pico-coder
commented
2 years ago Rough, off-the-cuff response.
Per the Raspberry PI PICO W datasheet the SPI interface to the WiFi chip is only single lane SPI, and per section 3.8 typically runs at 33Mhz.
The CYW43439 datasheet indicates support for up to 2KB data transfers per SPI operation if you use the special wifi functions, with only a 4B command overhead. So in theory the SPI link might reach ~30Mbit/sec of raw data rate. But there could be all sorts of bottlenecks (Hardware and Software) in the ability of the PICO to send the packets.
IIRC, your ideal use case was something like 16 channels at 3Msps with fairly low run length encoding success due to needing to sample a clock and due to all the variable delay line circuits etc. With no RLE, you need 48 Mbit/sec raw bandwidth, so you would still come up a bit short, but 30 is a bit higher than the 12Mbit/sec raw USB bandwidth.
Also, the RP2040 says SPI clock can reach half of sys_clk and the CYW43439 says it can support 50Mhz, so I think you might be able to boost the 33Mhz based to 50, but with a product that was just release a week ago I wouldn't start pressing my luck. Further we are still in the era of the chip shortage and these seem to be built from the typical rare earth metal "unobtanium" ;=). It's also fishing season so my screen time has been pretty light these days....
Side note: As I was looking into the broadcom specs for the CPUs used on the Raspberry Pi Model 3/Model 4, they all seem to have a DMA engine that can access their GPIOs, and thus in theory you should be able to build a logic analyzer with fast and well spaced sample rates without even using a PICO. Granted you lose analog sampling, though with the right parallel interface of a PICO feeding the GPIOs of the Model 3/4 you could likely get analog signals (up to 500Khz limited by the PICO) and higher digital sample rates.
But, what is strange is that I can't seem to find any reference to a DMA based raspberry pi model 3/4 logic analyzer on the internet, and it just seems strange that nobody has every done it. Perhaps people are just scared of programming DMA engines and just throw down the $16 for a logic analyzer and call it a day....
normanr
tony1tf
AapoTahkola
mtk11
With the new PicoW release last week, this seems like an ideal time to upgrade this logic analyser. Since the WiFi chip uses SPI for communication, I guess it is going to be faster than the USB serial port. Any thoughts about this Shawn?