I'm stress testing an Ethernet to Serial converter, mostly it works well, but when receiving larger Ethernet frames (bursts of frames) while also receiving SPI Rx data (that are being unloaded via DMA) I am getting SPI Overrun errors occasionally.
Ethernet is 100 mbps, SPI Rx is 2 mbps. As Ethernet frame size is reduced the SPI overrun problem reduces. With a very much slower SPI (300 kbps) speed there is no problem at any Ethernet Frame size.
My presumption is the MAC transfer into RAM is consuming too much bandwidth and I'm running almost flat out now.
So my latest test was to play with the SPI Rx FIFO disable setting - to my surprise it did not impact the overrun errors either way.
I'm looking for confirmation that in DMA mode the SPI does not use its FIFO.
(I need to convince the hardware folks to change the module to not use the SPI.)