Pin as ADC in and digital out

[BLL]

I have one A port pin currently driving a MOSFET to control an LCD's backlight. I need to read an analogue voltage but have no other spare port A input.

As the MOSFET gate impedance is high, it shouldn't shunt the sensor's input. Can I therefore just change the pin from digital output to ADC in, read the value and then put it back to digital output again?

PIC is an 18LF2550.

Thanks for the help.

[Mike Walne]

I see no problem as far as the PIC is concerned.

How will the MOSFET drive to the LCD behave as the MOSFET gate drive is taken through the range of sensor output voltage?

How will your voltage sensor react to being digitally driven?

Mike

[BLL]

Hi, Thanks for the reply. The sensor is an MPX4115 pressure sensor, buffered by an opamp, so I can put a diode from the opamp output to the pic pin, so no problem there. The LCD backlight will be turned on and this is the problem I hadn't considered!! There doesn't seem to be any way around that!!

[Ttelmah]

One thing you could do, though 'complex'. is have the PIC pin connect to the MOSFET via a comparator. Set this so it's output will go 'on' at (say) 2.5v, and then your analog source can drive the pin up to this voltage, without turning the FET on.
Other thing you have to consider is the effect of the diode drop on accuracy.

Best Wishes

[Mike Walne]

[Ttelmah]

and our answers complemented each other well, with 'alternative solutions'.

Best Wishes

[BLL]

The mosfet is simply driven via a potential divider. The idea of a time constant sounds clever - wouldn't have thought of that!

Brian

[Mike Walne]

One thing you will have to watch.

When doing an ADC, the OPA output is attenuated by the attached resistor network. The signal amplitude is dependent on the voltage at the FET gate. You will need to either, adjust the resistor ratio to make their effect negligible, or correct for it.

Mike

[BLL]

Thanks for that. The other possibility is to remove the mosfet from the port and drive it from a monostable triggered by one of the 3 pushbuttons which would leave the ADC input as sole user. Means an additional chip though.

I'll have to play with both ideas.

Thanks again

Brian

[Ttelmah]

One thought does occur.
You have been talking about just this one pin, but if you are controlling an LCD, then you have a minimum of four other pins available you can use.
The four data lines to the LCD, are ignored by the LCD, when it is not enabled. All you need is something like a bilateral switch, operated off the enable line, so the connection is made to the other circuitry, when the LCD is not being talked to.

Just a thought.

Best Wishes

[Mike Walne]

On the LCD theme.

Could you free up the R/W line? (Then you won't need extra hardware)

Mike

[BLL]

no, the R/W pin on the LCD is already grounded, so that's a non-starter! Why is it that Murphy always conspires to make sure that you are one line short?

I could try one of the LCD lines but the monostable would be no more complex.

[Ttelmah]

With a resistive drive, you can do it directly to one of the LCD data lines with no other circuitry at all. If you are not using R/W, the PIC is the only thing that can drive these lines, and will only do so, when you send data to the LCD.

So long as you choose a drive value, that is low enough to meet the ADC input requirements, but high enough that the PIC pin can still drive the line to the logic levels needed by the LCD (probably perhaps 2KR), then provided an ADC is available on on of the LCD data lines, all you'd need do is stop using the LCD for a moment, use 'output_float' on this line, wait for Tacq, read the voltage, then drive the line again.

Best Wishes