Waitstate

The 28C256 EEPROM, which holds the BIOS, has been our go-to choice since the early breadboard days. Before that, we relied on EPROMs, which required UV erasure before every upgrade—a time-consuming and inconvenient process. Switching to an EEPROM significantly improved our workflow, reducing turnaround time.

However, the 28C256 has a couple of drawbacks:

• It cannot be written in-circuit since it requires a 12V programming voltage, which the Steckschwein does not provide.
• It is slow, with an access time of 150ns, forcing us to introduce wait states to slow down the CPU when accessing the BIOS.

Because of these limitations, using ROM routines isn’t ideal from a performance standpoint. As a result, the BIOS’s sole function is to load the steckOS bootloader from an SD card into RAM and start it.

… on the way back to munich, we had some time to do a little code review of our gfx library. thinking about the cpu to video chip timings and again read the well known datasheets of the V9938/V9958. suddenly i got an enlightenment and we came to the following conclusion.

as described in the datasheet (V9958-Technical-manual_v1.0.pdf) of the V9958 there are different timings given for different kind of writes. so as far as we understand there are the following timings