“Pendulum”daughterboard for the V6Z80P

Following an enquiry from Spectrum emulation whizz Alessandro Dorigatti, I’ve made a small daughterboard for the V6Z80P that provides 28.000MHz and 28.375MHz clock signals – IE: perfect for the Spectrum and Spectrum 128.

Although we’ve had very respectable Speccy emulation before now, further development was becoming tricky since the internal FPGA clocking was a based on a kludge to obtain higher clocking from the 14MHz and 16MHz oscillators available by default on the V6 (these values were below the magic 25MHz at which the Spartan II’s internal multiplier/divider PLLs can be used.) One immediate result is that the Spectrum 128 emulator now shows pixels of the correct aspect ratio, but hopefully it’ll open up future development too (I’m glad I added those unused GCLK pin headers now:)

The board is at present a home-etched affair, but there are plans afoot to get it made professionally to better match the V6Z80P. It’s a very simple circuit based on biased 74HC04 gates – It just took a bit of experimentation to get them oscillating correctly (at the high end of the 74HC logic @ 3.3v, not all brands of logic IC are equal, it would seem.)