[adrian_b]

The easy to obtain FPGAs contain ancient ARM cores, but which are usable for implementing microcontroller tasks.

For example the AMD Xilinx UltraScale+, like in the AMD Kria modules and development kits (3-digit prices), include some Cortex-R5 cores, which provide deterministic operation, like Cortex-M.

Cortex-R5 are somewhat slower than Cortex-M7 at the same clock frequency, but they are available at a higher clock frequency than many Cortex-M7 implementations.

If you can implement some custom peripherals in the FPGA logic array, then you can obtain much higher performance than with a microcontroller alone.

[varispeed]

Yes, this will not offer meaningful performance improvement over "native" silicon. I am talking about computational power, access to fast memory etc. not peripherals, which can still be serviced by FPGA.

[adrian_b]

The AMD Kria modules include 600 MHz dual Cortex-R5 cores and 1.5 GHz quadruple Cortex-A53 cores, and also 4 GB of DDR4 memory.

So they are similar with an older Raspberry Pi and they have far more computational power than a Cortex-M7 or Cortex-M85 CPU, even if they are very slow in comparison with modern Cortex-A7x or Cortex-A7xx cores.

I have never heard of any FPGA containing better CPU cores than Cortex-A78, but even those with Cortex-A78 are extremely expensive, so they may be worthwhile only for their FPGA part, not for a CPU that is much slower than cheaper alternatives.

The same is true even for the cheaper modules with UltraScale+ FPGAs, like AMD Kria, which cost as much as one of the cheaper mini-PCs with a much faster Intel or AMD CPU, so they are worthwhile only if you can implement in the FPGA an essential part of the functionality.

There is however another advantage of the FPGAs with ARM cores, besides implementing fast peripherals with hard real-time requirements.

Unlike with most non-microcontroller ARM CPUs where the vendor keeps secret various things, including the boot loader, so you cannot be absolutely certain about what the vendor does, because ARM has followed the example of Intel and has introduced a potential Trojan horse in its CPUs, i.e. an execution mode controlled by the vendor, which is more privileged than even a hypervisor, in the FPGAs with ARM cores you have complete documentation and absolute control over what the CPU does, so you could implement with greater confidence some devices for which security is important.

[varispeed]

> 600 MHz dual Cortex-R5

Is slower than Cortex-M7. Missing the point completely.

Your proposal does not offer any improvement in terms of realtime performance.