PIC16F877 v PIC16LF877

It's probably the Brown Out Reset. If you look at the datasheet, VBOR is typically 4.0V, even for the LF.

I think you need to disable the Brownout detector to go below ~3.6V.

Jim

You don't have enough voltage left to drive the LED on. ~1.6 to 2v needed for most LEDS and you will need to adjust the resistor values. Assume there is 2V at the output, the resistor needs to be ( assuming 10ma if current and 1.6 Vf),

R= (2-1.6)/.01 = 40 ohms

Regards, Wayne

working

Please don't toppost. As far as led drive current is concerned, the simple answer is to use a current mirror, which can be very effective down to quite low voltages. You can get the reference for the current mirror from a diode drop and a resistor, after which you can use multiplicative effects to concentrate the current drain in the LED drive. Although the technique is primarily used in linear IC's, you can have the same effect with discretes, and especially with packages of multiple transistors.

I would expect to get something like 10% or better regulation of LED current with supply voltages in the 2 to 10 V. range.

working

Thanks Guys, Disabled the Brownout reset and all works fine. Sometimes you just can't see the wood for the trees! I'm not too worried about the Leds dimming, the circuit only has to work down to about 3V. How did we ever manage without Newsgroups and the 'net, Thanks again for your help. Dave.

"Dave" wrote in news:bdffsj$grk$ snipped-for-privacy@newsg3.svr.pol.co.uk:

The brown out problem has caught more than one of us. Some of the newer PIC18 family are pin compatible and have a programmable level for the brownout threshold.

The only difference between a 16F part and a 16LF is the promise. They are certainly manufactured from the same die.

Apparently with an EEPROM-trimmed internal bandgap reference.

Best regards, Spehro Pefhany