Electronics > Beginners
Attiny85 in Circuit Programming Issue
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vitormhenrique:
Hello Everyone,

I'm designing a small PCB to help a friend, it would basically be a buzzer for his drone that would function as a normal buzzer when it's connected to power, but on a event of loss of power it would beep at defined intervals to help him find his drone.

I have the schematic attached.

One problem that I'm fighting now is that if I populate the PCB the Attiny85 refuses to be programmed. I troubleshooted the issue with the Li-ion battery charger on the board (MCP73831) and could even reproduce the issue one a breadboard.

I don't understand why I'm having this issue, the ISP header has a power supply that should be different from the +5V and the +Bat voltage supply.

Does anyone has an idea of why is this happening? I can just put a solder jump on the board, and leave if disconnected before programming, but I really want to understand what is going on.

Thank you very much for any lead,

Vitor Henrique
IconicPCB:
Your MOSI line is held at +5V constantly.
The lines coming from the programing header need to be isolated from the rest of the circuit  ( typically using a resistor such that the programmer has a chance to drive the programing lines .

So disconnect your MOSI line from +5V or put a resistor from +5V to MOSI and you will be able to program the the micro.
vitormhenrique:
That is not it, there are diodes preventing the VCC voltage to backfeed to the +5V and +Bat voltages.

Tha a look on the top of the schematics, I'm 100% sure that the issue is with the MCP73831, when I take it out of the PCB it works, whenI solder it back it stop programing the Attiny85.

I just don't understand why.  |O
IconicPCB:
Mosi is connected to +5V directly.

Mosi is also connected to the battery charger directly

Programmer CAN NOT drive battery charger..

Shove a resistor between mosi and +5V this will reduce LOADING on programer mosi signal and allow it to program the processor rather than power up charger.
vitormhenrique:
Ahhhh! got it! now it makes sense! I'll try that! thanks!
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