I need to add to the above post what I think is happening/not happening:
On a 7474 FF the input D (pin 2) is clocked in when the clock (pin 3) goes from low to high. What is probably happening here is that there is a slight timing issue where the data has not settled when the clock makes the transition - result... always a '0' out, or Drive A selected.
Outputting QO34,01 is still seen as a 0 on the data line input D to the flip flop.
On Sunday, June 8, 2014 7:58:45 PM UTC-4, Thomas Owen wrote:All,
I had posted this to the thread dealing with ordering this card, but decided to post a new topic dealing with the problem I have had with final checkout of the board.
During assembly the board passed all the 'in progress' checks. The final steps were to output to different ports:
I have output the following and everything passes:
QO33,80 ; Configures ports A, B and C as output ports
QO32,2B ; Selects right hand pair of digits
QO30,01; display 01on right digit pair and work through each bit - all ok
QO32,2c; Selects middle pair of digits
QO30,01; Should display 01 on middle digit pair, work through each bit - all ok
QO32,2d; Selects left hand pair of digits
QO31,01; Should display 01 on left digit pair, work through each bit - all ok
All displays are correct.
Now the final step is the Drive Select and that is where I am having trouble:
Board always comes up with 'Drive A' selected
Now, QO34,0; no change
Reset, start again:
QO34,1; no change
I can never select drive B.
Several generous members (thank you David Fry) have made suggestions, and here is where I now stand:
My preliminary check this afternoon shows me that there is a timing issue at U15, the 74ls02. Using my logic analyzer and monitoring the inputs and the output (which clocks the drive select flip flop) I see a big timing difference between SEL_SECOND and WR.
What this means is that the output of the gate never goes high allowing the bit change to effect drive change/selection.
I am going to get some screen shots from the analyzer tomorrow, and if anyone has any suggestions I would greatly appreciate it.