Using RAM on the Alloy board

lawrie 2020-10-27 20:11:57 UTC #1

@folknology If the main language for programming the Alloy fpga is to be nmigen, I was wondering how we would access the RAM. The BRAM is no problem an nmigen supports that using Memory.(...). But i don't think it has support for the 128kb of single-port ram (SPRAM) on the up5k.

The only way i have used that is through @daveshah's code that use the Lattice primitive's - https://github.com/daveshah1/picorv32/blob/up5k/picosoc/up_spram.v, but I am not sure how you would use that from nmigen.

The discussion that I have seen on using SPRAM from nmigen was rather old, so perhaps there is a way of doing it now.

If there was a simple way of incorporating Verilog modules from nmigen programs (like the SpinalHDL Blackbox), then we could use that, but I have not seen any such facility. Do you know of a way of doing that?

The other type of ram that I believe you are going to have on Alloy is SPI RAM. Presumably, we will have to write a controller for that. I guess it will be similar to an SPI flash memory controller.

daveshah 2020-10-27 21:18:28 UTC #2

You can use Instances in nMigen for SPRAM. A random example is https://github.com/zignig/gizmotron/blob/20cb223ed72863fac6c7281505373cc91fe191d7/tpw/spram.py#L26

I have seen a few cores around for QSPI RAM but don't have a link to hand.

lawrie 2020-10-27 21:57:17 UTC #3

Thanks. I saw Instances used in migen, but did not know that you could use it in nmigen.

Folknology 2020-10-27 23:51:43 UTC #4

I believe you can also use Verilog modules from within nMigen but would have to look that up again tomorrow.

lawrie 2020-10-28 09:41:04 UTC #5

I believe the Instances solution works for any Verilog module.

Is the SPI RAM you are using the same as SPI PSRAM?

That was used on the Hackaday badge and I believe this may be the controller they used - https://github.com/Spritetm/hadbadge2019_fpgasoc/blob/master/soc/qpi_cache/qpimem_iface.v

I am not sure what QPI means as opposed to QSPI.

Folknology 2020-10-28 09:45:32 UTC #6

For the BlackIce V yes I was looking to add the SPI PSRAM, on Alloy we connect it to the ESP32-S2 and that has built in support so we do not need to write anything. Do you need SPI PSRAM for your project?

BTW there are Verilog SPI PSRAM drivers..

lawrie 2020-10-28 10:12:47 UTC #7

I had not remembered correctly what you are doing on the Alloy. I thought you had SPi Ram connected to both the ESP32 and the fpga, but I see it is just SPI flash connected to the up5k.

I would quite like more RAM available on Alloy, as it is useful for camera applications if you need framebuffers. and it is always useful for SoCs. I am not sure if PSRAM could be made to work with retro gaming.

However, Alloy is a small board and we can't have everything.

The 128KB SPRAM is just big enough for a 240x240 16-bit color framebuffer for the st7789 display.

Connected PSRAM via the pmod is a possibility - https://twitter.com/gregdavill/status/1073883636305293313