Approach, Design, and Implementation of my ECE306 Embedded Systems Project

You can download or view all of my code for ECE306: Introduction to Embedded Systems in my local git repository; all code is licensed under a Modified BSD 3-clause license and is thereby free to use, copy, modify, and redistrubute under the license’s terms

There is an (in)famous course in the computer engineering major at N.C. State university: ECE306 “Introduction to Embedded Systems” (mirror of the 2018 syllabus); the course is generally taken in the third year at State and advisors always recommend to take this course along with a light semester instead of cramming it in with other, equally demanding courses.

ECE306 is project-based: more than fifty percent of your final grade in the course arises from the various components of the main project (the other fifty percent is derived from your performance on three exams).

The Project and its Requirements

The project itself is actually very neat: you are given a TI-MSP430 microcontroller and several components (e.g. an IoT Wi-Fi booster board, debugging cables, an H-bridge, motors, and a handful of infrared detectors / emitters) which you are expected to use to complete 10 sequential projects (each of which builds upon the last) which include the following objectives:

  1. Solder all components to the TI-MSP430, booster boards and H-bridge
  2. Write an operating system which exposes and leverages the functionality of the MSP430 and attached hardware
  3. Design and fabricate a chassis (with room for the wheels and servo-motors) on which the MSP430 will be mounted
  4. Design and implement data structures and algorithms which permit the newly-constructed device to:
    1. Communicate and receive commands from a user via the wireless interface
    2. Navigate a 10-part obstacle course (while being driven by a user)
    3. Autonomously drive, intercept and follow an arbitrary black line on the floor

That’s all to say: the final project is an IoT-enabled RC-Car capable of operation in autonomous and manual modes.

Implementation Strategy

All code is written in C; IAR is the preferred IDE (I explain how I replaced IAR with a home-baked GCC toolchain later); the free version of IAR comes with several restrictions, the most problematic of these is an 8MiB hard-limit for compiled binaries.

Firstly, pieces of pre-compiled code are provided by the instructor at the beginning of the course: the first project focuses on installing this code on the MSP430; the subsequent 2 projects require you to replace the obfuscated binary with your own, self-written firmware. The below picture is taken from my lab-report for Project 1, which familiarized me with using IAR and the debugging tools to port software from a lab (Windows) machine to the MSP430 board.

P1 Board
Example of the outcome of Project 1

Projects beyond the first few introduce locomotion and precision into the car in addition to the black-line detection which is necessary for the final project. Eventually all the pre-compiled code is replaced except for the LCD microcontroller code.

Reverse-engineering the LCD

The LCD is attached to a breakout board which is mounted on top of the MSP430 itself; the board only sources a handful of GPIO signals (the LCD board itself uses a simple SPI interface to communicate with the host MCU) and it seems that only a fraction of the useful and fun functions of the LCD are exposed by the pre-compiled, instructor-provided code.

I hassled my professor a number of times about the code: how he did it, what documentation did he use, even if I could see the code itself; but he never allowed me to see even a snippet of his code, calling it a “dirty hack” and “not good practice”; this inspired me to research the LCD and begin experimenting with its functionality.

The LCD itself is a EA DOGS104-A (amber) (datasheet); from a cursory look at this datasheet, I was excited about how simple things looked, and the prospect of doing something which nobody had ever tried in the history of ECE306 at State excited and inspired me to replace the provided obfuscated binary with my own LCD code.

I began by researching everything about the LCD: especially I was interested in existing libraries written for the controller (I found no such code), downloading and filing everything meticulously on my personal computer. These late afternoons and weekends in the lab were meditative: I would sit with the MSP430 on the table to my right, wired with a pair or two of cables to an oscilloscope and a power supply on the lab bench. Whenever the documentation was not verbose, I would run the instructor’s obfuscated code and dump the resulting voltage waveform to the oscilloscope, zooming in at specific parts of the sample, trying to figure out what each part meant.

