Embedded Systems Programming

Plain old C is considered a "high level language" on embedded machines, and many commercial embedded products are still written in assembly language.  20MHz is considered "fast".  Integer multiply instructions are a rare luxury--floating point is almost never found. A $5 chip is considered "expensive".

A few families of embedded microcontrollers out there that I've used:

• The ARM microcontrollers.  Instructions are 32 bit RISC, with 16 32-bit registers.  A 64-bit transition is happening now.  Common in cell phones, and scale up to running Linux at GHz, or down to putting several in a hard drive at MHz.  Available as VHDL code, so you can drop one into your ASIC, with any custom hardware you want.  Used on the Raspberry Pi, BeagleBone Black, and at lower clock rate on the Arduino Due.
• Amtel's AVR line, as used in the amazingly popular open source Arduino platform.  The hardware is pretty conventional, but there's an excellent and very easy to use IDE and standard library, so it's much more accessible to novice programmers than other lines, which tend to have a tricky "toolchain" of compiler, linker, and device programmer.  Atmel instructions are 16 bit wide RISC instructions, with 32 8-bit registers.
• Microchip(tm) corporation's PIC line.  Instructions are fixed-size RISC, from 12 to 16 bits wide depending on the model, with one 8-bit accumulator register W and hundreds of I/O ports.  Higher end models can support a full embedded OS, low end models don't even have interrupts.  The popular BASIC Stamp controller is build on a PIC.
• TI's MSP430 line is very popular in the UAF EE department.  TI sells the "Launchpad" USB programmer board for just $4.30, although the software situation is still pretty confusing.
• The venerable 68HC11 microcontrollers.  Instructions are variable-length CISC, with two (A,B) or four (A,B,X,Y) registers.

I've written code for real projects on Arduino, PIC, and 68HC11.  I've used all of the above at least at the demo level.  I mostly use Arduino for one-off projects, due to the full C++ support and good standard library.

Embedded Inputs

There are a bunch of different input devices you can hook up to embedded microcontrollers.  Basically, input pins read voltage, so anything you can convert into voltage, you can input into the controller.  One really common device for this is a voltage divider, which is basically just two resistors in series, where you hook up the middle lead to the microcontroller.

For example, pushing a button normally closes a contact, bringing the resistance of the button from infinity down to zero.  Alone, that's useless.  But if you put 5V on one terminal of the button, and connect the other terminal to both the microcontroller input pin and a 1Kohm "pull-down" resistor (the other end hooked to ground), then you've effectively built a voltage divider with the button as the top resistor.  If the button is unpushed, its infinite resistance allows the pull-down resistor to pull the micro's pin down to ground, zero volts.  When the button is pushed, it shorts the micro's input pin up to 5v.  A little current leaks through the pull-down resistor, which is fine.  Light switches, limit switches, keyboard and mouse buttons, thermostat mercury bulbs, etc all boil down to just contact or no contact, and are interfaced in exactly the same way.

One caveat: pushing a button may result in several hundred tiny contact/no contact pulses a few microseconds wide.  You typically clean this up with a "debounce" circuit, either a hardware resistor-capacitor filter circuit, or a software function that only checks the button at 50Hz or so.

Another example, a "thermistor" is a resistor whose resistance varies with temperature.  If you set up a voltage divider as above, but plug a thermistor into the top half, you can convert temperature to voltage.

Analog TV, analog audio, and VGA signals are already just quickly-changing voltage patterns, so you can run them straight into an analog input pin of a microcontroller!

Embedded Outputs

Output pins output voltage, usually just either 0v or 5v, but can supply up to a few dozen milliamps.  This is just enough to light up a small LED, although you usually want a 1Kohm (or so) current limiting resistor inline with the LED.

To switch a useful amount of current, say to run a little motor, you usually need an interface device like a transistor.  "Signal" transistors can switch up to a few hundred milliamps, and "power" transistors can switch up to a few amps.  FET transistors can go up to dozens of amps fairly cheaply.  

One annoying thing about transistors is they only conduct current in one direction.  An electromechanical relay can switch AC current, although usually you need a transistor to push enough juice through the relay coil to get it to close.  I like relays, because they can't be hooked up backwards, and they're a lot tougher to fry than transistors.  Downside with relays is they're slow, energy intensive, and electrically and even acoustically noisy.

To turn a DC motor in either direction, you need an H-Bridge.  In theory, you can build these yourself from high power FET transistors, but in practice, it's way easier, especially at high power, to just buy a single-chip H-bridge (I really like the ST VNH3SP30TR, which switches up to 30 amps at 16 volts for $8), or even buy a prepackaged radio control electronic speed controller (ESC).

One cool output device is a servo, which is just a little motor, position sensor, H-bridge, and controller circuit integrated into one handy case.  You tell the servo what position to go to with a pulse-width-modulated signal: 5v for 1ms means all the way to the left, for 2ms means all the way to the right, and 1.5ms means halfway in between.  Most servos can seek to several hundred separate positions.  You usually repeat the seek signal every 15-30 milliseconds.  Servos use just three wires: black for ground, red (in the middle) for 5v, and white for the PWM position signal.  Servos are as low as $3 direct from China (although watch out for shipping!).

If talking to another big or little computer, you can speak USB (which is fast enough you usually need special hardware to speak it), plain slow serial (where bits are known fixed and *slow* times), I2C, SPIB, or any of a bunch of weirder protocols.

Embedded Programming Models

Typically, embedded programs look like this:

	... initialize hardware ...
	while (1) {
		... read my inputs ...
		... decide if anything needs to change ...
		... if so, write changes to outputs ...
	}

The main infinite loop is the "control loop".  The idea is microcontrollers usually are hardwired into known, fixed hardware, so they only have one job to do.  

Traditionally, embedded systems had no operating system: no files or permanent storage of any kind, no builtin networking, no threads or processes, and in general are missing all the junk we've come to expect from computers.  On the minus side, your "debugger" was a blinking LEDs and a voltmeter!

Now that the internet is considered kind of important, we're trying to figure out exactly how much operating system to add to embedded systems.  Some stripped-down form of Linux is becoming more common, and boards like the Raspberry Pi support a full GUI desktop. 

Raspberry Pi I/O Pins

#include <stdio.h>    // for printf
#include <fcntl.h>    // for open
#include <sys/mman.h> // for mmap
#include <unistd.h> 

long foo(void) 
{
	int fdgpio=open("/dev/gpiomem",O_RDWR);
	if (fdgpio<0) { printf("Error opening /dev/gpiomem"); return -1; }

	unsigned int *gpio=(unsigned int *)mmap(0,4096,
		PROT_READ+PROT_WRITE, MAP_SHARED,
		fdgpio,0);
	printf("mmap'd gpiomem at pointer %p\n",gpio);
	
	// Read pin 8, by accessing bit 8 of GPLEV0
	return gpio[13]&(1<<8);
}

(Try this in NetRun now!)