- PIC16F877 v PIC16LF877
- Posted by Dave on June 26th, 2003
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.
- Posted by Gary Kato on June 26th, 2003
It's probably the Brown Out Reset. If you look at the datasheet, VBOR is
typically 4.0V, even for the LF.
- Posted by James Beck on June 26th, 2003
In article <bdf6rl$h3r$1@newsg1.svr.pol.co.uk>,
dave@dsquibb.freeserve.co.uk says...
I think you need to disable the Brownout detector to go below ~3.6V.
Jim
- Posted by Wayne on June 26th, 2003
You don't have enough voltage left to drive the LED on. ~1.6 to 2v needed
for most LEDS and you will need
to adjust the resistor values. Assume there is 2V at the output, the
resistor needs to be ( assuming 10ma if current and 1.6 Vf),
R= (2-1.6)/.01 = 40 ohms
Regards,
Wayne
"Dave" <dave@dsquibb.freeserve.co.uk> wrote in message
news:bdf6rl$h3r$1@newsg1.svr.pol.co.uk...
- Posted by CBFalconer on June 26th, 2003
*** top posting corrected ***
Wayne wrote:
Please don't toppost. As far as led drive current is concerned,
the simple answer is to use a current mirror, which can be very
effective down to quite low voltages. You can get the reference
for the current mirror from a diode drop and a resistor, after
which you can use multiplicative effects to concentrate the
current drain in the LED drive. Although the technique is
primarily used in linear IC's, you can have the same effect with
discretes, and especially with packages of multiple transistors.
I would expect to get something like 10% or better regulation of
LED current with supply voltages in the 2 to 10 V. range.
--
Chuck F (cbfalconer@yahoo.com) (cbfalconer@worldnet.att.net)
Available for consulting/temporary embedded and systems.
<http://cbfalconer.home.att.net> USE worldnet address!
- Posted by Dave on June 26th, 2003
"Dave" <dave@dsquibb.freeserve.co.uk> wrote in message
news:bdf6rl$h3r$1@newsg1.svr.pol.co.uk...
Disabled the Brownout reset and all works fine. Sometimes you just can't see
the wood for the trees! I'm not too worried about the Leds dimming, the
circuit only has to work down to about 3V.
How did we ever manage without Newsgroups and the 'net,
Thanks again for your help.
Dave.
- Posted by Al Clark on June 26th, 2003
"Dave" <dave@dsquibb.freeserve.co.uk> wrote in
news:bdffsj$grk$1@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
- Posted by Spehro Pefhany on June 26th, 2003
On Thu, 26 Jun 2003 21:58:43 GMT, the renowned Al Clark
<dsp@danvillesignal.com> wrote:
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
