Question

If my design calls for relatively fast data transmission between one microcontroller and 10 unique sensors and the microcontroller only has 3 pins available for the communication bus, which serial protocol would you chose?

PWM

SPI

I2S

UART

I2C

Answer 1

Hi,

Let us examine suitability of each protocol:

1. PWM (Pulse-Width Modulation): While analog information can be encoded into a PWM signal, but for 10 sensors, we would need to produce 10 PWM signals on sensor side. Then these will have to be received and decoded on microcontroller side. It will be rather complex to multiplex 10 PWM inputs on 3 microcontroller input lines. So this one is ruled out.

2. SPI (Serial Peripheral Interconnect): At the very least it needs two lines - MISO (Master-In Slave-Out) and SCK (Serial Clock). Now as there are 10 sensors (which would be configured as slaves), we need an additional Slave Select (SS) per sensor. Therefore 2+10= 12 lines are needed. Clearly this one does not work here.

3. I2S (Inter-IC Sound): This protocol is specifically designed for transmission and reception of two channels of digital audio data. So it not meant for application at hand.

4. UART (Universal Asynchronous Receiver Transmitter): This communication protocol is meant for async (i.e. clock signal need not be transmitted) communication between two nodes. Only TX and RX lines (and a ground line) are used. In its most basic form it provides no addressability or slave selection. It is not inherently meant to be used in a bus or star topology, where multiple nodes connect to central microcontroller. Theoretically it can be used provided:

a) the 9-bit extension of this protocol is used by both the sensors and the microcontroller. This extension does provide for addressability and hence allow multiprocessor communication. However I have not come across any sensors that use this extension natively.

b) Build a layer of software both on microcontroller and the sensor wherein each sensor is assigned a unique address. All sensors receive all the queries from the microcontroller but respond only when the receive their own address.

In a nutshell, this is tedious too.

5. I²C (Inter-Integrated Circuit): This protocol uses two signals (SDA - Serial Data and SCL - Serial Clock) and a ground wire. It is a multi-master, multi-slave protocol. So you can easily connect 10 (or more) sensors provided each sensor has a unique I2C address. Also there is a huge variety I2C sensors available commercially. So this is the most suitable protocol for this application.

Hope this helps.