cfobel/i2c_demo

Overview

This project demonstrates some advanced I2C concepts, discussed [here] 1.

Timing

Using the default transmission rate, the I2C clock runs at 100kHz, which translates to ~96us/byte of actual payload. This timing takes the overhead padding the payload into account.

By using a clock rate of 400kHz instead, the transmission speed increases to ~28us/byte. As shown in the file master_writer/master_writer.ino, the clock-rate can be set by modifying the contents of the TWBR-register (search for TWBR [here] 1).

To demonstrate the usage of 400kHz clock-rate, load the master_writer project on one device and load the slave_receiver onto another device and connect a serial terminal to the slave_receiver device. You should see output similar to:

x is 1
x is 2
x is 3
x is 4
x is 5
...
...
...
x is 254
x is 254
x is 1
x is 2
x is 3
...

NB See i2c-SCL-TWBR_default.pdf and i2c-SCL-TWBR_12.pdf to show that the clock-rate does indeed change when changing TWBR.

Broadcast

By default, a slave receiver only responds to transmissions sent to the slave's specific address. However, as described [here] 1, by setting the lowest bit of the slave's TWAR register to 1, the slave will also respond to any transmissions to address 0x00, which is reserved as a special, broadcast address.

To demonstrate the sending and receiving of a broadcast message, load the control_board project on one device and load the extension_module onto one (or more) device(s) and connect a serial terminal to the extension_module devices. You should see output similar to:

broadcast: 1
broadcast: 2
broadcast: 3
broadcast: 4
broadcast: 5
...
...
...
broadcast: 254
broadcast: 254
broadcast: 1
broadcast: 2
broadcast: 3
...

NB Just to reiterate, the broadcast messages are not sent to a specific slave-device address. Instead, the slave-devices are configured to listen to broadcast messages and the master-device sends out a broadcast message by transmitting to address 0x00.