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Dave, The open collector / open drain buffers were mandated by your requirement of being able to connect the Propeller interface in parallel with the MTX keyboard. Without them you could have the situation of no keys on the add-on keyboard pressed, so the Propeller outputting a logic high on all the...

Tony,

The Propeller output cog does a "waitpne" on nine pins, not eight. The ninth pin is toggled by the keyboard decode cog whenever the state of the keyboard matrix changes. This causes the output cog to update the sense lines, even if the drive lines don't change.

Bill.

Dave,

The issue is not multiple keys pressed simultaneously. Existing code will cope with that. The problem comes with any games that activate more than one keyboard matrix drive lines at the same time. Tony suggested that some games may do that to speed detection of "any key" presses.

Tony, Interesting. It had not occurred to me to pack the results from multiple scan rows in order to minimise reads from hub RAM. This will be slower than what we currently have, but probably still OK for the MTX. For the MTX+ at full speed it will increase the number of wait states required. I had ...

Tony, The MTX ROM certainly only activates one drive line at a time. Not being a game programmer / player I don't have any real feel for whether games might activate multiple drive lines. Perhaps Dave's trials might identify any issues. It would not be too expensive to add one more test for all eigh...

The USB protocol is far more demanding than PS/2. Using the USB driver that I have found requires three cogs to read the keyboard. There are also some differences in the electrical connection required. Dave's current plan is to have a link (or links) which adapt the electrical interface appropriatel...

Dave wanted a description of the Propeller code ... The Propeller chip is a fast 32 bit micro-controller, designed for hardware interfacing, with eight largely independent cores, called "Cogs" in Propeller speak. It can be programmed in assembler, a high level language designed specificall...

Just to get the attributions right (as per Dave's web page):

Original idea - Dave
Suggestion of using Propeller - Martin
Propeller firmware - Me
First hardware test - Martin
PCB concept design - Dave

I too first learnt Z80 programming on a Sinclair ZX81. The books I used at the time were: Machine Language Programming made simple for your Sinclair - Anonymous Understanding your ZX81 ROM - Ian Logan Sinclair ZX81 ROM Disassembly Part A - Ian Logan Sinclair ZX81 ROM Disassembly Part B - Ian Logan &...

The timing does not have to be exactly cycle accurate, however the MTX tape input routine uses a Z80 busy loop to measure the time between CTC interrupts. If there is too much latency in the emulated interrupt response, then this timing becomes inaccurate, and cannot reliably distinguish between zer...