Thursday, February 3, 2011

Terminology Galore

If you're anything like me you hate terminology. You know, those special, magical technical words that people use that you don't know the definition of. Terminology. It'd be great if it weren't so imprecise. You'd think (well, hope) that a word has one definition. This is not even close to the case with regular English (and even worse with British English) but can't one hope for a more direct mapping from technically-minded people. A word should just mean one thing, right?

Take the word 'driver'. On your desktop PC you have drivers - all kinds of them. On an embedded system you have drivers - all kinds of them! But they're not the same kind of drivers - not exactly the same anyway. If you wanted to fill a job writing Windows drivers you might not want to fill it with someone who write embedded systems drivers or even Linux drivers. So if you saw such a job advertised you'd want to make sure what kind of job you were getting into.

So some recruiter calls you and asks 'Do you have driver writing experience?' And you really want to ask what he means but you know he doesn't know. The best answer you're going to get is "What kind of experience do you have?" I would respond with something like "I've written multiple device drivers for bare-metal microcontrollers and real-time operating systems, is that what you're looking for?" And if you're lucky the notes he writes about your experience will be something like "bear-metal.. big iron? Iron Man? multiple operati.. operation systems. Operation - I loved that game...." And what he tells you is "Absolutely absolutely, I'll get in touch with them and let them know. That's great that's great!"

And that's only the answer I would give now - because now I have some idea what a driver is and how it isn't a board-support package or hardware abstraction layer (I think). But if you're anything like me a month ago you're a bit lost. You see I had developed drivers before (I think), and just didn't know it. So let's define some terms!

I consider a driver (in an embedded system) something that hides registers for you. For instance, here's some code that configures a timer on on MSP430 for creating a servo control pulse:

//Clear timer A config

TACTL = 0x04; //TACLR = 1

TACTL = (0x02 << tassel =" 10">
(0x00 << id0 =" 00">

TACCTL0 = 0x0000;

//Configure compare and capture unit 1 for output compare mode 3
TACCTL1 = 0x0000;

TACCTL1 = (0x00 << cap =" 0">

(0x03 << outmod0 =" 011">

//Set CC1 to generate 1.5ms pulse - neutral

//MSP430 user's manual page 11-14

//?? what's going on here?

TACCR1 = 0x0000; //SET output line HI at 0x0000

TACCR0 = PULSE_1MS; //RESET output line (LO) at 1.5ms

//Set timer A period to 2ms


//Start timer

TACTL |= (0x02 << mc0 =" 0x01">

(0x01 << taie =" 0x01">

Thats... a lot. A lot of bits. A lot of hex, a lot of OR'ing. A lot of bad formatting. Oh my, I can't handle this.

I'd rather do something like this:

timera_cc_setpw(1500 /*us*/);

See? No bits. That's a driver.

Now, this is an internal peripheral. Those are easy. Well, easier. We know that drivers basically set registers. When it's an internal peripheral then accessing those registers is just as easy as saying 'register = value'. But it's harder if you have (for instance) a peripheral connected over SPI. You still have to set registers but that requires you writing data over SPI - usually commands like 'I WANT TO WRITE TO THIS MEMORY LOCATION. IT'S A CONFIGURATION REGISTER YO' and then the peripheral responds 'YO DAWG THAT'S COOL WHERE THE DATA AT?' and then with another SPI transfer you say 'HERE DA DATA AT!'. So basically you'll have a peripheral driver utilizing the SPI driver. It's a whole bunch of driver on driver goodiness.

So what about all the other crap? Like a Board Support Package. Well a BSP... supports a board. For instance the ez430-USB development kit has one LED on it (this is the extent of its on-board peripherals). It's located on P1.0 which is on physical pin 3 which can be accessed on port (yadayadayadayada). You don't want to know all of that - you just want a heartbeat LED. You want it to flash. So you have a smart guy write a function for you - a BOARD SUPPORT FUNCTION!

void bsp_led_toggle( void )
P1 ^=0X01;

This is great - I don't have to know where the LED is, I can just say 'toggle that please!' and it gets done. That's board support - it supports the board. Whatever's on the board needs functions so I don't have to know all about it.

And what about the dreaded HARDWARE ABSTRACTION LAYER?!?
The HAL just makes sure you don't actually need to know what your hardware looks like to use it. For example, you can turn a general-purpose I/O port into a TTL serial interface - you just have to be careful with timing and such but it's certainly possible. Now imagine on your Arduino you have the regular UART and a software-based UART. You want the same interface to both: get a byte, put a byte, turn it on, turn it off. So you write up a bunch of functions and then you just say something like:

byte = uart_get(fake_uart);
byte = uart_get(real_uart);

Same interface, same bytes, different underlying hardware. That's what a hardware abstraction layer does.

Hopefully with some of these definitions you'll be a little more educated about what all these weird definitions are. Good luck!

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