Post #4251367

2026-07-30 19:44 UTC

The most attractive MCU option is the STM32H757ZIY6. Digikey doesn't stock it, and has a 5000 piece MOQ for $57521, but if I can source it elsewhere it has exactly the specs I want: * ~5x5mm WLCSP156 package * 2 MB of flash * 1 MB of SRAM (including 64 kB ITCM/128 kB DTCM) * 99 total GPIOs leaving plenty free after the memory bus * 480 MHz M7 + 240 MHz M4 ST does not list *any* distributors though, seems special order only? You can't even get samples that I can see, or order small quantities direct. The same part is also available in a 7x7 ball UFBGA169 which is only 2mm larger and has 112 GPIOs but otherwise mostly the same specs and Digikey has in stock for $21.49 @ 1 or $15.36 @ 50. So tentatively, this seems a good choice one way or another. It has an internal LDO/SMPS for the digital core and can run IOs at either 1.8 or 3.3V, but there's only one IO bank so I'm stuck running everything at 3.3V assuming I need at least *some* 3.3V stuff, and then if I need 1.8 it will have to be via the FPGA. This is generally how I've built my board-level MCU+FPGA systems in the past, nothing unusual here other than using a higher density package. So far, totally doable just will need a higher density PCB than I've used in the past for these parts.

Replies (1)

@azonenberg@ioc.exchange 2026-07-30 20:08

With that out of the way, we're now looking for a small FPGA that has both 1.8 and 3.3V IO banks. I need a minimum of 24 IOs MCU-FPGA to run the FMC at all (hypothetically down to 22 if I remove the byte write enables but I would need to restructure some RTL to make that possible). Then up to another 10 to break out the full address space, and another 3 to get more chip selects for even more address space. So let's budget 37 pins for this and can always go lower. Candidates include: * Spartan-7 S6/15 in CPGA196(8mm) or S25 in CSGA225 (13mm). These provide 2 and 3 50-pin IO banks respectively. All are 1.8/3.3V compatible. * Artix-7 CPG236/8 (10mm). These go up to 50K LEs with two 6 Gbps transceiver lanes and 3-4 IO banks adding up to 106 or 112 pins depending on density. All are 1.8/3.3V compatible. * Kintex-7 is off the table, FBG484 (23mm square) is the smallest package. * Spartan UltraScale+. The SU10-35 are available in CMVA361 (10mm square) with 168 HDIO, 52 HPIO, and no transceivers and are supported by a not-enshittified Vivado version (2025.1) * Artix UltraScale+. The AU7P comes in FCVA289 (9mm square with 72 HDIO, 58 HPIO, 4 GTH. The AU10P and AU15P come in UBVA368 (11.5 x 9.5mm) with 24 HDIO, 104 HPIO, 8 GTH. Everything else is way too big. * Kintex U+: All options too large * Efinix Trion: 49/81 ball packages don't have enough IO (33 max, nothing left over after the FMC. 169-FBGA (9mm square) has 73 GPIO and goes up to the T20. 324-ball 0.65mm FBGA is 12mm and goes up to the T120 with 130 GPIOs. * Efinix Topaz: Tz50 WLCSP100 only offers 61 1.8V IOs and no 3.3 so it's out. Same die comes in 225 ball (10mm square) with 140 HSIO and 23 HVIO, still not enough for FMC. The Tz100 comes in 441 ball 11mm with 20 HVIO, 85 HSIO, and even transceivers but the 20 HVIO is really constraining. I don't like any of the Topaz options. * Efinix Titanium: Ti60 comes in nice small packages, but without much in the way of HVIO either. The 361-ball 13mm package goes up to Ti180 with also only 20 HVIOs. Looks like you need the 16mm 400 ball to get 74 HVIO. * Efinix Titanium Edge: Ti125 comes in 225 ball (8mm) with 36 HVIO and 110 HSIO, and even a hard DDR interface. No transceivers, but this looks promising - better logic and IO density in a small package than other Efinix offerings. * Efinix Titanium Transceiver: Ti135 comes in 441-ball (11mm) with only 20 HVIO, other options are too big.

Open ##4252379