2-wire Serial Interface (TWI, a.k.a. I2C) uses pins with SDA and SCL functions.

Referance datasheet 2-wire Serial Interface.

Arduino has twi.c and twi.h which was done in C, I did some modification and updates for avr-libc and avr-gcc (4.9). It uses an ASYNC ISR but does block (busy wait) while reading or writing.

With a serial port connection (see BOOTLOAD_PORT in Makefile) and optiboot installed on the RPUno run 'make bootload' and it should compile and then flash the MCU.

sudo apt-get install make git gcc-avr binutils-avr gdb-avr avr-libc avrdude
git clone https://github.com/epccs/RPUlux/
cd /RPUlux/i2c-debug
make bootload
...
avrdude done.  Thank you.

Now connect with picocom (or ilk).

#exit picocom with C-a, C-x
picocom -b 38400 /dev/ttyUSB0

Commands are interactive over the serial interface at 38400 baud rate. The echo will start after the second character of a new line.

rpu_address is taken from the I2C address 0x29 (e.g. get_Rpu_address() which is included form ../lib/rpu_mgr.h). The value of rpu_address is used as a character in a string, which means don't use a null value (C strings are null terminated) as an adddress. The ASCII value for '1' (0x31) is easy and looks nice, though I fear it will cause some confusion when it is discovered that the actual address value is 49.

Commands and their arguments follow.

identify

/0/id?
{"id":{"name":"I2Cdebug^1","desc":"RPUlux (17323^0) Board /w atmega328p","avr-gcc":"5.4.0"}}

Scan of I2C bus shows all 7 bit devices found. I have a PCA9554 at 0x38 and an 24C02AN eeprom at 0x50.

/1/iscan?
{"scan":[{"addr":"0x29"},{"addr":"0x38"},{"addr":"0x50"}]}

Note: the address does not include the Read/Write bit.

Set the I2C address

/1/iaddr 56
{"address":"0x38"}

Note: Set it with the decimel value, it will return the hex value. This value is used durring read and write, it will also reset the xtBuffer.

Add up to five bytes to I2C transmit buffer. JSON reply is the full buffer.

/1/ibuff 3,0
{"txBuffer":["data":"0x3","data":"0x0"]}

Show buffer data.

/1/ibuff?
{"txBuffer":["data":"0x3","data":"0x0"]}

Attempt to become master and write the txBuffer bytes to I2C address (PCA9554). The txBuffer will clear if write was a success.

/1/iaddr 56
{"address":"0x38"}
/1/ibuff 3,0
{"txBuffer":["data":"0x3","data":"0x0"]}
/1/iwrite
{"returnCode":"success"}

If txBuffer has values, attempt to become master and write the byte(s) in buffer (e.g. command byte) to I2C address (example is for a PCA9554) without a stop condition. The txBuffer will clear if write was a success. Then send a repeated Start condition, followed by address and obtain readings into rxBuffer.

/1/iaddr 56
{"address":"0x38"}
/1/ibuff 3
{"txBuffer":["data":"0x3"}
/1/iread? 1
{"rxBuffer":[{"data":"0xFF"}]}

Note the PCA9554 has been power cycled in this example, so the reading is the default from register 3.

Load the PCA9554 configuration register 3 (DDR) with zero to set the port as output. Then alternate register 1 (the output port) with 85 and 170 to toggle its output pins.

/1/iaddr 56
{"address":"0x38"}
/1/ibuff 3,0
{"txBuffer":["data":"0x3","data":"0x0"]}
/1/iwrite
{"returnCode":"success"}
/1/ibuff 1,170
{"txBuffer":[{"data":"0x1"},{"data":"0xAA"}]}
/1/iwrite
{"returnCode":"success"}
/1/ibuff 1,85
{"txBuffer":[{"data":"0x1"},{"data":"0x55"}]}
/1/iwrite
{"returnCode":"success"}

/1/scan?
{"scan":[{"addr":"0x29"},{"addr":"0x40"}]}

Command 0xE3 measures temperature, the clock is streached until data is ready.

/1/iaddr 64
{"address":"0x40"}
/1/ibuff 227
{"txBuffer":["data":"0xE3"]}
/1/iread? 3
{"rxBuffer":[{"data":"0x6A"},{"data":"0xC"},{"data":"0xC6"}]}

The first two bytes are the temperature data. The last two bits of LSB are status (ignore or mask them off). Some Python gives the result in deg C.

Stmp = 0x6A0C
Temp = -46.85   175.72 * Stmp / (2**16)
Temp
25.9

Command 0xE5 measures humidity, again the clock is streached until data is ready.

/1/iaddr 64
{"address":"0x40"}
/1/ibuff 229
{"txBuffer":["data":"0xE5"]}
/1/read? 3
{"rxBuffer":[{"data":"0x65"},{"data":"0x96"},{"data":"0xBC"}]}

The first two bytes are the temperature data. The last two bits of LSB are status (ignore or mask them off). Some Python gives the result in deg C.

Stmp = 0x6596 & 0xFFFC
RH = -6   125 * Stmp / (2**16)
RH
43.6