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SERIAL INTERFACE FOR EMBEDDED DESIGN

by Art Eck

Start ý Trying I²C ý Sources and PDF

TRYING I²C

The I²C serial protocol requires only two I/O pins, the least of the three major serial protocols (see Figure 3). This means that not only does the mixed-signal device using I²C serial protocol cost less, but the microcontroller it interfaces to can have a lower pin count. This, coupled with reduced board-space requirements, can offer a lower overall system cost. The system-cost reduction must be weighed against system-cost savings, because of the more compact code that SPI offers.

Figure 3ýGiven that I2C protocol devices are level triggered, noise immunity is likely to be superior when compared to edge-triggered protocols like SPI and Microwire.

Noise can be an issue with any mixed-signal design. Although the best solution is to keep serial buses far from noise, this is not always possible. Given that I²C protocol devices are level triggered, noise immunity is likely to be superior when compared to edge-triggered protocols like SPI and Microwire. Whenever noise is an issue, devices with Schmitt triggers should be considered.

Because of the benefits of reduced pin count and the popularity of the I²C serial protocol, it is likely to continue in the industry for a long time. Like SPI, many additional products will also be developed for this protocol. Although the I²C serial protocol may not enjoy as broad an offering of products in mixed-signal devices, it will continue to have a significant presence.

The I²C serial protocol tends to lack speed. Until recently, I²C serial protocol was not defined for operation above 400 kHz. Applications that require significantly fast data transfer may find the I²C serial protocol device not capable of meeting the requirements.

Besides these three major protocols, there are many serial protocols employed by suppliers that are proprietary to their product. Some of these approaches seem to have benefits for a given application but may not have value in the broader use. It is unlikely any of these approaches will gain the acceptance that SPI, Microwire, and I²C serial protocols have, making it difficult to justify investing the time to learn them.

Proprietary protocols lock the application into a particular supplier. The lack of competition is likely to mean higher cost over the life of the project. Even with reasonable prices today, later price reductions are not as likely for devices using standard protocols. Given that support of a proprietary protocol is less broad based, the time used to learn it is less likely to be a useful investment. Therefore, it is recommended that proprietary protocols be avoided whenever possible.

Each of the three major serial protocols offer different value points. Microwire offers a legacy as the oldest of the three. SPI offers greatest execution speed, most compact code, and a broad offering of products. I²C requires only two system I/O pinsýhalf that of SPI and Microwire.

Itýs up to the designer to determine the set of features that most benefit the application. Whatever the selection, the overall design can enjoy a cost reduction through the use of an appropriate serial protocol.

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