Interfacing to SD card using 4bit SD mode

[proprocks]

The press releases for the ESP32 mention an SDIO peripheral, however I'm having difficulties finding out much more information than that. The most I've read is that the ESP31 is a SDIO slave device (ie. SDIO used for a host micro to talk to the ESP31), while the ESP32 would be a SDIO master device (SDIO used to talk to an SD card.) We're searching for a wifi module that can read/write to a SD card at a decent rate (SPI being too slow.)

Can anyone confirm how the ESP32's SDIO is/can be used?

[iot-bits.com]

What is the speed that you could get using SPI interface with an SD Card? The ESP8266 easily managed around 10 Mbits/s with HSPI... so the ESP32 can do the same (SPI hardware is almost identical on both)

[ESP_Sprite]

The SPI bus can be clocked at 80MHz, so 10MByte/s is the theoretical max. It can also do 4-bit parallel reads/writes, so with that it should be able to shift 40MByte/sec in theory. It has DMA, so as long as you keep the buffers filled, you should be able to get pretty close to that theoretical maximum.

[iot-bits.com]

10M bits I meant. That's the max I could go. Over that, traditional SPI mode won't work as card has frequency limits of about 25MHz.