adc_ports(NO_ANALOGS); // Don't fix port for ADC
setup_adc(ADC_OFF); // Off A/D function
.
.

Please ! teach me .What mean off
1. #use fast_io(A). I don't understand.
2. #byte port_b=6. If not register port b .what happen or it's not stable ?

regards