Hello all
I have tested one of each of the above pics in a circuit which just lights a led on PortB,0 via a 470 ohm. The clock is a 4MHz crystal. Both stop working at 4.0V although the 'LF should work down to 2.0V. By stop working I mean the led goes out (suddenly) and the osc stops. I've tried 2 different LF chips with the same result. Config is Power on reset, Brown out reset and XT osc. Any ideas please? Dave.
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
aworking
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.
--
Chuck F (cbfalconer@yahoo.com) (cbfalconer@worldnet.att.net)
Available for consulting/temporary embedded and systems.
USE worldnet address!
a
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.
-- Al Clark Danville Signal Processing, Inc. -------------------------------------------------------------------- Purveyors of Fine DSP Hardware and other Cool Stuff Available at http://www.danvillesignal.com
Apparently with an EEPROM-trimmed internal bandgap reference.
Best regards, Spehro Pefhany
-- "it's the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.