As some of you know, Daqq pointed out the Mattel Hot Wheels Radar Gun which I have proceeded to spend way too much time on. I have it reverse engineered and am ready to give all my theories a try, but am failing in using its ISP connection with my (working) STK500.
The problem is that they use the MOSI, MISO, and SCK lines for generating the LCD backplane voltages (0, V/2, and V). Therefore there are two 56K resistors on each, one to ground and one to V. When they output a 0 or 1 they get 0 and V; when they switch to input/no pullups they get V/2. My guess is these resistors are causing my grief.
I've tried using the gun's power without Vtarget, Vtarget without the gun being powered on, have traced the ISP header to the ATmega88 pins, checked cables, tried the STK500 on other chips, etc. - everything I can think of, and all checks out. I've dropped the ISP clock to the lowest it will go (a little over 1KHz) and jacked it up to where it should be. Nada. Reading fuses, locks, or signature, I always get a message that there's an ISP error. A scope shows reset handled correctly and an attempt to talk (SCK and MOSI).
I really don't want to remove the resistors, since to do so would sort of defeat ISP plus be a lot of trouble. I could put jumpers to break the connections but that too is more than I hope to have to do. Yes, I've read AVR910, and they do show isolation from other active stuff using a series resistor on each line. But it seems that Mattel's manufacturer probably successfully programmed this doggie, although possibly not via ISP. It seems odd they would have the ISP solder pads there if it shouldn't work, but who knows.
Any comments or ideas would be greatly appreciated. Thanks in advance.