6502.org

Hi,

What would be the best way to make a ROM emulator, i.e SRAM being programmed In-System.

I was thinking of using 74HC245s to isolate the SRAM from the system while programming so that my programmer doesn't try to drive the bus etc.

Is this a good idea? or is there a better way to do this?

We are currently working on a circuit for an old video game (Interton VC 4000) that uses exactly this configuration. An Arduino is used to write data to an SRAM via 3 74HC595 shift registers. The advantage of those parts is, that you can put their putputs in high impedance mode.
The data and address bus between the SRAM and the console bus is connected through 74LS245 bus transceivers. becasue of the smaller address bus of the SRAM (13 lines) there are some inputs left to connect /CE and R/W also through the 74LS245.
Mario.

I built one for myself with a pic16f877 and 74 series 8 bit latches to hold the address while data is being written to the sram.

I use a dual-port SRAM (IDT7174) and a ATMEGA32. I think that gives you the lowest chip count. I use that combination on my breadboards.

I can't find any info anywhere about the IDT7174 .

If there's an error in one digit, maybe it's something on this page.

Sorry my bad. What I use is a IDT7134. This is a 4k x 8bit DP-SRAM available in PDIP48.

Thanks guys

I had a couple of 245s and an AS6C6264 already so I went with that, the dual-port SRAM would be much better though I'd say.

I had a hell of a time getting it to work though.. I had connected the OE pin from the buffers to a pulldown with my mcu pulling it high during programming, which meant my computer didn't work at all because the buffer was driving the bus even when the ROM wasn't selected.

OR'ing the ROM CS with the signal from the programmer fixed this.

My current circuit looks like this