Notes on I2C Programming for ESP32

I’m new to programming the ESP32 and I want to use the I2C peripheral to interface with the Hillcrest BNO080 sensor hub.

General Stuff

The ESP32 Technical Reference Manual has hardware architecture and register level information.

The esp-idf code base doesn’t seem to have any i2c driver. There are header files that give access to the I2C peripheral registers, though. These are detailed in “Findings in esp-idf” below.

Findings in esp-idf

in components/esp32/include/soc/i2c_struct.h, a structure is defined that maps onto the I2C peripheral. Two instances of this are declared, I2C0 and I2C1. And components/esp32/ld/esp32.peripherals.ld contains the lines:

PROVIDE ( I2C0 = 0x3ff53000 );
PROVIDE ( I2C1 = 0x3ff67000 );

So I think I should be able to access the peripherals from C with statements like:

I2C0.int_clr.rx_rec_full = 1;  // clear the interrupt
if (I2C0.status_reg.bus_busy) ...

There is also a file components/esp32/include/soc/i2c_reg.h. It has defines such as

#define I2C_INT_STATUS_REG(i)          (REG_I2C_BASE(i) + 0x002c)
...
/* I2C_BUS_BUSY : RO ;bitpos:[4] ;default: 1'b0 ; */
/*description: 1:I2C bus is busy transferring data. 0:I2C bus is in idle state.*/
#define I2C_BUS_BUSY  (BIT(4))

So I could also write code like:

if (I2C_INT_STATUS_REG(0) & I2C_BUS_BUSY) ...

Why they provide support for both styles, I don’t know.

The _reg.h files are used extensively in the ESP-IDF code base. The _struct.h are only used in 6 ESP-IDF source files.

So, yeah, it looks like I have to write register-level code to use the I2C bus.

Steps for BNO080 driver

System Initialization

Overall initialization sequence:

Configure I/O Mux
Enable I2C peripheral
Configure I2C peripheral for Master operation
Enable INTN GPIO pin for interrupt use
Enable interrupt on INTN

Writing I2C

Initiate operation to be processed by interrupts?
Start generation
Send address
Send data
Stop generation

Reading I2C

Initiate operation to be processed by interrupts?
Start generation
Send address
Read data
Stop generation

Detecting INTN

Configure I/O Mux
TBD

Controlling GPIO lines

Configure I/O Mux
TBD

Supporting modules

Write drivers to provide higher level access to features:

I2C master
GPIO output
GPIO input with interrupt
(SPI master)
(PWM generation for BLDC control)
(PWM generation for R/C servo)

ESP32 I2C