Hacker News Reader

Comment by mattthebaker | Hacker News Reader

Regarding point 1, this is a terrible design decision. Any low power or high performance uC that requires code in external SPI loads it into and executes from RAM. At 32Mhz it starts to make sense to have an ICACHE, with EMBEDDED Flash on a parallel bus. At 320Mhz you want to cache your RAM. With 320Mhz and external SPI (not even fast parallel NOR), you have to be insane. A single instruction cache miss will cost you 100s of clock cycles for a SPI read.

I really hope they just forgot to mention 100kB+ of RAM on that landing page, and the 16kB data is just a DCACHE.

View on HN

The limitation that I referred to (same as open-v) is that it's very difficult to get IP for embedded NVRAM, and requires more complicated processes. So this seems like the next best option. Yes, misses are going to be very expensive, but I'd rather have icache than manually paging code (plus you can use the 16KiB internal RAM for tight timing loops). More RAM would of course be better, but probably not the first thing I would add in the next revision (that would be more peripherals).

Also note that Quad SPI flash is 4 bits wide, and is NOR flash.

Yes, the process trade offs are not so good if you want embedded flash. You need a process that can withstand the high voltages for flash erase, which means thick oxides and slow transistors.

At 320Mhz, you should have icache and paged code. A process that can clock this high should have no problem with RAM densities to offer more than 16kB. Better to mirror what the entire industry does: low clock speed and embedded flash, or better process with faster clocks, more RAM and external flash.

Quad SPI NOR is still SPI, which requires serial timings. You need to serialize address and data with each transaction. Parallel NOR has parallel address and data, and an order of magnitude improved throughput and latency.

There is really no point to 320Mhz with such slow code memory.

Yeah, even with the Opus application I mentioned, 320Mhz is way overkill - 50Mhz would have been plenty. It kind of seems like for some reason they were able to get a modern process, but only a really tiny die. It's pretty interesting that Open-V made the same tradeoff - 160MHz, but only 8KiB of RAM shared between code and data (with no memory mapped SPI flash).

Reply on news.ycombinator.com