Are the two pics being run by the same power supply?  If not, are the
two power supply grounds connected to each other, and is there any
ground loop current?

-Adam

Benjamin Bromilow wrote:

>Hi all,
>
>I wonder if I can ask for some input on this query? I'm pretty sure my
>thinking is sound but so far I've blown the output stage of two 16F877s and
>its getting expensive! I'm also worried about putting the circuit in a
>finished product.....
>I'm driving a 16F877 to 16F84 link. I've got a 500 ohm resistor between the
>two pics and a 2k pull-down resistor on the 16F84 stage. The link is
>bidirectional (hence the series resistor). If the 16F877 doesn't get an ACK
>bit when it is expecting it, it knows to resend the data until it does.
>The only problem is I keep on blowing the pin I'm using on the 16F877 for
>driving the 16F84. I'm pretty sure I'm not going outside the parameters (I'm
>running at 5v) but other than two 16F877s both choosing RB0 to die on, I'm
>stuck..........
>Perhaps I'm just unlucky......
>
>16F877------[500 ohm]--------------16F84
>                                               I
>                                               I
>                                         2000 ohm
>                                               I
>                                             GND
>
>16F877 and 16F84 both GND- no current
>16F877 and 16F84 both 5v- 2.5mA 16F84
>                                             2mA 16F877
>16F877 5v, 16F84 0v- 10mA 16F877, 0v 16F84
>16F877 0v, 16F84 5v- 10mA 16F877, 16F84 12.5mA
>
>None of these go over the 20/25mA limit so why might my pic pins be blowing?
>I've tested the PICs with a simplistic program (ie set the pins as follow
>then test with a DVM) and they've definately died.... Why??
>I've tested for shorts BTW and there aren't any....
>
>Ben
>
>
>Ben
>
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