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I2C/TWI Programming Video. Creating the Code!

This is a 36 minute video on me showing the programming process in the Atmel Studio of the I2C communication with an accelerometer and fully testing using a logic analyzer. If you wish to see the video, please purchase using the credit card form after registering or logging in.

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The initialization portion of the program will include two main registers that serve two purposes, the prescaler and the actual bit rate. The bit rate is a rate of speed that each bit will be sent over the SDA line. The SCL is the heart beat which pulses at the bit rate.

To make things really easy, it is recommended that the prescaler stay zero. This is not because dealing with a prescaler is hard, but because the TWSR (Two Wire Status Register) holds the prescaler and also serves as the status during transmission. If a prescaler was mixed into this number, then the status would be more difficult to determine.

If you do wish to modify the prescaler because your microcontroller has a very high CPU clock rate, then you can shift the TWSR status register so the prescale drops off when you need only the status, or you can determine the values of the status with the included prescaler. See the list of status values below. If you use a prescaler, just add that value to the status value.

TWSR (TWI Status Register (Prescaler Bits)) -
TWSR - TWS7 | TWS6 | TWS5 | TWS4 | TWS3 | –---- | TWPS1 | TWPS0
TWSR - --7--|---6--|---5--|---4--|---3--|---2---|---1---|---0-----
TWSR - (----------Status Bits-----------)-------(-Prescaler Bits-)
TWSR will be 0x08 after a start condition
The SCL clock should be 100kHz

Calculating the TWBR is relatively straightforward. First, you need to know the desired speed that you intend the SCL to be. For standard mode, this speed is a maximum of 100 KHz and for fast mode the maximum speed is 400 KHz. Notice that these frequencies are maximums. That means that you can still communicate using I2C or TWI at lower speeds. In the example I show in the video on this page, I use a frequency of 50 KHz. To determine the TWBR (Two Wire Bit Rate Register), you can use this formula:

Bit Rate Generator Unit
TWBR (TWI Bit Rate Register) - will be 2 for a 50kHz SCL
TWBR - --7---|---6---|---5---|---4---|---3---|---2---|---1---|---0----
SCL Frequency = (CPU Internal Clock Frequency) / 16 + 2 (TWBR) * 4 ^ TWPS

You see that TWPS? You're saying to yourself, what is that! That is the prescale bits found in the TWSR register. Notice the bits in the TWSR where I describe the TWSR above. The bits are called TWPS0 and TWPS1. Let's do some algebra to make it easier to find TWBR. Welcome to algebra class!

SCL_Clock = (CPU_Clock_Rate) / 16 + 2(TWBR) x 4 ^ TWPS

SCL_Clock = (CPU_Clock_Rate) / 16 + 2(TWBR) (We can remove the 4 ^ TWPS since the prescaler is 0)

Cross Multiply:
SCL_Clock x (16 + 2(TWBR)) = CPU_Clock_Rate

Divide SCL_Clock by both sides to get it to the other side of the equasion:
16 + 2(TWBR) = CPU_Clock_Rate / SCL_Clock

Subtract 16 from both sides:
2(TWBR) = (CPU_Clock_Rate / SCL_Clock) - 16

Divide both sides by 2:
TWBR = ((CPU_Clock_Rate / SCL_Clock) - 16) / 2

There! Now you have a simple formula you can apply in the program.

Or you can use the following table to make it easier.


This video will explain the programming I2C or TWI in detail. An accelerometer with I2C functionality serves as the slave device and an ATMega 324p is the master in this example scenario.

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By purchasing this exclusive content, you can expect to see a video that demonstrates the programming associated with I2C or TWI between an Atmel microcontroller and the ADXL 345 accelerometer. The first video reads the ID on the ADXL 354 by using I2C/TWI control such as start, repeat start, sending slave address and write and read bits, sending data and receiving data, sending ACK or NACK and stop. All of these controls are explained in detail and you can follow along with me in the programming and testing of these features. If you haven't seen the free videos that server as prerequisites to this video, please follow this link.