This process, while tedious, gave me the hands-on experience of reverse-engineering and SPI programming that everyone else missed out on; an example of my notes (annotated with my post-analysis) is posted below:

LCD Dump
An annotated view of the waveform traced from a lab oscilloscope; using this (and 10 other sketches) I was able to reverse-engineer the entire initialization routine

It appeared that information travelled to the LCD from the MCU in small chunks (with delays in-between); 10 such command-chunks were sent in my professor’s initialization routine. I meticulously sketched each command in my lab notebook and matched up the signals with the SSD1803A specs, which is the actual controller and driver for the LCD segment.

Cross-referencing this 69-page technical document I was able to reverse-engineer all 10 of the commands sent during the initialization routine in my instructor’s binary just over a weekend in late October. It happens that almost none of the commands sent are necessary; many steps simply re-assert the power-on-default parameters of the chip; perhaps this initialization is not an “initialization” but a “reset” function, I wondered; but it didn’t matter as nobody used their LCD like this anyhow (and if they were stressing their LCD to the point it required soft-reset they have bigger problems to worry over).

It seems that the code provided to us used delays to ensure that the commands reached the LCD; this is not optimal for a number of reasons. Most importantly, code which uses delays often blocks execution of other critical tasks; if the blocking code is used in an ISR then events can be missed and normal operation can break down. I was surprised to find evidence of blocking delays in my professor’s obfuscated code (especially because he warned us not to use such busy-waits); writing to a screen using his library, it seemed, could introduce several milliseconds of delay, potentially causing events to be missed elsewhere. This delay could be eliminated entirely if one wrote code to use a ring-buffer or a similar structure instead of feeding the bytes one-by-one.

After decoding the initialization routine and distilling it down to only the necessary commands, I decided to write my own library of functions for the SSD1803A. I only bothered writing code to leverage the functions I wanted to use, however it is trivial to expand this library to include all functions exposed by the chip; below are a few snippets from the final version of my version of LCD.c: it’s worth noting that I provided no mapping of my instructor’s function calls to my function calls so the library is not strictly interchangeable (and it’s likely that the hardware for future ECE306 courses will change so such a mapping is likely to be deprecated soon). Here is a snippet of the head of the file:

/*
*  Provides the microcontroller an interface with the LCD screen
*  Wesley Coakley
*  Nov 2018
*/

#include "macros.h"
#include "data-types.h"
#include "functions.h"
#include "msp430.h"
#include "globals.h"
#include <string.h>

#define SPI_BUF_LEN	(0x3F)

#define LCD_MODE_1	(0x00)
#define LCD_MODE_2	(0x01)
#define LCD_MODE_3	(0x02)
#define LCD_MODE_4	(0x03)

/* LCD Start Bytes */
#define LCD_DATAWRITE_START	(0xFA)	// RS=1, RW=0
#define LCD_DATAREAD_START	(0xFE)	// RS=1, RW=1
#define LCD_CMD_START	(0xF8)	// RS=0, RW=0
/* Extended instruction set */
#define LCD_ROM_SELECTION	(0x72)

/* 0x01: LCD Clear Display */
#define LCD_CMDMASK_CLEAR	(0x01)

/* 0x02: LCD Return Home */
#define LCD_CMDMASK_RETHOME	(0x02)


/* 0x04: LCD Entry Mode Set (RE=1) */
#define LCD_CMDMASK_EXT_ENTRYSET	(0x04)
#define LCD_ARGMASK_BDC	(0x02)
#define LCD_ARGMASK_BDS	(0x01)

/* 0x08: LCD Display On / Off Control (RE=0) */
#define LCD_CMDMASK_DISPLAYCONT	(0x08)
#define LCD_ARGMASK_D	(0x04)
#define LCD_ARGMASK_C	(0x02)
#define LCD_ARGMASK_B	(0x01)

/* 0x08: LCD Extended Function Set (RE=1) */
#define LCD_CMDMASK_EXTFUNCSET	(0x08)
#define LCD_ARGMASK_FW	(0x04)
#define LCD_ARGMASK_BW	(0x02)
#define LCD_ARGMASK_NW	(0x01)

