This CPU upgrade module uses the bigger version of the default ‘F92 processor. The eZ80F91 chip has more internal memory, works with up to 50 MHz clock, and includes an Ethernet controller (not used here). This greater complexity and the 144-pin SMT package made me initially not choose it as default processor for the Z20X computer. More information about eZ80F91 can be found on ZiLOG’s website.

The module is relatively straightforward, however it needs to include a separate PLL clock generator that is able to generate higher frequency clock for the ‘F91. This requirement resulted in a few extra components needed on the board.

The R2 component (a 0 Ohm resistor) determines the main CPU clock. When installed, it will be 50 MHz. If R2 is missing, the CPU will be working with 40 MHz clock.

The PCB for the module is a 38.6 x 38.6mm (approximately 1.5” x 1.5”) 4-layer PCB which hosts the ‘F91 processor on top side, and the PLL clock and several other passive components (0805 size), on the bottom side