STM32 I2C Lecture 37 -Exercise : Testing repeated start

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Exercise : Testing repeated start

 

 

In this article, let’s understand the concept of repeated start. Why repeated start is used and how to change our APIs to make use of repeated start all these things we are going to learn now.

The waveform obtained from the master receive data exercise is, as shown in Figure 1.

Figure 1. Trace of master receive data exercise.

 

If you analyze the waveform in Figure 1, the first I2C transaction is master write, which starts from the first green circle and ends at the first red circle, as shown in Figure 2.

The stop condition is generated at the end to stop (or end) the first I2C transaction. The stop condition in Figure 2 is not required. Instead of raising the stop condition at the end of the first I2C transaction, you can go ahead and generate one more start condition in order to begin the next I2C transaction.

Figure 2. Trace for first I2C transaction (master write).

 

Look at Figure 3. The green circle pointed with the cursor is the beginning of the second I2C transaction, and the red circle is the end of the first I2C transaction.

Figure 3. Trace that marks the beginning of the second I2C transaction and end of the first I2C transaction.

 

Observe Figure 3, the bus is released for 5 microseconds, and both the SCL and SDA lines are also released, allowing another master to claim the bus. Therefore, the stop condition is generated at the end of the first I2C transaction because you may not be able to complete all other transactions on time since the SCL and SDA lines are released.

Generating a stop means giving a chance for another master to claim the bus.

If you think your data transaction is critical and you have to complete all the I2C transactions on time, then go ahead and generate one more start condition instead of generating a stop at the end of the first transaction. When you generate one more start condition, that will be called a repeated start.

Repeated start means generating start condition without generating stop condition for the previous I2C transaction.

 

Now let’s make the required changes to the code to include repeated start:

  • To include the repeated start, you have to avoid generating unnecessary stop conditions. To achieve the repeated start, avoid the generation of stop condition at the end of the master send data API (Figure 4). This can be done using an if statement, as shown in Figure 5. Here Sr is an argument taken from the user.
Figure 4. Generation of stop condition at the end of MasterSendData API.

 

Figure 5. Code to avoid the generation of stop condition at the end of the MasterSendData API.

 

  • Create one more argument for master send data API, as shown in Figure 6.
Figure 6. Creation of one more argument for MasterSendData API.

 

  • Add the argument Sr in the prototype of master send data API (Figure 7).
Figure 7. Changes made to the prototype of MasterSendData API.

 

  • In the if statement, instead of comparing Sr with zero (Figure 5), compare Sr with I2C_NO_SR macro, as shown in Figure 8.
Figure 8. Comparing Sr with macro I2C_NO_SR.

 

  • Define I2C_NO_SR and I2C_SR macros at the beginning of the I2C header file.
Figure 9. Code to define the macros.

 

Now in the driver.c, if it is I2C_NO_SR, then stop will be generated; otherwise, the stop will not be generated.

  • Similarly, change the master receive data API, as shown in Figure 10, Figure 11, Figure 12, and Figure 13.
Exercise : Testing repeated start
Figure 10. Generation of stop condition in the MasterReceiveData API.

 

Exercise : Testing repeated start
Figure 11. Generation of stop condition in the MasterReceiveData API.

 

Exercise : Testing repeated start
Figure 12. Creation of one more argument for MasterReceiveData API.

 

Exercise : Testing repeated start
Figure 13. Changes made to the prototype of MasterReceiveData API.

 

The name I2C_NO_SR and I2C_SR are a little confusing; instead, use I2C_DISABLE_SR and I2C_ENABLE_SR (Figure 15), respectively.

Exercise : Testing repeated start
Figure 14. Replacing macros I2C_NO_SR and I2C_SR with I2C_DISABLE_SR and I2C_ENABLE_SR.

 

Similarly, replace every I2C_NO_SR and I2C_SR with I2C_DISABLE_SR and I2C_ENABLE_SR.

  • Go to the application and give the last argument for MasterSendData and MasterReceiveData APIs, as shown in Figure 15.
Exercise : Testing repeated start
Figure 15. Passing the last argument for MasterSendData and MasterReceiveData APIs.

 

Look at Figure 15. The I2C_DISABLE_SR is used only in the second MasterReceiveData API because that is the place where the I2C transaction must end.

  • Test the application by capturing the data using a logic analyzer.

In Figure 16, you can see a couple of repeated starts, not a repeated stop. There is only one stop at the end of the message transfer.

Exercise : Testing repeated start
Figure 16. I2C transaction with repeated starts.

 

The master starts transactions from the first green circle, and it won’t generate any stop condition in between until it finishes all transactions.

In Figure 17, you can observe that when the clock is high, there is a transition of the SDA line, which is called a start condition.

Exercise : Testing repeated start
Figure 17. Trace for I2C transaction with repeated starts.

 

In this case, the master will hold the bus for 2ms (Figure 18). Until the master finishes all the I2C transactions, no other master can trouble this master. Therefore, the repeated start is the best choice while using the I2C bus in multi-master communication.

Exercise : Testing repeated start
Figure 18. Master holds the bus for 2ms.

 

In the following article, let’s see I2C IRQ and interrupt discussion.

 

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