Olimex ESP32 EVB. SDMMC with LAN can cause LAN freezes.

[dukess]

I tried read files from SD card and send they through self-written web server (https://github.com/fredlcore/BSB-LAN). When files transferred through WiFi all works fine (on Arduino Mega and Due boards code works without problem too), but when i start to use built-in LAN i got these problems:

1. ICMP packets are lost time-after-time.
2. Low LAN performance when transferring application-generated data.

The investigation allowed us to solve problem by reducing SDMMC frequency (fredlcore/BSB-LAN#390 (review)) from SDMMC_FREQ_HIGHSPEED to SDMMC_FREQ_DEFAULT.

I want propose to add one more parameter in begin() function which allow to change SDMMC frequency from application rather SDK. Backward compatibility will be preserved. Also i want to propose to add ETH_CAN_INTERFERE_WITH_SDMMC definition which can switch default speed between SDMMC_FREQ_HIGHSPEED and SDMMC_FREQ_DEFAULT for boards with LAN adapter.

Patch: https://github.com/dukess/arduino-esp32/commit/f068626c12f7cdbe16077a758fe59d043f65eea8

Hardware:

Board: Olimex ESP32 EVB
Core Installation version: 1.0.6
IDE name: Arduino IDE
Flash Frequency: 40Mhz
Upload Speed: 115200
Computer OS: Windows 7/10