- Introduction to the I2C Bus
- The I2C Bus: Hardware Implementation Details
- The I2C Bus: Firmware Implementation Details
- The EFM8 Series from Silicon Laboratories
Help from Dedicated Hardware
If you've read the articles listed under “Supporting Information,” you are well aware that I2C is a relatively complex protocol that requires detailed communication procedures and a specific circuit configuration. Nevertheless, I2C is widespread, and rightly so, because in return for this complexity it enables flexible, robust, low-pin-count serial communication among multiple independent integrated circuits. It is true that designing an I2C interface “from scratch”—i.e., as low-level hardware or pure firmware—would be a bit burdensome; fortunately, though, countless microcontrollers from numerous manufacturers incorporate peripherals that greatly facilitate I2C implementation. In this two-article series we will discuss in detail how to incorporate I2C communication into firmware written specifically for an EFM8 microcontroller, though the concepts and code presented here are applicable to other Silicon Labs devices. Furthermore, it would not be difficult to adapt this information to any other microcontroller—after all, I2C is a standardized protocol that does not vary from one manufacturer to another.
Configuring the Hardware
We will implement I2C using the SMBus peripheral, which supports both protocols (SMBus is actually an extension of I2C, as discussed in a previous article). Microcontrollers often function as I2C masters, so master functionality will be our focus, but slave functionality can be incorporated with similar firmware. This screen capture from Simplicity Studio summarizes the SMBus peripheral configuration:
- The source for the serial clock is chosen as Timer1, and then Timer1 is configured for an overflow frequency of 100 kHz.
- The SMBus is enabled.
- In this particular system we want to keep everything as straightforward as possible, so “free timeout detection,” “SCL timeout detection,” and “setup and hold time extension” are all disabled (you can read about these features in the EFM8 reference manuals, e.g., this one).
- “Slave inhibit” is enabled because in this system the EFM8 functions only as a master. You need to disable this feature if you want the EFM8 to generate an interrupt when another device on the bus attempts to address the EFM8 as a slave.
- Slave-address properties are not altered because this example does not incorporate slave functionality. If you want the EFM8 to function as a slave instead of (or in addition to) a master, you must set the address that a master device will use to establish communications with the EFM8. (Refer to the reference manual for an explanation of the “general call” and “address mask” features.)
- “Hardware acknowledge” is shown as disabled, though in many cases it is simpler to enable it. When enabled, the SMBus peripheral will generate an ACK or NACK without real-time firmware intervention when the EFM8 is addressed as a slave or when it receives a byte during a master/read operation. More information on the hardware acknowledge functionality is provided toward the end of this article.
- The “start detection window hold time” feature allows you to modify the timing specifications applied to the detection of a start bit. This can be used to compensate for major impedance mismatches between the clock (SCL) and data (SDA) lines. In most cases no such compensation is necessary.
Now that the peripheral is configured, we need to enable SMBus0 in the crossbar so that SCL and SDA are connected to I/O pins. Both pins should be configured for open-drain input/output, as follows:
This particular pinout shows which pins would be assigned to SCL and SDA if you also had the SPI peripheral enabled.
As discussed in a previous article, I2C transactions must follow a specific sequence of events. It is not surprising, then, that Silicon Labs expects you to implement I2C communications in the form of a state machine: each event leads the firmware from one state to another, and the code executed at each stage of the transaction is determined by the value of the state variable. This is intuitive enough, but it is important to understand something that might not be immediately apparent—namely, that the SMBus interrupt flag is a dominant factor in the operation of the state machine. The interrupt flag functions like a traffic light, governing the interaction between firmware traveling east–west and hardware traveling north–south:
- The SMBus hardware sets the interrupt flag after each event in the transaction.
- When the flag is set, the hardware holds SCL at logic low, which is essentially the “inactive” clock state because SDA is allowed to transition when SCL is low. Thus, the bus remains “stalled” as long as the flag is set.
- I2C-related firmware executes during this “stalled” condition; this clever arrangement ensures that the processor has completed its tasks before the bus becomes active again with the next rising edge of SCL. Even slow processors or lengthy firmware routines are not a problem with this technique because (as discussed here in regard to “clock synchronization”) the I2C protocol is designed to support extended clock-low periods.
- The SMBus interrupt flag must be cleared by firmware; in fact, the action of setting the flag bit to zero is what initiates the next hardware event in the I2C transaction. This is why clearing the flag bit is the final command executed in the code block assigned to each portion of the state machine.
Silicon Labs provides some very helpful state-machine and event-sequence diagrams in its EFM8 reference manuals. Let’s start with a master/write operation:
The diagram identifies each event and its associated interrupt in a typical I2C transaction; the letters-in-blue-circles correspond to code blocks described in the corresponding state-machine flowchart (see below). All the data bytes are transmitted from master to slave, so each ACK is generated by the slave. This is why the interrupts are the same regardless of whether hardware ACK is enabled or disabled—the master does not generate any ACKs in a master/write operation.
(Note: In some, perhaps all, of the EFM8 reference manuals, this diagram has a typo indicating that the R/W bit should be set to 1, instead of 0, for a write operation. The above diagram has been corrected.) Take a good look at this flowchart, which is highly informative. Again, notice how each hardware event in the transaction is followed by an interrupt, and thus each interrupt corresponds to the transition from one state to another. The key to efficient, clean, reliable I2C code is a state machine driven by interrupt events: the interrupt service routine is called when the interrupt flag is set, and then a switch statement is used to execute a block of code corresponding to the current state of the transaction. The final statement in each code block resets the interrupt flag bit to zero, thereby initiating the next hardware event.
Here is the event sequence for a master/read operation:
Note the difference in when the interrupts are generated: If hardware ACK is enabled, the interrupt is generated after the master (automatically) generates ACK or NACK based on the preexisting state of the ACK/NACK bit in the SMBus control register; the action required by firmware in this case is storing the received byte in preparation for the next byte. If hardware ACK is disabled, the interrupt is generated immediately after the last bit of the data byte is received. The “ACK request” bit in the SMBus control register is set to indicate that firmware intervention is required, and in response the firmware should write the appropriate value to the ACK/NACK bit. Then execution continues with storing the received byte and clearing the interrupt flag.
Here is the master/read flowchart:
Note the differences here: 1) R/W is set high instead of low; 2) the master reads from (rather than writes to) the SMBus data register; and 3) the master controls the ACK/NACK response in order to confirm that a byte was successfully received (via ACK) or to indicate that no more data is needed (via NACK). The EFM8 reference manuals also provide event sequences and a flowchart for slave functionality.
We have covered the most important characteristics of the EFM8’s SMBus/I2C peripheral, and we have also established the foundational concepts for designing an efficient, robust I2C firmware interface. Part 2 will explore code development in more detail; sample code for I2C master functionality will be provided, and we will look at scope traces for the SCL and SDA signals.
Next Article in Series: Implementing I2C with an EFM8 Microcontroller, Part 2