3V causes problem

Started by upand_at_them March 1, 2005

I have a 16F628 running at 5V, but when I switch it to 3V it doesn't
work. It uses the UART and the MAX232 is supplied 5V. When I change
the PIC back to 5V it won't work again and I have to reprogram the
chip. I have LVP disabled.

I have, however, verified that I can run a simple program in the '628
at 3V. But something is happening when I use the more complex
circuit.

Any ideas what could be happening?

Mike


> I have a 16F628 running at 5V, but when I switch it to 3V it doesn't
> work. It uses the UART and the MAX232 is supplied 5V. When I change
> the PIC back to 5V it won't work again and I have to reprogram the
> chip. I have LVP disabled.

If I understand this correctly you have 3V PIC connected to 5V MAX232,
right?

PIC16F628 Data Sheet says (page 136) that "Input High Voltage" is
approximatelly 0.8 VDD (min) to VDD (max). Thus, when powered with 3V
anything above 3V on PIC inputs might cause problems. In other words, when
powered with 3V, PIC MCU is not "5V tolerant".

I just designed some circuit with 3.3V PIC and I used MAX3221E on UART.
Maybe you should try something like that? Please note that MAX3221E has only
one receiver and one transmitter thus it can deal with only RX and TX line
while other RS-232 lines can't be used.

Regards,
Igor
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Thanks, Igor. I guess I'll have to get one of those 3V MAX's.

Mike


Here's a quick tip for anyone using MAX232 type chips --

Only buy MAX3232 devices. They are totally pin compatible with the
MAX232 and work from 3V to 5.5V. They are available from several
manufacturers (Maxim, TI, ST, Sipex to name a few). I usually use ST as
they have seem to be the cheapest for me in small quantities every time
I go to order them. Last time I ordered from Mouser they were less
than $1 in small quantities. I mostly use narrow SO-16 packages and
keep a tube around most of the time.

Matt Pobursky
Maximum Performance Systems

On 3/2/2005 7:25:11 AM, wrote:
> Thanks, Igor. I guess I'll have to get one of those 3V MAX's.
>
> Mike