Scott Shawcroft Aims to Replace 7400-Series Logic with a STEMMA QT FPGA Board Design

Scott Shawcroft has shown off a new board for the STEMMA QT I2C ecosystem, designed to house a Lattice MachXO2 32QFN field-programmable gate array (FPGA) and replace 7400-series logic chips with a programmable glue logic system.

"This is a STEMMA QT board. So, it's got the two-in and -out there, and then it's got logic level shifters and a regulator in case it's five volts, and then this is a MachXO2 FPGA," Shawcroft explains of the design during an Adafruit Show and Tell session. "I was thinking what i would do is add software so that it would act like a 7400-series logic chip."

"7400-series logic is like classic glue logic, which is like very basic sort of Boolean logic. I was thinking it would be actually really handy to have a board like this for, like, 20 bucks that can basically be any one of the 7400 series that you want."

The idea, Shawcroft explains, is that the FPGA would be accessible via I2C from a controlling CircuitPython device for quick reconfiguration into different combinations of 7400-series logic parts. At present, though, it's still a work in progress: "It doesn't work well," Shawcroft admits of the current prototype. "Like I said, this is what FPGAs do best is is not quite work and then they work great. You know, there are only two states of FPGAs: not working at all, and like 'oh my god this is amazing.'"

The Lattice MachXO2 "bridging FPGA" picked by Shawcroft to drive the project comes in its smallest form with 256 LUTs and 2kb of static RAM (SRAM); the XO2-1200 variant offers 1,280 LUTs, 7 EBR RAM blocks totalling 64kb, 10kB of SRAM, 80kb of flash memory, and a single phase-locked loop. It's not yet clear if Shawcroft's board will support the larger bariant.

More information is available on the Adafruit Show and Tell video; the KiCad design files for the board can be found on Shawcroft's GitHub repository.