If you are using a PIC with the brownout detection enabled, you shouldn't have any problems, the PIC resets when the voltage level is below 4 volts. Therefore, the PIC would be in reset until the second battery is installed and a voltage greater than 4 volts is acheived. -----Original Message----- From: Lewis H. Cobb To: PICLIST@MITVMA.MIT.EDU Date: Monday, June 01, 1998 9:10 PM Subject: Re: Lithium Batteries & PICs Question >Hi Dennis - thanks for the reply. I've clarified a few things below as I >wasn't too clear in my first posting - > >> >>Hi Lewis >> > the batteries are in series, and are currently being replaced >>(Old one in while a new one in the other location is being inserted), then >>the old cell should have a high enough impedance to not provide enough >>current to run the circuit. When the older battery is removed, the power >>will go and the PIC will enter a reset state when the new battery is >inserted. > >The batteries are in series in the holder so that you get a full 6V (6.3 >open circuit) on the terminals of the holder. The sequence that I have >some concerns about is as follows; >1) The battery holder is empty - no batteries are installed. >2) User slips in the first coin cell - the holder now has only one battery, >the voltage on the terminals of the holder (and thus the micro) is 3.0V. >3) The user installs the second battery - the holder now has two batteries >in series and has 6 volts on the terminals of the holder - and thus the micro. > >It's the period of time between 2) and 3) that I am wondering about - i.e. >am I going to cause "funny business" with the micro and then when the >second cell is installed, the micro has gone "south" on me or something >like that. > >The code is being written so that it sets up everything and then goes to >sleep when powered up. When it awakes (via the change on port B) it does >it's main task as long as the button is held down by the operator - then >when the button is released - it goes to sleep again. > > > >>Things to look out for:- >> >>1/ CMOS latch up, ensure that the PIC is not attempting to drive high >>capacitive or inductive loads during the power up phase > >No problem here - in fact, all other "circuits" are being powered by the >port pins on the PIC and therefore won't be fired up until the micro does >it in the main task code. > > > >>2/ Check the operating range of the PIC, some will operate down to 2.5V > >I want to use the '620 series but the F84 is also on the plate for usage as >well. From the spec sheets it's 4-6V for the Vdd on the F84- I'm running >them at 4MHz. The '620 looks better in that it specs a Vdd down to 3V - >this would quell any concerns about powering up the micro with one lithium >cell for a brief time before the second one was loaded into the holder... > >>3/ On power up (Check the status bits) DON'T trust the reset circuits >>to fully enable all the on board stuff. i.e. Do your own resetting! > >Not sure what you mean by this. I'm new to PICs - is there a bit in the >status register to check for a power-up condition? *grabbing for the data >book as I type this :) > > >>4/ Don't trust the on board RAM, i.e. The RAMS contents may be corrupted >> > >No problem here - there's only a handful of bytes and they are all loaded >with counter values etc. each time the routines use them... > > >>The last two are very important, if the PIC loses power, then the internal >>reset (Not like some other micros) only resets the internal stuff and >>attempts to set the oscillator is running. The RAM may have had data written >>any where, so be ready for this. > >Thanks for the tips - will keep this in mind. > >>On the oscillator, if you are having problems choosing the correct C and R >>(For Xtal) then look at using an RC, these are cheep, start faster and if >>designed correctly use less power for the same frequency (Up to 500KHz), >>note that this excludes the 32KHz stuff > >I want to bit-bang a port bit for a serial data stream coming out of the >micro - no receiver - just transmit. Was thinking of the ceramic >oscillator approach here - cheaper than xtals but more accurate than RC - >will be fixed at 4800 baud... > > > >> >>For a last piece of advice (Standard thing that I give to all people using >>battery powered stuff) Always ensure that the internal stuff and the TRIS >>registers are constantly re enabled. > >Duly noted - thanks for assistance! >