Pico RMII Ethernet library, Neon Chrome Edition (NCE)

Introduction

This is an update to the existing pico-rmii-ethernet library by Sandeep Mistry. Please see README_orig.md for a description of that library. Note that the RMII board modifications to use an external 50 MHz clock are still necessary.

Improvements present in this library:

1. Achieves 94.9 Mbit/sec when Pico is overclocked to 300 MHz, as measured by iperf.
2. Inter packet gaps correctly inserted into transmit stream.
3. Greater flexiblity in choosing system clocks: 100, 150, 200, 250, 300. Higher system clock frequencies should work; untested.
4. iperf available in the examples directory

Discussion

This library uses DMA driven ring buffers for both transmit and receive. The transmit side is entirely DMA driven, while the receive side uses a per-packet interrupt to finalize a received packet. Performance does vary with system clock speed, and the memory region the executable is placed:

System Clock MHz	Iperf Mbit/sec (SRAM)	Iperf Mbit/sec (flash)
100	1.38	1.27
150	2.81	Link not established
200	65.4	31.4
250	83.4	69.3
300	94.9	85.9

The MDIO interface was changed from polled to interrupt driven, thus freeing up more processor time for packet processing.

Resource Utilization

The library uses:

1. Five DMA channels: 2 receive, 3 transmit. Two channels are used per Tx/Rx for ring buffer management, and the third Tx channel assists in generating the Ethernet Inter Packet Gap.
2. Two interrupts: 1 shared for MDIO, and 1 exclusive for the end-of-packet processing.
3. Two 4KB aligned memory regions for Tx/Rx data, 32/64 long word pointer buffers, and a 256 long word CRC table.
4. One PWM timer used as MD clock
5. One DMA timer used to assist with IPG generation
6. 12 PIO instructions for Tx, 6 for Rx, total 18

At 300 MHz, almost all of core 1 is used. It is possible to use about 6 usec per packet poll, verified by placing a sleep_us(6) call in netif_rmii_ethernet_loop() and running iperf. Core 0, of course, remains available for user applications.

Customization

Edit main.c in examples/{http, lwiperf} to set the pins used for the RMII interface, and set the target_clk variable to the desired system clock, one of: 100/150/200/250/300 MHz.

Limitations

** For best performance, must be run directly from RP2040 SRAM **, using:

        pico_set_binary_type(pico_rmii_ethernet_lwiperf no_flash)

and loading via a debugger or

        pico_set_binary_type(pico_rmii_ethernet_lwiperf copy_to_ram)

in the CMakeLists.

Acknowledgements

Sandeep Mistry - who determined that using an external clock for the RMII board would enable much better performance.