Well, if you’re connecting an RP235x to (say) a 6502, you’ll have a 16-bit address bus, an 8-bit data bus, and (at least for Atari 8-bits) another 8 for control lines (clk, /ref, rd4, rd5, /halt, r/w, /rst, /irq). That’s 32 lines or 64 resistors, and even though it’s pretty low frequency (it’s cycling at ~1-3MHz), guidance I’ve seen is that a few kilohertz is where parasitic capacitance comes into play, so really you need the capacitor pairings on the resistors as well, and now you’re up to 128 components. That is a bit of a hassle…I've never understood why people find adding two resistors in a voltage divider such a problem. It's only at very high speeds that an active level shifter is needed.I believe 2 is as well - we just spent the time testing and qualifying. Note this means input can go to 5v (as long as the chip is powered), but output is still 3v3.
And yes, I know this is exactly where a level-translator is perfect (eg: the awesome high-speed, low latency, auto-directional SN74CB3T16211DGGR) but “it said it was 5v tolerant”…
Most of the things I’d want to do with such a board are plug-in boards (eg: the parallel bus on the 8-bit Atari’s or the cartridge port on the 16-bit ones). It sure would be nice to make them hot-pluggable, meaning the 5v supply is up and running when the PCB containing the RP235x is plugged in, so
.. using diodes to power VCCIO via voltage drop might be an option, but for the sake of two £1.33 bidirectional auto-sensing level translator chips, I’ll probably go with the level translators. Who knows how good the voltage regulation to 5v is on 40-year-old PSUs and their 40-year-old capacitors?… so in practice, it’s not really what I want, though it is probably very useful for othersYour +5v will take to to come up too, so typically you will be fine else add 3 diodes in series between the 5v rail and the 3.3v rail to help out.Also, how "phut" are we ? If the RP235x VDDIO is powered by a 3.3v LDO which is fed by a 5v supply coming from legacy hardware, is that "phut" ? There'll be some delay between the 5v going into the LDO and coming out as 3.3v... Not a long time, but ...
Statistics: Posted by ThrudTheBarbarian — Mon Aug 04, 2025 2:22 am