Program wont run without download cable attached

[BrendanP]

I have a 40X1 on a pcb that downloads/sertxd backs and runs its the program fine/solid with the download cable attached.

Remove the cable and the program wont run.

I have decoupling caps on the power supply pins. 100n & 10n. I have tried putting a 10n cap on the reset pin. Im running the 40X1 on around 2.7V so I thought to try a lower value pull up R on the reset, (1K) all to no avail.

Can anyone give me any pointers?

 

[hippy]

Often it's the case of an incorrect download circuit, Serial In not pulled-down to 0V when the cable is disconnected. Probably worth checking everything is wired as it should be.

 

[BrendanP]

Thanks Hippy will do.

I had a embarrassing experience with this thing. I had it going fine hooked up to the notebook.

I tried to demonstrate it to a guy running on its own and it did nothing. Didnt occour to me to check running it stand alone before doing so. Lesson learnt.

 

[BrendanP]

"....Serial In not pulled-down to 0V when the cable is disconnected...."

Yep, a quick glance at the pcb design prog. confirms that is exactly what the problem is.

The board is preety packed tight and I didnt realsie I had left some ground plance disconnected from the rest. Now I see Rev ed is doing a smd 40X1 so the board is going to get smaller yet......
Thanks Hippy once again