ESP8089-SPI

Linux kernel module to use ESP8089 / ESP8266 over SPI. Intended for use with Raspberry Pi Zero.

Hardware

If using an ESP8089, no changes are required to be made to the chip. If using an ESP8266, the SPI flash must first be desoldered. This is the 8-pin SOIC component near the 32-pin QFN die. The ESP-201 variation works well for this. Alternatively, one can use an off-brand NodeMCU or WeMos D1 Mini, albeit a version that doesn't use the ESP-12.

What pins go where:

Raspberry Pi	ESP8266 / ESP8089	Function
BCM 13	CHIP_EN	esp_reset_gpio
BCM 16	GPIO10 / SDIO_DATA_3	esp_cs0_pin
BCM 19	GPIO7 / SDIO_DATA_0	MISO
BCM 20	GPIO11 / SDIO_CMD	MOSI
BCM 21	GPIO6 / SDIO_CLK	SCLK
BCM 26	GPIO8 / SDIO_DATA_1	esp_interrupt
3.3V	GPIO15 / MTDO	boot select
3.3V	GPIO0	boot select
GND	GPIO2	boot select

It may be advisable to add a resistor across each I/O pin (33 ~ 330 Ohm).

Software

Start with a fresh install of Raspbian.

Step one: prerequisites

sudo apt-get -y update

sudo apt-get -y upgrade

sudo apt-get -y dist-upgrade

sudo apt-get -y install raspberrypi-kernel-headers gcc git make

Step two: install

mkdir ~/esp

cd ~/esp

git clone https://github.com/notabucketofspam/ESP8089-SPI.git

cd ESP8089-SPI

sudo make install

Step three: configure

sudo su

echo "options esp8089-spi esp_reset_gpio=13 esp_cs0_pin=16 esp_interrupt=26" > /etc/modprobe.d/esp.conf

echo "esp8089_spi" >> /etc/modprobe.d/blacklist

echo "spi_bcm2835" >> /etc/modules

echo "spi_bcm2835aux" >> /etc/modules

echo "esp8089_spi" >> /etc/modules

echo "dtoverlay=spi1-1cs,cs0_pin=16,cs0_spidev=disabled" >> /boot/config.txt

reboot

How it works

The ESP8266 is basically just a rehashed ESP8089, which is a wireless chip commonly used in many Unix-based devices. The ESP8089 is designed to load its firmware over SPI / SDIO when the device in question boots, whereas the ESP8266 is intended to load firmware off of the integrated SPI flash component. The pins used for this operation, though, are exposed on most varieties of the chip. These pins can be therefore utilized to load any custom firmware onto an ESP8266; in fact, this is what the eagle_fw#.h files are. The SPI flash must first be removed to do this.

Upon boot of the host device, the ESP chip is power cycled using the CHIP_EN pin (held low) and subsequently set to load code over SPI via GPIO15 (held high). During this time, GPIO0 must be held high and GPIO2 must be held low to select the correct boot mode.