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VIL maximum and VIH minimum for Mega Drive/Genesis?

MrD

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Does anybody know the effective VIL and VIH thresholds for a normal model 1/2 Mega Drive?

I am developing some hardware for the cartridge port, and my first approach was to do it 'properly' with the logic output levels of my device being 0V and 5V, using a 74LVC164245 dual-supply 5V/3.3V transceiver to separate the on-cartridge 3.3V logic from the 5V Mega Drive motherboard logic.

However, I've looked at several datasheets for the 68000 and they all agree that the VIH minimum is 2.0V. Does this mean that I can interface the data bus with 3.3V logic output? * I'd raise that by 10% to account for losses, capacitance and speeds and so on, but it seems like a 5V-tolerant, 3.3V-output device like the SN74LVCH16652 transceiver-register would be able to communicate with the Mega Drive motherboard as well. Am I understanding correctly? Or are there some devices in the Mega Drive motherboard that have higher VIH that raises the VIH of the whole system to somewhere >3.6V?

(*I assume that's what some existing Flash-based hardware does, given that one page that analyses the 5V/3.3V interfacing of various carts.)
 

rama

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Generally, this works.
You need to check each line that you plan on driving with 3.3V for additional components on the motherboard that could attenuate the signals.
Especially check for pull-up resistors, and consider what happens with additional expansion devices that could be connected.
There will be a bit more jitter than in a pure 5V system, due to the threshold being on the high side for the 3.3V part.
 

MrD

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Thank you for confirming my understanding. :)
 
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