HX512: Can it work like this...?

Spork Frog

I just placed an order on an HX512, just have a little curiosity in advance.

Would it be possible to create an alternate firmware that would use the onboard memory to create, on its own, a VGA signal and output it via 16-23 with the Hydra? I'd suppose the interface would then have to be serial or something similar, but is it possible?

epmoyer

Spork Frog,

The IO pins (16-23) which are shared with the VGA interface are the "data bus" for the E512K, and are directly connected to the SRAM's data pins. There is no way for the propeller (or the PLD) to write data to the SRAM without driving the VGA interface, since they are one and the same, and there is no way for the PLD to read the SRAM without putting its data on the VGA interface, because again they are one and the same.

If you managed to construct a serial interface between the prop and the PLD using the 3 control pins available, and tried to send the screen bitmap data serially to the PLD, the PLD still would not be able to write the data to the SRAM, because any write operation means that the write data shows up on the VGA interface, which would undoubtedly mess up the VGA timing.

Spork Frog

Ok, I wasn't aware that the data lines went directly to the SRAM, thought they might have gone through the CPLD. Thanks for clearing that up.