65c265 Disable Internal ROM via hardware

[RebeccaBlue]

I saw in another thread a mention about using the BE/RDY and RESB pins to disable the internal ROM on the 65c265.

Can someone explain how this works? I'd like to use the '265, but I'd also like to avoid the crazy boot sequence with $8000 / WDC String.

Thanks!

[drogon]

RebeccaBlue wrote:

I saw in another thread a mention about using the BE/RDY and RESB pins to disable the internal ROM on the 65c265.

Can someone explain how this works? I'd like to use the '265, but I'd also like to avoid the crazy boot sequence with $8000 / WDC String.

Thanks!

My understanding (of the '134) is that you need to pull BE low before Reset goes low, then keep it low for some time after Reset goes high. Look for a timing value called "tBR" in the data sheet. For the '134 it's defined as a minimum of 35ns at 4Mhz. There is no maximum specified, so maybe there is internal widgetry to ignore BE if it starts out low (which might make sense if you want to use BE for something.

So try simple pulling BE low then take it out of reset...

Good luck.

-Gordon

[RebeccaBlue]

My understanding (of the '134) is that you need to pull BE low before Reset goes low, then keep it low for some time after Reset goes high. Look for a timing value called "tBR" in the data sheet. For the '134 it's defined as a minimum of 35ns at 4Mhz. There is no maximum specified, so maybe there is internal widgetry to ignore BE if it starts out low (which might make sense if you want to use BE for something.

So try simple pulling BE low then take it out of reset...

Good luck.

-Gordon

Thanks- I thought it was a simple RC kind of thing, so we'll probably just do the WDC string thing.

[tokafondo]

This operation mode doesn't disable the internal ROM but just put the complete system in a way it can be traced from outside.

It's called In Circuit Emulation (ICE).

That I'm writing here is what I understood from emailing WDC back in the day about this very same matter.

At last, the only way to disable internal rom is what you say: by using the WDC string