STM32F4 Cortex-M4 is bonkers, and I want one

I posted at LeafLabs about ST Micro’s recent release of their ARM Cortex-M4 implementation, the STM32F4, but forgot to post here. I am a geek. I love computer hardware, and microcontrollers are great because I can afford to get them and try to make my own computers.

What could be more fun? Oh, yea, you have  a long list. Okay, let me press on.

There are a few things which microcontroller fans might like about ST Micro’s new product:

1MByte Flash, and 192KBytes of RAM. I remember reading the Unix edition six source, and IIRC, it would run in 128KBytes RAM, and that was a multi-user , multi-process operating system, with a proper hierarchical file system (none of the A: nonsense). That amount of memory is quite luxurious for Arduino users. It is 32 Arduino UNO’s worth of Flash, and 96 Arduino UNO’s worth of RAM.

Floating Point Unit. Modern PC’s have hardware floating point. For some problems, it is much easier to write a solution using floating point numbers than whole numbers (integers). For example manipulating angles, or digital audio signals. A floating point number is one with a decimal point, like 3.1415926, or that is tiny, like 10^-126, a number which makes a yoctosecind seem like an eternity (seriously, the age of the universe, measured in yoctoseconds is only 4.3×10^41 yoctoseconds), or big, like 10^127 (that’s the digit 1, with 127 zero’s after it).

The nifty thing about floating point numbers is they keep track of the position of the floating point for you. Most microcontrollers don’t have floating point hardware, so programs have to do all the bookkeeping and arithmetic in software. You don’t have to write the code because the compiler will do that for you. This can be a bit weird because it looks just the same as integer arithmetic, but the program is bigger and slower.

ARM publish a few benchmarks which suggest Cortex-M processor with hardware floating pont might be 10x faster than the same processor doing floating point arithmetic in software.

That is quite a big deal, if you want to use something like trigonometry, where calculating sine or cosine are themselves a bunch of floating point calculations. We really might see that 10x speed change.

Why do I care? Well, I am very interested in things which might inspire and motivate children and adults to learn programming. Sound and music are an area that lots of people love. Simple notes can be described by sine curves. So it would be very handy to write simple programs which generates values on a sine curve, and pumps them out through a Digital to Analogue Converter (DAC), through an analogue audio chain, so that we can hear it.

Something like:

const float frequency = 440.000; // A above middle C
const int sample_freq = 44100; // CD audio sample frequency, 44.1KHz
const int samples = sample_freq/frequency; // number of samples/cycle
while (true) {
    for (int i=0; i<=samples; i++) { 
        dacWrite( sine(i * 2.0*PI / samples) ); 
        delayMicroseconds(1000000/sample_freq); 
    }
}

That is too rough to work well (the delay might not be accurate enough, and it ignores the time used to do the calculation and loop) but I hope it is enough to illustrate the idea. Fast floating point should make maths and music easier to program.

STM32F4DISCOVERY is the low-cost development kit that STM are offering. Farnell are taking pre-orders, it is £9.26+VAT! They are about $16 in the USA, but I didn’t see any distributors with the stock-arrival date.

That is an amazing price if you were to compare to something that has to be sold at a profit, like an Arduino UNO. Of course ST Micro don’t have to make a profit on this, they might regard it as the cost of a marketing campaign. I much prefer this approach to a fancy, glossy, high visibility marketing campaign. I don’t want to look at fancy ads. I want the product at a price which I can’t ignore. Thank you STM.

I’ll try to pick up again, and mention some of the other nifty stuff, but if your interested, please check out the datasheets and reference manual.