Tutorial 19: Going Beyond the UART

We've already looked at asynchronous serial communication, and seen how useful it can be for a scientific instrument as a data logging system. Serial communication has a lot more power, however, especially when we begin looking at the "control" aspect of the microcontroller.

There are a lot of sensors and devices designed to work with microcontrollers that use synchronous communication protocols. The primary difference for these modes of communication are the inclusion of a single clock between the sender and receiver, rather than having to rely on precise and accurate clocks. Having to share a clock limits the separation to a fairly close proximity, but it does simplify the communication process a great deal.

The number of protocols designed for serial communication is quite large, but a few have stood out as standard for many devices. The MSP430 has many devices that, for example, have built-in hardware for communication using the Serial Peripheral Interface (SPI) and the Inter-integrated Circuit Bus (I²C or I2C -- pronounced either as eye-two-see or eye-squared-see, your preference).

Most MSP430 devices have one of two forms of serial communication hardware. The less expensive devices have the Universal Serial Interface (USI), which can do SPI and I2C. Larger devices come with the Universal Serial Communication Interface (USCI), in at least one of two flavors: USCI_A handles asynchronous communication (UART), and USCI_B handles synchronous communication (SPI/I2C). If for any reason you must use a device without either of these modules, you can resort to bit-banging, but aside from the previous tutorial on UART we won't cover that any time soon; most modules have at least USI, and it's far more economical to use the built in hardware.

To work with this tutorial, you'll need at least one of the following:

• Two devices that communicate via SPI and I2C, such as a serial EEPROM produced by Microchip. 
• USB-SPI/I2C interfaces, such as the FTDI devices. (Their chips include one that is great for UART at speeds other than 9600 bps too!)
• A second MSP430 device.

Owners of a LaunchPad already have two devices that include USI, but keep in mind you'll need a few other components to power the chip off of the LaunchPad. I'll do tutorials that will demonstrate all three methods, but for the beginning I will be working with Microchip serial EEPROM chips. (I will use the 25AA080C for SPI and the 24AA08 for I2C, but any size will do.)

Of the two synchronous protocols used by the MSP430, I2C will likely be the one used most frequently (at least by me), for the reason that the protocol is designed to work with multiple devices on a single communication line. Many sensors and external peripherals are already available that use I2C. However, SPI is a little simpler to understand, and we'll start with that. Once I've gone over the USI peripheral, I'll begin working on a new experiment that will make use of a pressure sensor, and design an MSP430-driven 4-digit LED display. Both will communicate over I2C as a digital pressure gauge. The project will require designing and fabricating a couple of printed circuit boards, so we'll go over a little of that process as well.