08m2 supply voltage

[oracacle]

I know this has been posted before but as you can imagine anything with "supply voltage" or the like returns rather a lot of posts to go through
any ideas how low a 08M2 can go and still be useful?

Thanks

 

[lbenson]

Depends (among other things) on what "useful" is. In this thread, post 51, an LED was still blinking with an 08M once every 7 seconds at 1.87V after 11 years on the same 3-AAs.

 

[AllyCat]

Hi,

For the 08M2, the data sheet for the "base PIC" says that the default (enabled) brownout detection voltage is about 2.45 volts (+/- 100mV) and it is NOT recommended to disable it. The data sheet also specifies minimum ("guaranteed") supply voltages of 2.3 volts if < 16 MHz clock, else 2.5 volts. The other M2s are specified significantly lower, down to around 1.8 volts, but I've found that the FVRs may "misbehave" below about 3 volts.

Cheers, Alan."

 

[oracacle]

I am looking at around 2.2-2.4v
I suspect I will have to do a little experiment to see if my project idea is going to work.
As for useful, its going to be reading 2 digital inputs and one analogue. If the 2 digital go high, then when the ADC raises above a certain amount, it changes the ADC to an output, send a pulse, and then reverts to an input. There might be a few changes to try and avoid switching between ADC and output

Thanks for your input

 

[AllyCat]

Hi,

What reference voltage do you plan to use for the ADC ? I don't believe the FVR2048 is stable down to 2.2 volts and the FVR1024 (or anything below 1.8 volts) is not recommended to use with the ADC (but does appear to work reasonably well in practice). So strictly, you should use the (default) supply rail as the ADC reference and then correct the result (mathematically) using the CALIBADC10 command. I've discussed some of the issues in this thread which has further links to aspects of the FVRs limitations which might be relevant.

The 08M2 "probably will" work down to 2.2 volts (maybe by risking a DISABLEBOD) but the "safe" solution would be to go to a 14M2. The digital threshold voltages are all specified in section 30.4 of the base PIC data sheet (e.g. the minimum upper threshold level is 0.8 + Vdd/4 volts) for a "normal" TTL input.

Cheers, Alan.

 

[lbenson]

I am looking at around 2.2-2.4v

One question is why this specific voltage? What gives this voltage? If 2 rechargeable AAs or AAAs, why not just non-rechargeable ones? The circuit linked to above monitored one input--monitoring a second would not make much difference. How often do you need to check the ADC reading? One of the tricks the linked circuit used to minimize battery drain was to poke the ADCON0 register to turn off the ADC module after any use, which might or might not make a difference depending on how frequently ADC is read.

 

[Hemi345]

FWIW, I had weird issues programming an 08M2 operating on a 2.5V supply. Even the hard reset method didn't work. I wanna say it might have been due to insufficient current from the vreg. I have a thread on that here somewhere.

 

[oracacle]

I don't want to include the download circuit, so will be programmed out of final circuit and then swap into the final thing so that soulmate be an issue.
That is good info to have for future reference, thanks

 

[hippy]

FWIW, I had weird issues programming an 08M2 operating on a 2.5V supply. Even the hard reset method didn't work. I wanna say it might have been due to insufficient current from the vreg. I have a thread on that here somewhere.

The 08M2 chip claims to be programmable across its entire operating voltage range, however, at low voltages, serial communications may become unreliable, which may be the problem.

Current required for programming should be less than 10mA added to what the normal current usage is.

 

[Hemi345]

I don't want to include the download circuit, so will be programmed out of final circuit and then swap into the final thing so that soulmate be an issue.
That is good info to have for future reference, thanks

Make sure to include a pulldown resistor on the serin pin in your final circuit