At 08:27 PM 25/05/98 -0300, you wrote:


>Hello - I'm designing a small serial data based controller system and have
>been considering the use of a PIC as it appears that the ports can be used
>to power other peripherals as they can sink/source some 20mA (can't do that
>with the old standby 8051 variants I'm used to).
>
>My question is this - what is the voltage level on a port pin that is set
>to "high" and is sourcing near it's  maximum current?  The data sheets in
>the microchip databook only show a Vdd-0.7 for a source current of -3mA.  I
>will be sourcing at least 10 mA and perhaps 15.
>
>Thanks for any assistance people can provide.
>
>Lewis
>cobb@zeus.ee.unb.ca
>
>

26/05/'98

Hello Lewis

Firstly the 8051 quasi bidirectional ports can sink 20mA, it can only source
around 100uA on the standard I/O pins (Perfect for TTL loads with an
external 2k7 pull up).
At maximum source current the PIC will deliver a TTL HIGH, and at maximum
sink it will be a TTL LOW (This is all that can be guaranteed). Be careful
not to exceed the maximum power dissipation when using the PIC in this
manner. Note that there is no problem if you "TIE" several I/O pins together
to provide the current that you need. To improve the dynamic power response
you will need to use large decoupling caps in the PIC. You may also need to
clamp the I/O line to VDD with a diode as the switch off current is fast and
will cause voltage spikes, in this case it is not a good idea to trust the
diodes in the PIC to clamp the spike (Small problem, but still may cause
heartache).

For the sake of a small FET, this problem can be overcome, however it may be
nice to use the port pins in a battery powered situation (I still use a FET
however, as this then allows me to decouple the micro from the main power
and control the "Brownout" when the batteries are low ie. Peripherals often
draw more power under certain situations, this may cause a dip in the
supply, that the micro can then detect and do something about (Given: there
are many ways to skin this "cat").


Dennis

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Dennis Plunkett: Embedded Hardware, Software design
NEC Australia DRMASS
Line Interface cards
TRX software
ISDN interface card
ph 03 9264-3867

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