Post by feedingdragon on Sept 30, 2019 1:46:46 GMT
Is there a location on a 128DCR that will always either be +5V or 0V when powered on, but can be changed by setting a bit somewhere in RAM?
The only place I could find would be the Tape port motor control, but would prefer not to be turning the tape motor on & off while my program runs. I thought of using the tape port data line, but am not sure of how stable the startup state is.
The plan is to put a 50K program onto a Function ROM. It would use this point to switch the Function ROM bank as needed. If push comes to shove, since any Tape access would only take place after the program had finished using it anyways, I can just make the first block of both banks identical, then have it force it to where I need it first off. But not really a fan of doing it that way if there is another way to do it.
I think it's U3 that is spare. You would need a latch but you can write to a particular memory address (or read) and it decodes to a pin on this chip. You could use the R/W line to set on off and latch on address access. You can even use the data bus and some of the address lines if you want.
Basically just decode the IO space further than it is, like second sids etc do and use that for your latching.
Post by feedingdragon on Sept 30, 2019 10:06:50 GMT
Further searching, I think I can use either GAME or EXROM on the cartridge port. They are set to +5v when the system is turned on, unless a C64 cartridge is installed which would bypass the Function ROM anyway. They can be controlled by I/O address $D505 (bits 4 & 5.) Could even use it to control a total of 4 banks instead of just 2.
Using the EXROM pin on the expansion port works great For the internal ROM, just run a jumper cable and clip to the pin. For the External ROM, just run a trace (if designing your own board cartridge.) If using an REU don't connect pin-1 of the chip and solder a wire from it to the edge connector (there's enough room to solder it to the back and still insert the REU into the port.) Can make it 4 banks by using the GAME pin as well Though, that would mean a different adapter, as I don't know of any 128KB chips that are pin compatible.