ISP programming of the (tr)uSDX (trusdx) showed that filtering on the MOSI pin in that kit distorted the MOSI signal significantly and suggested a workaround (reducing SCK rate) for reliable programming.
Some correspondence prompts a little more information on the nature of the ATmega328P ISCP signals.
The line protocol used is actually SPI, quite a common protocol.
ISP uses SPI MODE 0 (CPOL=0, CPHA=0), shift out on the falling edge of SCK, and capture input on the rising edge.
Let’s look at a three channel capture of SCK, MOSI and MISO of a AVRDUDE / USBasp driving an Arduino Nano.
The capture shows SCK at around 750kHz rate, the default (-B1) rate for AVRDUDE in this setup. Continue reading ISP programming of the (tr)uSDX – more on SPI