/* 0x10: LCD Display Shift */
#define LCD_CMDMASK_SHIFT	(0x10)
#define LCD_ARGMASK_SC	(0x08)
#define LCD_ARGMASK_RL	(0x04)
/* 0x10: LCD Double Height (RE=1) */
#define LCD_CMDMASK_EXT_DOUBLEHEIGHT	(0x10)
#define LCD_ARGMASK_UD2	(0x08)
#define LCD_ARGMASK_UD1	(0x04)
#define LCD_ARGMASK_BS1	(0x02)
#define LCD_ARGMASK_DHP	(0x01)

/* 0x20 Common Arguments */
#define LCD_ARGMASK_DL	(0x10)
#define LCD_ARGMASK_N	(0x08)
#define LCD_ARGMASK_RE	(0x02)
/* 0x20: LCD Function Set */
#define LCD_CMDMASK_FUNCSET	(0x20)
#define LCD_ARGMASK_DH	(0x04)
#define LCD_ARGMASK_IS	(0x01)
/* 0x20: LCD Function Set (RE=1) */
#define LCD_CMDMASK_EXT_FUNCSET	(0x20)
#define LCD_ARGMASK_BE	(0x04)
#define LCD_ARGMASK_REV	(0x01)

/* 0x40: LCD Set CGRAM Address */
#define LCD_CMDMASK_SETCGRAMADDR	(0x40)
#define LCD_MAXCGRAMADDR	(0x3F)

/* 0x50: Power Control */
#define LCD_CMDMASK_POWERCONTROL	(0x50)
#define LCD_ARGMASK_ION	(0x08)
#define LCD_ARGMASK_BON	(0x04)

/* 0x60: Set Follower Control */
#define LCD_CMDMASK_SETFOLLOWERCONTROL	(0x60)
#define LCD_ARGMASK_DBON	(0x08)
#define LCD_MAX_RESISTOR_RATIO	(0x07)
#define LCD_RESISTOR_RATIO	(0x06)

/* 0x70: Contrast Set */
#define LCD_CMDMASK_CONTRASTSET	(0x70)
#define LCD_MAX_CONTRAST	(0x0F)
#define LCD_DEFAULT_CONTRAST	(0x0F)

/* 0x80: LCD Set DDRAM Address */
#define LCD_CMDMASK_SETDDRAMADDR	(0x80)
#define LCD_DDRAMLEN_ONELINE	(0x50)
#define LCD_DDRAMLEN_TWOLINE	(0x68)
#define LCD_DDRAMLEN_THREELINE	(0x54)
#define LCD_DDRAMLEN_FOURLINE	(0x74)

/* LCD ROM Spaces */
#define LCD_ROMA	(0x00)
#define LCD_ROMB	(0x01)
#define LCD_ROMC	(0x02)

/* LCD struct */
#define LCD_MODERR	(0x01)
#define LCD_ROMERR	(0x02)
#define LCD_WRITERR	(0x04)
#define LCD_POR_ERR	(0x00)

/* LCD Modes */
#define LCD_EXTENDED_MODE	(0x01)	// Bit 0
#define LCD_SPECIAL_MODE	(0x02)	// Bit 1
#define LCD_DISPLAY_MODE	(0x0C)	// Bits 2 & 3
#define LCD_DATALENGTH_MODE	(0x10)	// Bit 4
#define LCD_MODE_NW	(0x04)
#define LCD_MODE_N	(0x08)

/* Power on reset defaults */
#define LCD_POR_CURSROW	(0x00)
#define LCD_POR_CURSCOL	(0x00)
#define LCD_POR_MODE	(0x00)
#define LCD_POR_PMODE	(0x00)

/* LCDRow */
#define LCDROW_NOSCROLL	(-1)
#define LCDROW_FOURCOL_ADDR(row) ( \
	(row > 2) ? 0x60 : \
	(row > 1) ? 0x40 : \
	(row > 0) ? 0x20 : 0x00 )
#define LCDROW_FOURCOL_LEN	(0x14)

