FPGA selection and options for a "ROM emulator", and 5v to 3.3v conversions  - Page 1
Forum section: electronics / beginners

author: mclarkson
author_rank: Contributor
timestamp: August 12, 2023, 01:25:50 am
content: Hi All, I'm kicking the tires of building a ROM board replacement for an old Sega System 32 main board (Arcade game system where the rom board can be interchanged for different games).The ROM board contains 4 busses, each has it's own sets of 27C160 sets of ROMS. Roughly 42 lines per bus, plus their own logic around the TTL chips around the roms with their own chip selection bits. So roughly 200 I/O pins between all busses. The various busses have their own clocks by the looks of it (but I will have to check myself to see what the timing on each of the busses is in respect to ROM access), with the CPU running at 16Mhz, and the GPU running at 50Mhz.I've been looking at different FPGA's, and I think I've settled on a few Trion options, the T20 dev kit can handle quite a bit of I/O, and I could possible do a couple of these. Or I could go up to a T120 dev kit which has ample I/O.T20 Dev Board-https://www.digikey.ca/en/products/detail/efinix,-inc./T20F256C-DK/10654491?utm_adgroup=&utm_source=google&utm_medium=cpc&utm_campaign=PMax%20Product_Low%20ROAS%20Categories&utm_term=&productid=10654491&gclid=Cj0KCQjwuNemBhCBARIsADp74QQ_eJGQijUp8gjJOlDqh3v5-gR63nmE6uEtFFky0NSMze45hQjqR4saAj83EALw_wcBT120 Dev Board -https://www.digikey.ca/en/products/detail/efinix,-inc./T120F576C-DK/11685805?utm_adgroup=&utm_source=google&utm_medium=cpc&utm_campaign=PMax%20Product_Low%20ROAS%20Categories&utm_term=&productid=11685805&gclid=Cj0KCQjwuNemBhCBARIsADp74QSw8e4gRqm4YXMo4F4yZxcs01neN84nSCc8hkT8zPcYSjP3oe5nHP8aAlpBEALw_wcBSince I am starting fresh with this, I have a couple questionsAre these FPGA chips a good option? (Price point and IO looks good)How do I properly convert down from 5v to 3.3v like these Trion's need? Equally important, how do I convert back up to 5v for my output lines (I've seen voltage dividers, and IC's to go from TTL -> CMOS, but I'm looking for the best fit for this application, and can do things within the timing that I need)Thanks a ton for anyone who can help me out on this

author: Buriedcode
author_rank: Super Contributor
timestamp: August 12, 2023, 05:56:37 pm
content: 5 to 3.3, and visa versa is pretty common, and what you use depends on the application - speed, drive capability (fan out, how many IO its driving) and direciton, be it unidirectional, or bidirectional.For unidirectional signals I tend to use the LCX series logic for 5V to 3.3, since its cheap, common, and has most types of logic in that family. Another option is the VHC which is slower. Both can accept input voltages of up to 7V max regardless of supply voltage. VHC can run from 2 to 5.5V, where-as LCX is 2 to 3.6V (going from 5 to 3.3V both will be powered by 3.3V anyway). Note the 74VHC245 is NOT 5v tolerant on the IO just the control signals.Going back from 3.3 to 5v I use the standard HCT series - 4.5-5.5V power only, but the input thresholds are proper TTL, so <0.8V for a 0, and >2.0V for a 1. This guarantees 3.3V logic meets the thresholds.For faster signals VHCT, but that is becoming a rarer variant. There is also the AHCT which is comparable to the VHCT (I'm not sure but I think VHCT is just a specific brand of AHCT).As for actual devices/part numbers, the 74xx541 is handy since all the IO are in a row. 573 for bus holds and latches.Then there are of course dedicated level shifters from various manufacturers, some bidirectional, some for specific applications. Given your application I doubt the signals are anything beyond what standard HC/HCT will cope with, so I'd say 74LCX541's for the 5 -> 3.3, and 74VHCT541 or even just 74HCT541 for the 3.3 - 5V.Edit: forgot to mention, the above '541 has 8 IO per chip. There are bus level shifters with 16 or even 32 IO per chip, which might work out cheaper for you. Examples are SN74ALVC164245, SN74LVCH16T245, these are bidrecitonal I believe - but if you just want one-way you can tie the relavent direction pin to VDD?GND.

author: mclarkson
author_rank: Contributor
timestamp: August 13, 2023, 01:34:39 am
content: Thanks a ton for the reply.Let me do some researching on all this. Extremely helpful, thanks again!