Embedded Systems Bit Count

Question

I do apologize if this is a duplicate even though I did search around here for a similar question, I only found one.

So my programming team in my Engineering class currently use a 32-bit 72MHz ARM Cortex-M3 microprocessor. We're all seniors in high school, and we're struggling to use the libraries and whatnot, mostly due to poor docs from the manufacturer of the Bioloid Premium we're using. However we are about to purchase an 8-bit 16MHz AVR microcontroller because it has a wider range of support online and an easier-to-use library + more documentation. My question here is, would the decreased bit-count as well as the lower processor speed really matter to us? We're not going to be doing a lot of process-intensive programming, but more like a basic robotics class. So, main differences between an 8-bit 16MHz AVR microprocessor and a 32-bit 72MHz ARM Cortex-M3 microprocessor? Also, (if it holds any relevancy):

1. We're using a Bioloid Premium by Robotis w/ CM530 (ARM), about to switch to CM510 (AVR).
2. We'll be using Embedded C instead of Robotis' RoboPlus IDE as our instruction set.

I have googled around, found out what a bit-count was, and more about it's impact on processor speed, but not a lot of documents about it give a clear and concise answer and that's why I came here, because it's for clear and concise answers. (So please don't tell me to Google it when I've spent the past twenty minutes doing so.)

Answer 1

My question here is, would the decreased bit-count as well as the lower processor speed really matter to us? [...] So, main differences between an 8-bit 16MHz AVR microprocessor and a 32-bit 72MHz ARM Cortex-M3 microprocessor?

What a cool project! This is a great opportunity to learn a bit about how processors work and what bit-width and clock speed mean.

Clock speed is conceptually the easiest to understand. Microcontrollers like the AVR and ARM use a clock crystal that sets the speed the circuitry operates at. With a faster clock, the processor can execute more instructions in the same amount of time. The 72MHz clock is more than 4x the 16MHz one, so the ARM processor is going to be able to run 4x faster than the AVR. But what does "run faster" really mean? Processors execute instructions. At the basic level, these are instructions like "add two numbers" and "make the voltage on this pin high". The ARM processor is going to be a lot faster here, but consider what hardware it's going to be talking to: servos. Servo motors listen to a fairly low-speed PWM signal, so at that speed the difference between 72MHz and 16MHz isn't going to become that relevant.

But what about bit-width? This one is a bit more tricky. It doesn't really affect the speed at which your processor runs, but it affects the complexity of the instructions it executes. Let's say that you want to add two really big numbers together. Numbers like 100,000 and 200,000. When we add those together on paper, it's just one step. But an 8-bit processor like the AVR can only operate on numbers as large as 65,536. So in order to operate on numbers that large it'll need to break up the addition into several smaller steps. The 32-bit ARM, on the other hand, can work on numbers that large. So it does the addition in one step. I hope that makes sense.

Anyway, I've done a lot of work with servos on even slower processors than your 16MHz AVR. It'll most likely be just fine for what you want to do, and like you found it has a much more active hobbyist community. And if you're looking for quick examples of code, the Cornell 4760 page has some great projects that you could learn from.

Answer 2

We're using a Bioloid Premium by Robotis w/ CM530 (ARM), about to switch to CM510 (AVR). We'll be using Embedded C instead of Robotis' RoboPlus IDE as our instruction set.

I looked around at the products you refer to, and your question seems to be missing the issues you should really be concerned with.

The Bioloid Premium kit looks pretty sweet, with all the parts put together and configured for you already. Much of robotics courses are usually concerned with designing the hardware. You are not going to be doing any of that. So your tasks really come down to programming the hardware you are given.

That said, there is a world of difference between the RoboPlus IDE, which seems similar to the Lego Mindstorms drag and drop interface, and writing code in C using AVR Studio!

I have used AVR Studio before, but there was a major change in versions recently. You might need to modify the example programs to work in the latest version, and you will probably need some help with that.

It looks like they supply you with enough example code to use the periperpherals, but I don't see right away how to write a main() function to do something like follow a plan. Perhaps, there are some examples online.

But to answer your question, you are probably not going to run into any limitations in terms of processor capacity. They switched to a cheaper and more powerful processor to write the newer version of their control software, but the old hardware will be great, too. Working in C, you will become familiar with how to actually use an MCU, and that knowledge will transfer to other chips. The AVR family is a great one to start with. It has lots of features and is pretty sensible in how it works, with lots of documentation and third-party support. Definitely download the datasheet from Atmel for the chip you are using, although it is a dense and difficult read. You will only need to read parts of it. Also, check out the AVR Freaks forums.

This sounds like a fantastic high school course. Have fun with it!