PIC16F877 v PIC16LF877

Do you have a question? Post it now! No Registration Necessary

Translate This Thread From English to

Threaded View
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.



Re: PIC16F877 v PIC16LF877
Quoted text here. Click to load it

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


Re: PIC16F877 v PIC16LF877
snipped-for-privacy@dsquibb.freeserve.co.uk says...
Quoted text here. Click to load it

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

Jim


Re: PIC16F877 v PIC16LF877
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

Quoted text here. Click to load it
a
working



Re: PIC16F877 v PIC16LF877
*** top posting corrected ***

Quoted text here. Click to load it

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 ( snipped-for-privacy@yahoo.com) ( snipped-for-privacy@worldnet.att.net)
   Available for consulting/temporary embedded and systems.
We've slightly trimmed the long signature. Click to see the full one.
Re: PIC16F877 v PIC16LF877

Quoted text here. Click to load it
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.



Re: PIC16F877 v PIC16LF877

Quoted text here. Click to load it

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.
We've slightly trimmed the long signature. Click to see the full one.
Re: PIC16F877 v PIC16LF877
On Thu, 26 Jun 2003 21:58:43 GMT, the renowned Al Clark

Quoted text here. Click to load it

Apparently with an EEPROM-trimmed internal bandgap reference.

Quoted text here. Click to load it

Best regards,
Spehro Pefhany
--
"it's the network..."                          "The Journey is the reward"
snipped-for-privacy@interlog.com             Info for manufacturers: http://www.trexon.com
We've slightly trimmed the long signature. Click to see the full one.

Site Timeline