ZX Spectrum With HDMI Out On The Pipistrello

Jozsef Laszlo, AKA “joco” has come up with a very good implementation of the 1982 ZX Spectrum computer using his Papilio-based Pipistrello board. Joco says he wanted to do the ZX Spectrum project to “prove that a Spectrum clone can have a modern HDMI video output. Pipistrello was a good choice for this, because it has the HDMI port, plus, the FPGA is large, and has enough static RAM to contain the Spectrum’s entire memory.”

Joco says his ZX Spectrum clone on the Pipistrello is built on some great work done previously to his efforts. Firstly, Miguel Angel Rodriguez Jodar’s cycle-exact implementation of Spectrum ULA over at the Opencores website. And second, Hamster’s VGA-DVID implementation which was critical to this project in terms of making the HDMI interface. (Look for the links at the bottom of the post.)

Well, old spectrum fans know, that timing is really important for special effects. Unfortunately, the original spectrum only had PAL composite video output (or U-V-Y), and it used 7MHz as pixel clock. It isn’t compatible with any HDMI resolution or frame rate, but close enough to one, that is 720×576@50Hz. According to the standard, this one uses 27MHz pixel clock. Which sucks, because we’d need something “compatible” with 7MHz. I took the definitions for 27MHz, and calculated everything for 28MHz (because 4*7=28, so that would be a perfect clock). I have found that HDMI TVs can handle 28MHz without any issue. (There is also a slight difference in vertical timing, because I produce 624 lines instead of 625, but the TV is happy with it, too.)

Make sure to check out Joco’s project page for how he resolved the above issues, and please take a look at the other reference materials for the Pipistrello ZX Spectrum:

(via Jozsef Laszlo)

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