First prototype. Note that there are only three lines that are used in the connector back Spectrum: 5V, ground, and the reset signal, we use because the uC has the ability to reset the Spectrum from the keyboard (yes, with the famous three-finger-salute: D)

Prior to the F343, and with the false asumption that the F320 would not be fast enough, I considered using the F352, which is somewhat faster than the F343 and with enough XRAM, but has two disadvantages: it only has 17 I/O pins, and it needs an external regulator. I even made a PCB for this uC, but couting with the F343, the whole thing becomes easier (and cheaper).

Silicon Labs C8051F352.

F352 and the SIMD 3.3V external regulator.

This is the schematic for the F343 version. The amazing thing is that you do not need anything else, just the uC and a few additional components: a small RC cricuit for the reset and the JTAG interface, filter capacitors, and pull-up resistors to the keyboard lines and ZX Spectrum address lines (half-rows).

Schematic (clic to zoom)

Half-row lines are configured as open collector pins (8 pin header), and data as push-pull pins (5 pin header). SV1 carries the three signals you have to wire on the ZX Spectrum PCB: 5V, ground and reset, the latter optional, only if you want to use the reset keyboard.

The half-row lines go directly to the port 2 of 8051, and the column lines go to port 1. Port 0 is left to control signals, in this case, the two signals needed by the keyboard (clock and data) and an output signal directly to the Z80 reset pin. Clock signal go to one interrupt pin. The firmware in the 8051 ties an interrupt service routine, so it's run whenever a falling edge happens on clock line, reading one data bit everytime the interrupt service routine is invoked. The 8051 provides SPI, but the data format is not compatible with that used by the keyboard.