Hello,

I ran into a strange problem last night.  I am using a 3-to-8 decoder
(74HC138) and alternately selecting Y1 and Y6 outputs.  I am somehow
getting spikes on the Y0 output.  The Y0 output should be high when
not selected but the negative spikes go low enough to register as a low
on other HC devices.  Decoupling the Y0 output to ground via a 1nF cap
seems to get rid of the spikes.

Has anyone ever run into some similar problem??

I have a 100nF decoupling cap very close to the power pin and other 100nf
caps spread around the board.  Only the Y0 output seems to be affected but
I haven't tried other input addresses.

Is this a common practice to have decoupling caps on the outputs of the
HC138 (Y0-Y7)??????

I am using a PIC16C84 running at 4MHZ.  Porta selects the HC138 address.
Portb is data.

Just using some sample code to test circuit.....something like:

        movlw   0x06            ;select HC138 Y6 output
        movwf   porta
        movlw   0x00            ;Set up data on Y6 as 00
        movwf   portb
circle  movlw   0x01            ;select HC138 Y1 output
        movwf   porta
        movlw   0xff            ;device on Y1 has data toggled to ones
        movwf   portb
        movlw   0x06            ;select HC138 Y6 output
        movwf   porta           ; but do nothing to data....data on Y6 should
                                ; stay at 00
        movlw   0x01            ;select HC138 Y1 output again
        movwf   porta
        movlw   0x00            ;device on Y1 has data toggled to zeros
        movwf   portb
        goto    circle

I know this is not an efficient way of coding (hey, I'm a newbie okay), but
I am using this to test my circuit.

I am getting the data on Y0 also toggled so it mirrors the data on the
Y1 and Y6 outputs.

Any hints or clues.  Should I stick to the solution of 1nF caps on the on
outputs of the HC138???

Huh???

-Randie

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*/ Randie Ohtsji                                                     /*
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