As microcontrollers drop in price and offer more capabilities, designers have found it more costeffective to utilize multiple small controllers in both single-board and multiboard systems. Such auxiliary processors can relieve the main processor of timeconsuming tasks such as scanning keyboards, display controllers, and motor control. These controllers can also be configured as a wide range of application-specific peripherals.
Recently, I was given the task of developing an interface (software/hardware) that could easily be adapted to many applications and be based on an industry standard commonly found in embedded processors. After reviewing some of the typical applications, I came up with a list of requirements to help zero in on a hardware interface.
- Common on both 32- and 8-bit processors
- Supported by many off-the-shelf peripherals
- Peripheral interface code less than 0.5 kbyte
- Low pin count
- Data bandwidth up to 10 kbytes/s
- Low RAM usage
- Support multiple peripherals on a single bus
- Easy API to use
- No external interface drive hardware required
Because of the low pin-count requirement, a serial interface was mandatory. Some of the more common serial interfaces found in today’s processors include SPI, I2C, USB, and RS-232. After weighing the various pros and cons, I settled on the I2C because of its simplicity, flexibility, and availability on most low-cost controllers. Low pin count and flow control also give I2C a big advantage over SPI if higher speed isn’t required.
How I2C works
I2C is a two-wire bidirectional interface consisting of a clock and data signals (SCL and SDA). A dozen devices or more may be included into a single bus without additional signals. The spec calls out three speeds of operation: 100 kbits/s, 400 kbits/s, and 3.4 Mbits/s. Most common controllers only support the 100- and 400-kbit/s modes. The spec allows for both a single master with multiple slaves or a multi-master configuration.
One very important attribute of I2C is that it supports flow control. If a slave can’t keep up between bytes, it may halt the bus until it can catch up. This is very useful for slaves that contain minimal I2C hardware and must support part of the protocol in firmware. The I2C bus specification supports both a 7- and 10-bit address protocol. I’ve found that the 7-bit addressing is more than sufficient for most applications.
Before starting to write code, we need a good understanding of how the I2C bus works. The I2C bus will always have at least one master and at least one or more slaves. The master always initiates a transfer from the master to the slave. The I2C interface has only two signals, no matter how many peripherals are attached to the bus.
Both signals are open-collector with pull-up resistors of about 2.7k to VCC. The SDA signal is bidirectional and can be driven by either the master or slave. The SCL signal is driven by the master, but the slave may hold it low at the end of a data byte to hold off the bus until the slave can process the data. The master releases the SCL line after the last bit of the byte, then checks to see if the SCL signal goes high. If it doesn’t, the master knows that the slave is requesting the master to hold off until the data is processed.
When data is being sent on the bus, data transitions occur only when SCL is low. When the SCL signal is high, the data in either direction should be stable (Fig. 1).
When the bus is idle, neither the master nor the slaves pull down the SDA and SCL. To initiate a transfer, the master drives the SDA line from high to low while SCL is high. Typically, the SDA line doesn’t change state when SCL is high, except for a start or stop condition. A stop condition occurs when SCL is high and the SDA line changes from low to high (Fig. 2).
The I2C bus transfers data in 8-bit increments. Each time a byte is transferred, it must be acknowledged by the device receiving the data. All data is transferred most significant bit (MSB) first.
At the beginning of each transfer, a START initiates the transfer, then a 7-bit slave address, followed by an R/W flag. The I2C standard also supports a 10-bit address, but this application requires only a 7-bit address. If a slave recognizes the address, it will pull down the SDA line during the ACK state, then release it.
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