I want to customize the software on a modular nixie clock. I did purchase an AVRISP mk. II. I’m wondering if any one can point me to instructions to update the firmware using AVRISP through the ICSP, and not having to remove the at mega chip. Hopefully this can be done MacOS, but if A PC is required I could manage. I have populated the ICSP headers.
I don't have any direct experience of the AVRISP, but if you have a 6 pin header on the programmer, it should not be a huge deal to start programming. On all modern boards (and I guess yours is up to date) there is a 6 pin ICSP header on the board. For the Modular it is between the Hours and Minutes tubes.
All you have to do it put a 2x3 header in it, and the rest should be fairly straightforward. Set the programmer type in the "tools" menu, and then you should be able to upload using the "sketch -> upload using programmer" option.
I find it helpful to turn on the verbose output for compiling and upload in the "File -> Preferences" menu.
Mac seems to work quite well with modern programmers.
If you want to flash a new chip, don't forget that you have to set the fuses! You don't need a boot loader.
I have an AVRISP Mk.2 I bought 10+ years ago and have used numerous times on Ian's modular clocks as well as on other projects. I still use it with various versions of Windoze and also on Ubuntu occasionally, but never tried MacOS. I can't add much to what Ian has already said but if you run into problems I might be able to help.
You'll have to pull out the 1h and 10m tubes to get the connector connected. The "key" on the AVRISP plug should point toward the separator LED / neon, not toward the Atmega chip. There's an LED inside the ISCP end of the AVRISP that lights green if all is good and flashes red if you have the connector on backwards. The board must be powered in order to get the green light.
I think I may have had to install libusb before the AVRISP would work, but that was a long time ago so my memory is imperfect!
Look into it later when the dust is clearing off the crater.