/* LCD Abstractions */
char display_line[LCD_MAX_ROWS][LCD_MAX_COLS];
char spi_buf[SPI_BUF_LEN];
struct LCDRow framLCDScreen[LCD_MAX_ROWS];
struct serial_buf framLCDSBuf;
struct LCD framLCD;

As you can see, I leveraged the preprocessor macros in C to make the commands and arguments easier to read; the increased readability allowed me to easily write the code which sends these commands. After declaring these constants I go on to define some useful high-level functions; here is a function which resets the LCD to a know state (e.g. after power-on-reset or after a brownout):

// Initialize the SPI bus for the LCD
void Init_LCD(void) {
	// Reclaim SPI buffer
	struct serial_buf *sbuf = &framLCDSBuf;
	sbuf->buf = spi_buf;
	sbuf->len = SPI_BUF_LEN;
	sbuf->wi = 0; sbuf->ri = 0;

	// Draw up the screen
	struct LCDRow *screen = framLCDScreen;
	for (int i = 0; i < LCD_MAX_ROWS; i++) {
		struct LCDRow *r = &screen[i];
		r->chars = display_line[i];
		r->scroll = LCDROW_NOSCROLL;
		r->len = LCDROW_ONSCREEN_COLS;
		r->addr = LCDROW_FOURCOL_ADDR(i);
	}

	// Configure the LCD environment
	struct LCD *l = &framLCD;
	l->mode = LCD_POR_MODE;
	l->printmode = LCD_POR_PMODE;
	l->error = LCD_POR_ERR;
	l->ROWS = LCD_MAX_ROWS;
	l->screen = screen;
	l->sbuf = sbuf;
	l->curs_row = LCD_POR_CURSROW;
	l->curs_col = LCD_POR_CURSCOL;

	UCB1CTLW0 = UCSWRST;	// Temporarily suspend eUSCI for configuration

	// Control-word 0 configuration
	UCB1CTLW0 &= ~UCCKPH;	// Change then capture
	UCB1CTLW0 |= UCCKPL;	// Inactive state is high
	UCB1CTLW0 |= UCMST;	// Master mode
	UCB1CTLW0 |= UCMSB;	// MSB order
	UCB1CTLW0 |= UCMODE_0;	// 3-pin SPI
	UCB1CTLW0 |= UCSYNC;	// Asynchronous
	UCB1CTLW0 |= UCSSEL_2;	// Source from SMCLK

	UCB1BRW = 40;	// /40 (divide SPICLK down to 100kHz)

	UCB1CTLW0 &= ~UCSWRST;	// Re-initialize SPI Bus with our configuration

	// Special functionality (IS=1, RE=0)
	lcdFunctionSet(&framLCD,
		// Data length -> 8 bits
		LCD_ARGMASK_DL
		// Display lines -> 4 / 2 setting lines
		| LCD_ARGMASK_N
		// Special Instructions
		| LCD_ARGMASK_IS);
	lcdClearDisplay(&framLCD);
	lcdContrastSet(&framLCD, LCD_DEFAULT_CONTRAST);
	lcdPowerControl(&framLCD, LCD_ARGMASK_ION | LCD_ARGMASK_BON);
	lcdSetFollowerControl(&framLCD, LCD_ARGMASK_DBON, LCD_RESISTOR_RATIO);

	// Extended functionality (IS=0, RE=1)
	lcdFunctionSet(&framLCD,
		// Data length -> 8 bits
		LCD_ARGMASK_DL
		// Display lines -> 4 / 2 setting lines
		| LCD_ARGMASK_N
		// Enable extended functions
		| LCD_ARGMASK_RE);
	lcdExtendedFunctionSet(&framLCD,
		// 3~4 line mode
		LCD_ARGMASK_NW);
	lcdEntryModeSet(&framLCD,
		// Shift DDRAM normally; segments shift in reverse
		LCD_ARGMASK_BDC);
	lcdDoubleHeight(&framLCD,
		LCD_ARGMASK_BS1
		| LCD_ARGMASK_UD1
		| LCD_ARGMASK_UD2
		| LCD_ARGMASK_DH);

