Re: PIC 16F84A Problems

From: Tim Wescott (tim_at_wescottnospamdesign.com)
Date: 07/08/04 

Date: Thu, 08 Jul 2004 09:06:27 -0700

Cason wrote:

> Thats a good question....I was wondering that myself. Does anyone know the
> answer to this? I just saw in some other guys schematic that he didnt use
> resistors....Seemed a little strange, but it seems to work, or not in my
> case...=) The LED's dont seem like they get the color they do when you hook
> them staight up to a battery without limiting the current.
>
> What kind of output do the pins of the 16F84A have as far as Voltage and
> Current when they are high/low? I use 4.5 Volts to run my circuit.
>
> Another question: is low(logical 0) differentiable from no voltage?
>
>
> "Andrew Holme" <ajholme@hotmail.com> wrote in message
> news:966757eb.0407080559.272efa15@posting.google.com...
>
>>"Cason" <casonclagg@cox.net> wrote in message
>
> news:<1v7Hc.59801$rn1.22303@okepread07>...
>
>>>...LEDS connected directly to PORTB....
>>>...the count will not go past the third LED (the number 7)....
>>>...It just starts the program over again or so it seems....
>>
>>I don't know the processor but -
>>
>>Do you need current limiting resistors (e.g. 1K) in series with the
>>LEDs?
>>
>>Even if the outputs are current limited i.e. able to drive an LED
>>without a series resistor, the addwf PORTB instruction might not read
>>the correct value if the I/O pin is held too low by the LED.
>
>
>
_Always_ use current limiting resistors, if not for the $0.10 LEDs then
for the $2.00 processor.

Externally a logic low is not reliably differentiable from 0 volts,
particularly on CMOS. Internally (i.e. reading back the port) depends
on the processor -- some companies read back what you've commanded, some
read back the physical port (so if you yank it around with short
circuits and the like it messes it up). I don't know what Microchip does. 

-- 
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

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