I've been using a simple address offset from the start of the screen rather than coordinates to simplify the PicoBlaze code, because the PicoBlaze does not have a multiply instruction, and to do a multiply would take a few cycles (especially as I'd need to multiply two 10-bit values).
However, a thought struck me: I'm mulitplying by a constant (800) to convert an X and Y value to an address. This can easily be done in the core of the FPGA itself.
In order to implement this, I added sB the PicoBlaze registers that were available to the core, and set the registers as follows:
I also changed the instruction set to:
All other commands are currently ignored (NOP).
The code is actually a bit quicker in theory, since when it increments to go to the next address, it only needs to change 2 registers, rather than 3.
There is another, fairly significant advantage - I can now implement line and circle drawing to the PicoBlaze code, rather than in the PIC, which means it would be a lot faster. Although that does assume I've got enough memory for that...
The only downside is that it uses a multiplier - however, there's another 15 left on this chip, so I'm not too worried by that. It could be done with 3 adders instead if I need it (and the delay isn't too much of a problem).
I've written the code for the line drawing algorithm here, but here's a picture of some lines that have been plotted over yesterday's work:
And the command set was extended with:
Note that I've got some problems with the clock synchronisation in that the vertical green line looks like it's missed every other X coordinate - this isn't the case, but it's not picking up those pixels for some reason. I'll look into that next.
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