Analog pins

[Tagge]

Hi, Im wondering how I can "mask" an An port?
Its on a PIC18F97J60, and I just want to use for example AN8-11 as analogs, the rest of portF and portA should be normal I/O?
There is no option in the project-set-up for that..
To make this I have to use AN0 to AN11

[FvM]

Generally, the said device has no option for arbitrary analog pin configuration as some newer PIC18 have. As you said, configuring AN8-AN11 to analog implies that all AN pins must have analog configuration. As the datasheet clarifies, it's no necessary in any case, to use analog configuration for analog inputs. The purpose of this option is only in disabling the digital input buffer to reduce exessive current consumption with analog input voltages. On the other hand, pins configured to analog can still work as digital outputs.

[electrogen]

Look on the datasheet on the "analog to digital converter module" description for possible configuration of the analog port pins.

You can't just assign any pin on the port as digital or analog! There are possible set of configurations though, it's stated on the datasheet!
_________________
chips are forever

[eskachig]

Sorry to resurrect this thread, but I'm still a bit confused. What exactly happens to my digital inputs when I am forced to use setup_adc_ports(all_analog) to get access to analog channel 13? Are they all automatically low?

The project I'm working on has this bizarre setup where I have to sample input on channels 12 and 13, but still have to use pin A0 as a digital input.

[bungee-]

[eskachig]

[Ttelmah]

Let me summarise:

The possible 'layouts' are:
1) Pin set as 'pure' digital input.
2) Pin set as 'pure' digital output.
3) Pin set as analog input, used as analog input.
4) Pin set as analog input, used as digital output.
5) Pin set as analog input, used as digital input.

Now, '3' is the preferred route for analog, but the first question is can you use 4 and 5, and what happens if you do?.

'4', works fine, but with two caveats:
A) There is slightly more tendency to crosstalk from this digital pin to the analog circuitry.
B) The Vref, then must be up close to Vss (or use Vss). A pin connected to the analog multiplexer, must not go more than 0.6v above the Vref level. So you can't (for instance), use 2.5v Vref, or you will get the digital signal tending to modulate the Vref...

The remaining question, is can '5' be used?.
Generally, no.
Selecting the analog input mode, disables the logic input buffer.
This has to be done, since otherwise excessive current can be drawn in this, if the input signal sits in the 'transition' region.

If you try to use them as digital inputs, what you read, will depend on the logic in the 'disable' circuitry, but on most of the older chips, the input is built using an 'OR' gate, with the 'analog enabled' signal, inverted, and fed into the other input of the gate. So when analog is enabled on a pin, it reads as '0' if you try to use the logic input.

On latter chips like the 18 series, this is described quite well in the data sheet, where the 'I/O pins' table, lists what happens for each pin, according to the settings on TRIS, and the analog input setting.

A 'classic' section in this (f just one pin), reads:

[libor]

What is your application doing with the analog inputs ?
If you can afford measuring their value not too often, you can switch your port configuration to all-analog only just before the ADC process (allow some time for the ports to settle) and then go back to the digital configuration during runtime for the rest of your operations.

[Ttelmah]

The downside of this approach, is that then the pins with analog signals attached, will run into the excessive current in the input buffer problem....

Alternatively, just read the ADC value on the pin, and convert this to a logic level. Use an arbitary level (like half the input range), and if it is above this, return a '1', or below, return a '0'.
Doenside is it takes a fw uSec to read the pin, but no problems then with configuration.

Best Wishes