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Work on a Better Graphics Card

A project log for Z80 overpowered computer

I am building a computer with a 4MHz Z80, 32KiB EEPROM, 32KiB RAM, IDE interface and UART.

Dylan BrophyDylan Brophy 11/15/2017 at 17:011 Comment

I haven't yet gotten the CF card to work... but I had already been working on a graphics card using the UPD7220.  I got the boards and am working on drivers.

here is the assembled board atop my motherboard:

The card overclocks the upd7220 to 6.25Mhz and uses a 12.5mhz pixel clock. It has 64KB of RAM (more than the rest of the computer!) and can display 16 colors.

The driver is giving me back some interesting information.  It consistently tells me that 4 commands are sent without overflowing the UPD's FIFO, that horizontal blanking is active, and that the FIFO is not full nor empty. The last is odd, the FIFO should be empty before the first command.

The driver tries, as a test, to write and read memory at an address in VRAM.  The UPD7220 never sends data back, so I think there is a problem sending the commands to the UPD7220.  Interesting!

Discussions

lezanderson wrote 12/17/2019 at 11:37 point

Cool !

The upd7220 / NEC 72020 Chips were extremely powerful  VDPs for the early 80s. It just never caught on ... probably was too expensive for 'Home' Computers of the time.

Do you have schematics to download, for those wanting to build their own  upd7220 Graphics Card ?

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