Hi everybody,
I'm doing a repro of Chrono Trigger using 27C322. I've read the thread about this and analyzed the scematics, but I prefer my solution of using 74HC257 muxers to convert the EPROM's 16bit data-bus to the SNES' 8 bit data-bus.
Anyway, my questions are related to timing:
* which input-to-output delay does the MAD-1 add? I mean, how long does it take the chip to assert ROM /OE, ROM /HI or ROM /LO when the address appears in the input?
* why does my Seiken Densetsu 3 repro work using -120ns memories, but not Chrono Trigger? Both are FastROM but Chrono Trigger only works with -100ns EPROMs. I'm nor using any external decoder, just using MAD-1 outputs as needed.
I guess that Chrono Trigger uses lots of MVN and MVP instructions, which move data from ROM at 6 cycles per byte (i.e. 120ns per byte) and Seiken Densetsu uses more DMA transfers (i.e. fixed 8 cycles per byte)... Am I wrong? Is there any other reason?
* which one is faster? 74HC257 ot 74LS257? which one is the proper one for muxing 27C322 data bus? Both have aprox. 20ns of propagation delay, so timing from address on SNES bus to data back on the SNES bus is:
100ns (27C322 access time) + 20ns (74HC257 propagation delay) = 120 ns
I only take into consideration propagation delay on 74HC257 because both muxers are enabled using ROM /OE output from MAD-1 (pin 4), and it is asserted before data on 27C322 bus appears.
Thanks!
I'm doing a repro of Chrono Trigger using 27C322. I've read the thread about this and analyzed the scematics, but I prefer my solution of using 74HC257 muxers to convert the EPROM's 16bit data-bus to the SNES' 8 bit data-bus.
Anyway, my questions are related to timing:
* which input-to-output delay does the MAD-1 add? I mean, how long does it take the chip to assert ROM /OE, ROM /HI or ROM /LO when the address appears in the input?
* why does my Seiken Densetsu 3 repro work using -120ns memories, but not Chrono Trigger? Both are FastROM but Chrono Trigger only works with -100ns EPROMs. I'm nor using any external decoder, just using MAD-1 outputs as needed.
I guess that Chrono Trigger uses lots of MVN and MVP instructions, which move data from ROM at 6 cycles per byte (i.e. 120ns per byte) and Seiken Densetsu uses more DMA transfers (i.e. fixed 8 cycles per byte)... Am I wrong? Is there any other reason?
* which one is faster? 74HC257 ot 74LS257? which one is the proper one for muxing 27C322 data bus? Both have aprox. 20ns of propagation delay, so timing from address on SNES bus to data back on the SNES bus is:
100ns (27C322 access time) + 20ns (74HC257 propagation delay) = 120 ns
I only take into consideration propagation delay on 74HC257 because both muxers are enabled using ROM /OE output from MAD-1 (pin 4), and it is asserted before data on 27C322 bus appears.
Thanks!