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'PIC 16C64 current source limits'
1995\11\18@210302 by

I know that there is a 20 mA/pin source limit on the '64, but is there
a per port or per chip limit on current sourced?

In short, can I directly run two 7 segment displays (one per port, ~10 mA
per segment) continuously without a problem?
>I know that there is a 20 mA/pin source limit on the '64, but is there
>a per port or per chip limit on current sourced?
>
>In short, can I directly run two 7 segment displays (one per port, ~10 mA
>per segment) continuously without a problem?

Current limits of this nature are specified in the "Absolute Maximum
Ratings" section of the data book:
Maximum output current sourced by any I/O pin: 25 mA
Maximum current into Vdd pin: 250 mA.
Maximum current sourced by PORTA, PORTB, and PORTE (combined): 200 mA
Maximum current sourced by PORTC and PORTD (combined): 200 mA
etc.

So it looks like a total of 140 mA from two ports at 10 mA per pin is OK.
Since the pins are lightly loaded, there should be very little Vdd-Voh and
you won't be close to the total power dissipation limit of 1 Watt (even if
Vdd-Voh were a full 5V, the power dissipation would be .01A * 5V * 14places
= 700 mW).  The only difference between source and sink current limits
regards the Vdd and Vss pins.  Assuming the current through the PIC logic
itself is negligible, you can source a grand total of 250 mA per chip, or
sink 300.  This would presumably require splitting the heavy loads up
inbetween the A,B,E group and the C,D group.

-Mike
Paul Christenson wrote:

> I know that there is a 20 mA/pin source limit on the '64, but is
> there a per port or per chip limit on current sourced?
>
> In short, can I directly run two 7 segment displays (one per port,
> ~10 mA per segment) continuously without a problem?

Paul:

The absolute maximum current sourced per pin is 25 mA, not 20 (at
least, according to section 16.0 of the 16C6X Data Sheet).

Miximum sourced by porta A, B, and E (combined) is 200 mA, and the
maximum current sourced by ports C and D (combined) is also 200 mA.

Also, total power dissipation is shown as 1 Watt.

-Andy
Andrew Warren - fastfwdix.netcom.com
Fast Forward Engineering, Vista, California
>In short, can I directly run two 7 segment displays (one per port, ~10 mA
>per segment) continuously without a problem?

May I suggest a better way to accomplish this?  Multiplex the two 7 seg.
displays using a transistor to switch them.  The base of the transistor can
be driven by the eighth pin (presently unused, I presume) on one of the I/O
ports.  You can now control two seven seg's w/ just one 8-bit port, leaving
the other I/O port free.

Anyway, just my \$0.02 worth.

-mike
--
Mike Goelzer
<mgoelzerus.net>
>>In short, can I directly run two 7 segment displays (one per port, ~10 mA
>>per segment) continuously without a problem?
>
>May I suggest a better way to accomplish this?  Multiplex the two 7 seg.
>displays using a transistor to switch them.  The base of the transistor can
>be driven by the eighth pin (presently unused, I presume) on one of the I/O
>ports.  You can now control two seven seg's w/ just one 8-bit port, leaving
>the other I/O port free.

That's what I ended up doing.  (Actually, I had a third display that was
always either 0 or 5, controlled by one line.)  Now all three displays are
fully programmable.  At least in theory; it hasn't made it to the proto
board stage yet.

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