SPI Bus: Theory and Implementation

SPI stands for “Serial to Peripheral Interface” and is a {hardware} and firmware communication protocol developed by Motorola and later adopted by all. The SPI bus is simply used on the board. I am certain a few of you’ll ask: “Why is the SPI bus solely used on the PCB? What prevents us from utilizing it exterior of the PCB space?” The SPI bus is particularly designed to trade knowledge between completely different IC chips at very excessive speeds; say at 180MHz or much more. Due to this high-speed side, the bus strains can’t be too lengthy as a result of their reactance will improve an excessive amount of and the bus will develop into unusable. Nonetheless, if you’d like you need to use the SPI bus off the PCB at low speeds, however that is not very sensible – the SPI bus requires 3 or 4 communication strains, which is a bit too many in comparison with the 1 or 2 strains that sometimes wanted to speak effectively with area units positioned off the PCB.

Anyway, on the PCB, the SPI bus is superb as a result of we will join virtually as many ICs (or units) to the bus as we would like. My apologies for not offering an image of the SPI bus, however relaxation assured you do not want one: the SPI bus is so easy, you will perceive all of it in phrases.

The following query is: “Why is that this SPI bus significantly helpful?” Along with exchanging knowledge between completely different IC chips, the SPI bus is a technique of multiplying the pins of the microcontroller. In different phrases, you probably have a tiny 8-pin microcontroller, you could possibly management a couple of hundred digital inputs and outputs with this little monster. That is spectacular and I am certain that many will doubt my phrases. Let’s clarify.

The SPI bus accommodates three strains that may reside on any frequent I/O controller pins. These bus strains are: Clock, Information-In and Information-Out. Additionally, every IC related to the SPI bus wants its personal allow line. Issues work like this: Suppose we have now 4 units, A, B, C and D; All of them are wired to the SPI bus strains, and the bus itself is wired to seven controller pins – that is 3 bus strains plus the 4 allow strains. Once we need to ship a message to gadget C, we first activate its Allow line, then we ship the message serially, one bit at a time. On the similar time, units A, B and D do precisely nothing as a result of they aren’t activated.

The great thing about the SPI bus is that it’s synchronous, which implies that when the controller is sending the message to an IC, it will also be receiving knowledge from that IC on the similar time. This explicit side of the SPI protocol lends itself significantly nicely to microcontroller-to-microcontroller communication.

Now we have now seen {that a} small 8-pin microcontroller can management 4 7-pin units (ICs). When you think about {that a} sort A, B, C, or D gadget may have eight and even sixteen I/O ports, that is nonetheless a far cry from the lots of of inputs and outputs I promised you. The following good factor in regards to the SPI bus is that one gadget IC will be serialized with many others of the identical sort! For instance we may have B1, B2, B3, B4, B5 and so on. All B# sort ICs are serialized collectively and solely require 4 microcontroller pins to work; the Allow line is frequent to all. Subsequent, we will use every sort A, B, C, and D gadget as a gaggle of ten comparable ICs.

The activation pace of every I/O port on the SPI bus is slower when the microcontroller’s pins are multiplied. Nonetheless, all the time think about that I/O area units don’t essentially require speeds of say 1000 ON/OFF activations per second. just because most of them cannot deal with that pace. Nonetheless, there are few very sensible firmware strategies like “barrel shift” options that permit us to keep up excessive pace messages on the SPI bus even when we have now lots of of I/Os. On the similar time, the “barrel shift” options permit for higher time administration within the microcontroller, giving it extra time to carry out different duties – is smart to me! In closing, I feel it is now clear that we will really construct lots of of environment friendly I/O strains on a small 8-pin controller.

Other than this common presentation of the SPI bus, try to be conscious that the majority ICs implement the SPI protocol in a particular method. For detailed and sensible functions, I recommend you go to my house web page below Corollary Theorems. There you will discover a good textbook on working with {hardware}, firmware – together with barrel-shift sort options – and software program design basically, and some neat and helpful implementations of the SPI bus particularly.

Many microcontrollers have built-in SPI bus {hardware} modules, however I’ve by no means been very taken with utilizing them. What I do, I all the time design – on the board and for a microcontroller – one, two or extra customized SPI buses as a result of my customized implementations are way more versatile. Additionally, virtually implementing a customized SPI bus, each in {hardware} and firmware, is very easy – belief me on this one!