PORTA.4 as analog input

[kblim]

Hi all,
I'm having problem to use the porta.4 as the analog input for PIC16F88... When i connect the signal line (0.1v to 0.9v) to porta.4, the signal voltage will become 0v. I have tried to put a 10M series resistor to porta.4 and connect another 10M resistor to the ground. Anyone have the idea?

I also tried to connect the signal line to other AN, for example AN2. Now the voltage on the signal will be .2 lesser than the original voltage. I really don't have any idea on this. Please help!!

Thanks!

[Charles Linquis]

Without knowing more, all I can tell you is that 10M is far too high a resistance for the input of a PIC to measure. If you want any kind of accuracy, you should keep the input impedance to below 4K or so. Use two 5K resistors
in series between Vcc and GND and connect the junction to the A/D input, then read the A/D and see what you get.

[kblim]

Without knowing more, all I can tell you is that 10M is far too high a resistance for the input of a PIC to measure. If you want any kind of accuracy, you should keep the input impedance to below 4K or so. Use two 5K resistors
in series between Vcc and GND and connect the junction to the A/D input, then read the A/D and see what you get.

Thanks for your reply... I just try for 2.2k resistor but also give me the same result. The Signal voltage to measure also drop to 0v...

From the internet surfing...... the PortA.4 is open collector.... maybe this is the reason.....? Or do i need to put a pull up resistor or something like that?

Thanks!

[Melanie]

Have you properly configured the pin for INPUT (eg set the TRIS for that pin)?

Have you DISABLED the COMPARATORS that use that pin?

Have you ENABLED that pin for ANALOG input?

What EXACTLY makes you think that RA4 is Open Collector? Some dubious second-hand information that you got off the internet? Just because RA4 is Open Collector on one PIC, doesn't mean it is on another... so find out if that statement is true or not.

And if it was Open Collector, what has that got to do with using the pin for INPUT?

There is ONLY ONE definitive source of reliable information for any PIC chip. That is the manufacturers DATASHEET downloadable from Microchips website. Go do it.

[kblim]

Thanks for the reply.

Below are my declaration

OSCCON = %01101110 ' 4 MHz, primary clock, internal RC

PIR1 = %00000000 ' clear all flags particularly A/D IF bit 6
SSPCON = %00000000
SSPBUF = %00000000
TXSTA = %00000000
RCSTA = %00000000
OPTION_REG = %00000000 'weak pullups enable

'pin IO 76543210
TrisB = %10000100
TrisA = %01010101

cmcon =7 'disable the comparator register


ADCON1 = %10000000 ' right justified, AD Clock divided by 2,Vdd = Vref+, Vss = Vref-
ANSEL=%1010100'only the RA4/AN4, RA2/AN2, RB7/AN6 as analog input
CCP1CON = %00001100

The adc are working on RA2 and RB7, but not on RA4.

Please help.

[Melanie]

OK...

Your settings look good, the ones that you have shown us... but here are some comments...

If you have problems, follow the instructions in the Datasheet.

They tell you that ANSEL should be set before ADCON1.

Also, if you are setting an entire Register using the Binary notation %, and the Register is eight bits wide, then you really should be setting all eight bits... ANSEL=%01010100 and not just seven bits.

You've not shown us your setting of ADCON0.

Have you also checked that your RA4/AN4 pin is not shorted out against any other pin or part of your circuit? Measure the Resistance between that pin and 0v, then unplug the PIC and measure it again on the socket. This way you will determine whether the excessive loading is internal to the PIC (indicating a possible setting error or other problem with the PIC), of if it is external on your PCB or breadboard layout.

Finally, beware of settings like...

OPTION_REG = %00000000 'weak pullups enable

That does a lot MORE than just set weak pull-up's.

OPTION_REG.7=0 ' Set weak Pull-up's ONLY (leave the other settings alone!)

Take a look at the OPTION REG in the Datasheet to see what I mean.