gameofyou1 said:
The pin at location 22, which is titled "CPU A0" is incorrect. I don't think that CPU A0 is present on the connector at all.
First, I wasn't sure about that point, too. But with a closer look to the PAR carts, this pin is connected to A0 of both EEPROM chips. So as the A-Bus is a 16-bit bus, it is addressed in words, not bytes. The naming is kind of misleading, since the A-BUS is accessed by the SCU, not the CPU. The Police Officer Smith cart proved that right.
gameofyou1 said:
It looks like, from RockinB's pinout, that you have 20 address lines available (A0 - A19).
I tried to guess the location of the other address lines. I assumed that there are actually 25 address line in total. Now this would lead to 64 MByte adressable storage, which would equal CS0 + CS1.
Code:
MCP: 20V8H pin 17(I 12) A21? SLOT_B32(_H73) SCU pin 45
MCP: 20V8H pin 12(I 9) A23? SLOT_B31(_H75) SCU pin 47
PAR: 16V8 pin 6 A20? SLOT_A33(_H72) SCU pin 44
PAR: 16V8 pin 2 A22? SLOT_A32(_H74) SCU pin 46
PAR: 16V8 pin 5 A24? SLOT_A31(_H90) SCU pin 203 next to
But I'm not sure whether there are 24 or 25 address lines. I was thinking, that CS0 and CS1 were mapped to one and the same memory location. No one ever spoke about 64 MByte carts, only 32 MByte. And by accessing CS0 or CS1 you just implement accessing different chips with different read access time. The SCU allows to specify seperate "Normal cycle wait number" (and other stuff), the BIOS sets 3 for CS0 and 15 for CS1 (maximum). Resulting read access times, depending on system speed:
Code:
- A-bus: 16bit, 28.63636 MHz / 26.8465875 MHz
-> each cycle: 34,92064 ns / 37,24868 ns
-> CS0: <= 104,762 ns / 111,746 ns
-> CS1: <= 523,810 ns / 558,730 ns
ExCyber said:
It could be done without a microcontroller at the cost of lower speed
That's true. The SPI-interface to the SD-Cart could be driven from a CPU inside Saturn.
ExCyber said:
On a related note, do you have any decent idea where the A-Bus CS1 signal (the one used for the cart ID region) might be? That could save a chip for a two-device cart and would neatly solve the address space issue. It seems to be connected to the 16V8 in the AR4MP (the input to the diodes that supply the ID code to the data bus is routed over to the 16V8), so I think it might be parts side pin 19 on your pinout (the one marked "MCP: 20V8H pin 27(I 13) / PAR: 16V8 pin 12"). I might test this later, but I need to get caught up on homework first...
Well, what I like about that assumption is, that it is the only spare signal, that I haven't assigned a guessed assignment yet (A20-A24, WE and this here).
Code:
MCP: 20V8H pin 27(I 13) SLOT_B48(_H80) SCU pin 53
What about the A24 guessed above, it could maybe be used for CS0/CS1, too. But, maybe even 2 pins are used, since there is CS2 and CS dummy, too.
BTW: what do these high capacity flash chips cost?