IoT with ESP8266

The recent release of ESP8266 has opened new possibilities for the age of IoT.  Its low-cost, versatility and simplicity make it instantly popular.  I fetched a module for less than $4.

ESP-01 Module

The Chinese made ESP8266EX is a complete WiFi chip, including RF and a Tensilica processor.  The programming of the processor is supported by the GNU toolchain.  On the module, it includes a 4Mb SPI flash PROM for mass storage, a crystal, a few discretes and a printed antenna.  There are one red LED for power and one blue LED to UART TX.  The 8-pin connector has the following connection,

Pin	Name	Description
1	GND	Ground
2	U0TXD	UART0 Transmit, GPIO1, (blue LED)
3	GPIO2	U1TXD, I2C_SDA, has internal pull-up
4	CHIP_EN	Chip Enable, active high
5	GPIO0	Has internal pull-up, (for Flash programming mode)
6	EXT_RSTB	External reset signal, active low, pull-up resistor
7	U0RXD	UART0 Receive, GPIO3, has internal pull-up
8	VDD	+3.3V power input

Pull GPIO0 low at power up to enter the Flash programming mode.

The ESP-03 has more I/O pins.  The board is 17.4mm x 12.2mm with 2mm pin spacing.

ESP-03 Module

The additional pins are

	GPIO12	MTDI, HSPI_MISO, PWM0
	GPIO13	MTCK, HSPI_MOSI, UART0_CTS
	GPIO14	MTMS, HSPI_CLK, I2C_SCL, PWM2
	GPIO15	MTDO, HSPI_CS, UART0_RTS, PWM1
	GPIO16	Deep-Sleep Wakeup

Module Schematic

For normal operation, CHIP_EN should be high and GPIO15 low.  In standby, the power draw is about 1mA; in the receive mode, the power draw is about 70mA.  The transmit power draw is over 100mA, maybe as high as 200mA.  So make sure the voltage supply can provide this peak current.

For this version of the firmware, the baudrate is 9600.  It had to be determined by looking at the scope of the TX signal.  (Note that some documentation says 115200, but that may be for an earlier version.)

The ESP module starts as an access point, which can be connected from a PC WiFi adapter.  Open a server TCP port on the PC, using nc.   We can start the communication,
AT+CWMODE=2 # set mode AP
AT+CWSAP? # returns AP: ssid, password, channel, encryption
AT+CIFSR # returns the IP address
AT+CIPSTART="TCP","192.168.4.100",6789 # open a TCP connection
AT+CIPSEND=4 # send 4 bytes, type in the 
The message shows at the PC terminal running nc, and anything you type in nc is displayed at the serial terminal.

To create a TCP server
AT+CIPMUX=1 # must be 1 for server
AT+CIPSERVER=1,6789  # create server port 6789
At this point, we can connect to the server from the PC and whatever we send is displayed.
To send data back,
AT+CIPSEND=0,5 # send 5 bytes, must specify id

Another way to try is to set up an access point (AP) on a PC and have the module connect to it.  The command sequence is as follows,
AT+CWMODE=3 # set mode to station and AP
AT+CWLAP    # list the available APs
AT+CWJAP="AP","Password"  # to join an AP
AT+CIPSTART="TCP","192.168.12.1",1234 # open a TCP connection
AT+CIPSEND=4 # send 4 bytes, type in the 

Next we check the compiler toolchain and the SDK.

The toolchain is xtensa-lx106-elf.  The latest SDK is esp_iot_sdk_v0.9.4.  The default app IoT_Demo is built by running make and gen_misc.sh in the app directory.   To upload through serial port, set GPIO0 low and GPIO2 high and run in the bin directory,

~/esptool-py/esptool.py --port /dev/ttyUSB0 write_flash 0x00000 eagle.app.v6.flash.bin 0x40000 eagle.app.v6.irom0text.bin

Connecting...

Erasing flash...

Writing at 0x00008c00... (100 %)

Erasing flash...

Writing at 0x00067000... (100 %)

Leaving...

esptool.py is a python program to communicate with the ROM bootloader.  The baud rate is non-standard 74880.

The at_v0.20 only works with esp_iot_sdk_v0.9.3 .  It can be built and uploaded to the module like above.  The default baud rate is 115,200.  After spewing some garbage, it is ready to accept AT commands.