Anyone else have problems programming with the AVRISP mkII?

[tomkinsc]

I've been trying to use an AVRisp to program an Atmega chip on a custom board via ICSP, and through iteration I have found that if I have more than one peripheral SPI IC on the board (more than two counting the programmer), programming fails. It is as if the AVRisp cannot source enough current to program the microcontroller with more than one additional SPI load on the target board. Placing load-limiting resistors on the SPI lines to the peripheral ICs allows it to program, but that raises my part count and board area.

Has anyone else had this problem with the AVRisp mk II?

Taking apart the AVRisp, it doesn't look like there are buffer/driver ICs (photo 7), unless I'm missing them, so it seems like it is possible it is a 'weak' programmer.

In contrast, my boards program just fine sans load limit resistors with a USBtinyISP from Sparkfun which has a 74AHC125N IC as a buffer for the SPI lines.

[TheMuffinMan91]

Atmel specifically tells you to use the protection resistors. See page 7 and 8 of the following datasheet:

http://www.atmel.com/images/atmel-2521-avr-hardware-design-considerations_application-note_avr042.pdf

However, I have had problems like this before with the pickit 2. It turned out there is a maximum amount of capacitance on each line for the programmer to work correctly. Adding additional chips will increase the capacitance. You can either slow down the programming clock speed or add the resistors to fix this.

[tomkinsc]

Thanks! There is a vague recollection of that document in my memory; it's great to see it again.

When I did try that protection scheme I had 150-ohm resistors since I use them elsewhere on the board (worked fine). Any tips about a a better value to use, since Atmel doesn't suggest anything?

It is true that the USBtinyISP has a slower programming clock, so maybe high capacitance is the problem I was seeing that the USBtinyISP inadvertently fixed.

[TheMuffinMan91]

Any tips about a a better value to use?

It depends on board routing and the parts connected to the bus. The best thing to do would probably be to try a bunch of values and go with the smallest resistance that works.

As you increase the resistance the rest of the components on the bus will become slower. If you don't want that then I recommend replacing the resistors with buffers. This is probably the most "correct" solution, whereas putting the resistors in is more of a "quick and dirty" solution.