Discussion

dmitrygr: > Above is the logic path isolated as one of the longest combination paths in the design, and below is a detailed report of what the cells are.which is an argument that "fpga_pio" is badly implemented or that PIO is unsuitable for FPGA impls. Real silicon does not need to use a shitton of LUT4s to implement this logic and it can be done much more efficiently and closes timing at higher clocks (as we know since PIO will run near a GHz)

Retr0id: PIO is unsuitable for FPGA impls, that's what the article says.> If you’re thinking about using it in an FPGA, you’d be better off skipping the PIO and just implementing whatever peripherals you want directly using RTL.

bunnie: Hello again HN, I'm bunnie! Unfortunately, time zones strike again...I'll check back when I can, and respond to your questions.

dmitrygr: very cool. tiny processors everywhere. but be nice to PIO. PIO is good :)

mrlambchop: I loved this article and had wanted to play with PIO for a long time (or at least, learn from it through playing!).One thing jumped out here - I assumed CISC inside PIO had a mental model of "one instruction by cycle" and thus it was pretty easy to reason about the underlying machine (including any delay slots etc...).For this RISC model using C, we are now reasoning about compiled code which has a somewhat variable instruction timing (1-3 cycles) and that introduces an uncertainty - the compiler and understanding its implementation.I think this means that the PIO is timing-first, as timing == waveform where BIO is clarity-first with C as the expression and then explicit hardware synchronization.I like both models! I am wondering about the quantum delays however that are being used to set the deadlines - here, human derived wait delays are utilized knowledge of the compiled instructions to set the timing.Might there not be a model of 'preparing the next hardware transaction' and then 'waiting for an external synchronization' such as an external signal or internal clock, so we don't need to count the instruction cycles so precisely. On the external signal side, I guess the instruction is 'wait for GPIO change' or something, so the value is immediately ready (int i = GPIO_read_wait_high(23) or something) and the external one is doing the same, but synchronizing (GPIO_write_wait_clock( 24, CLOCK_DEF)) as an alternative to the explicit quantum delays.This might be a shadow register / latch model in more generic terms - prep the work in shadow, latch/commit on trigger.Anyway, great work Bunnie!

bunnie: Agreed! The PIO is great at what it does. I drew a lot of inspiration from it.

dmitrygr: What are your thoughts on efficiency? BIO vs PIO implementing, say, 68k 16-bit-wide bus slave. I know i can support 66MHz 68K bus clock with PIO at 300MHz. How much clock speed would BIO need?

bunnie: The idea of the wait-to-quantum register is that it gets you out of cycle-counting hell at the expense of sacrificing a few cycles as rounding errors. But yes, for maximum performance you would be back to cycle counting.That being said - one nice thing about the BIO being open source is you can run the verilog design in Verilator. The simulation shows exactly how many cycles are being used, and for what. So for very tight situations, the open source RTL nature of the design opens up a new set of tools that were previously unavailable to coders. You can see an example of what it looks like here: https://baochip.github.io/baochip-1x/ch00-00-rtl-overview.ht...Of course, there's a learning curve to all new tools, and Verilator has a pretty steep curve in particular. But, I hope people give the Verilator simulations a try. It's kind of neat just to be able to poke around inside a CPU and see what it's thinking!

bunnie: As a side note about speed comparisons - please keep in mind the faster speeds cited for the PIO are achieved through overclocking.The BIO should also be able to overclock. It won't overclock as well as the PIO, for sure - the PIO stores its code in flip-flops, which performance scales very well with elevated voltages. The BIO uses a RAM macro, which is essentially an analog part at its heart, and responds differently to higher voltages.That being said, I'm pretty confident that the BIO can run at 800MHz for most cases. However, as the manufacturer I have to be careful about frequency claims. Users can claim a warranty return on a BIO that fails to run at 700MHz, but you can't do the same for one that fails to run at 800MHz - thus whenever I cite the performance of the BIO, I always stick it at the number that's explicitly tested and guaranteed by the manufacturing process, that is, 700MHz.Third-party overclockers can do whatever they want to the chip - of course, at that point, the warranty is voided!