Voltage Monitoring

[Megahertz]

I have a RFmodule & a PIC (16F877A). They both can run upto 3V to 5V. One pin of the RF module gives a constant voltage of 2.8V.

I plan to run the PIC on 4 AA batteries, rechargeable. (appx 1.2v - 1.3v when fully charged).

I want to take this lower voltage (2.8V) as a reference and check when battery pack reaches 3.6. Can someone please guide me how I can take the lower voltage as a ref and check the supply voltage. Thanks

[mackrackit]

One way would be to use a voltage divider on the battery pack so when it reaches 3.6 the divider returns 2.8.

[Charles Linquis]

If you set Vref to Vdd and read the A/D, you will find that the constant 2.8V will give you an A/D count of LARGER than 716 when the battery voltage drops below 4V. No resistors needed.

[Megahertz]

If you set Vref to Vdd and read the A/D, you will find that the constant 2.8V will give you an A/D count of LARGER than 716 when the battery voltage drops below 4V. No resistors needed.

Since it is a battery powered application. This seems a better option. Could you please explain more. I not an expert on this subject.

[Megahertz]

What setting of ADCON1 should I choose and what to connect where? I need maximum number of PortA pins as digital. Thanks

[Charles Linquis]

I think you can read the datasheet as far as ADCON1 goes.

Most PICs let you choose between two sources of Vref. You can choose AN3 or Vdd. If you choose AN3, then you must connect a reference voltage to that pin. All A/D readings will be compared against that reference.
If the reference is 3.0V the a 10 bit A/D will read 3.0V on any other A/D input as 1023. The "count" (1023) is equal to (input/reference) * ((2^10) - 1).
If you apply 2.0V to an A/D input, and the reference is 3.0V, the count will be 2/3 * 1023 = 682.
Everything is compared AGAINST the reference.

If you set Vdd as your reference, then every A/D conversion will be compared to Vdd.
Since your external chip puts out a constant 2.8V, if Vdd (the reference) is 5.0V then the A/D will output 2.8/5 * 1023 = 573
If Vdd drops to 4.0V, then the A/D will output 2.8/4 * 1023 = 716
If Vdd drops even further, the A/D output will go higher than 716


If you set

[Charles Linquis]

Additionally, if you have any other voltages to measure, connect your 2.8V to AN3. You can change Vref on the fly. Set Vref to AN3, and you can measure the other voltages accurately without regard to the battery voltage. Then set Vref to Vdd and measure the battery voltage, switch it back and measure your other voltages...

I use this technique often when reading absolute voltages (which must be compared against a constant reference) and ratiometric sensors, whose output varies as a percentage of their supply voltage (such as Hall-effect current sensors).

[jellis00]

I have a RFmodule & a PIC (16F877A). They both can run upto 3V to 5V. One pin of the RF module gives a constant voltage of 2.8V.

I plan to run the PIC on 4 AA batteries, rechargeable. (appx 1.2v - 1.3v when fully charged).

I want to take this lower voltage (2.8V) as a reference and check when battery pack reaches 3.6. Can someone please guide me how I can take the lower voltage as a ref and check the supply voltage. Thanks

I didn't check the Data Sheet for the 16F877A to see whether it has an HLVD module or not. If it does, you can easily use the technique I described in an article on this forum to check the supply voltage. Check it out at: http://www.picbasic.co.uk/forum/cont...oltage-monitor . It has schematic, layout, code, et al. Hope this helps.