	// Normal Functionality (RE=0, IS=0)
	lcdFunctionSet(&framLCD,
		LCD_ARGMASK_N
		| LCD_ARGMASK_DL);
	lcdDisplayControl(&framLCD,
		// Turn on Display
		LCD_ARGMASK_D
		| LCD_ARGMASK_B
		| LCD_ARGMASK_C);
}

… as you can see there are several helper functions which are called by this initialization routine; each helper function sends one or more commands via SPI to the LCD driver. In additon, I liberally leverage struct and other C abstractions to organize and structure my code better, in a way which is easier to read and which is similar to an object-oriented (OO) approach to the problem; it seems that the pre-compiled code did not take this approach to the problem. Below are examples of high-level helper functions:

// Toss a string on to the LCD
void lcdPrint(struct LCD *l, char *s) {
	for (int i = 0; s[i]; i++) lcdPutChar(l, s[i]);
}

// Erase all characters on the LCD
void lcdEraseContents(struct LCD *l) {
	// Reset cursor to zero position
	lcdReturnHome(l);

	// Remove characters
	do { lcdWrite(l, ' ');
	} while (l->curs_addr > (*l->screen).addr);
}

// Write to the cursor position
void lcdPutChar(struct LCD *l, char c) {
	if ((c == '\n' || c == ' ') &&
	l->curs_col == l->screen[l->curs_row].len - 1) {
		return;
	}
	switch(c) {
	case '\n':
		lcdLineFeed(l); break;
	default:
		lcdWrite(l, c);
	}
}

// Advance LCD cursor to the beginning of the next line
void lcdLineFeed(struct LCD *l) {
	unsigned char row = l->curs_row;
	row = ++row % l->ROWS;
	l->curs_row = row;
	l->curs_col = 0;

	lcdSetDDRAMAddress(l, l->screen[row].addr);
}

// Advance cursor to the beginning of an arbitrary line
void lcdCursorToLine(struct LCD *l, unsigned char line) {
	if (line > l->ROWS) {
		lcdTossError(l, LCD_WRITERR);
		return;
	}
	l->curs_col = 0;
	l->curs_row = line;
	lcdSetDDRAMAddress(l, l->screen[line].addr);
}

… these are examples of high-level helper functions, which are normally called by code elsewhere within the MCU firmware; they make life easier for the progammer (me). Contrasting the high-level functions, I also programmed a class of low-level functions which do the heavy-lifting by taking care of things like SPI communication and also handle the quirks of the chip which I discovered from the datasheet:

/* Low-level LCD functions */

void lcdSendCmd(struct LCD *l, unsigned char instruction) {
	struct serial_buf *s = l->sbuf;
	unsigned char revi = reverseInstruction(instruction);
	writeSBuf(s, LCD_CMD_START);
	writeSBuf(s, (revi >> 4) << 4);
	writeSBuf(s, revi << 4);

	// Initialize SPI bus transfer using TX interrupt
	if (!(UCB1IE & UCTXIE0)) {
		UCB1TXBUF = readSBuf(s);
		UCB1IE |= UCTXIE0;	// Enable transmit ready interrupts
	}

}
unsigned char reverseInstruction(unsigned char instruction) {
	return ((instruction & 0x01) << 7)
	| ((instruction & 0x02) << 5)
	| ((instruction & 0x04) << 3)
	| ((instruction & 0x08) << 1)
	| ((instruction & 0x10) >> 1)
	| ((instruction & 0x20) >> 3)
	| ((instruction & 0x40) >> 5)
	| ((instruction & 0x80) >> 7);
}

/* Clear Display */
void lcdClearDisplay(struct LCD *l) {
	lcdSendCmd(l, LCD_CMDMASK_CLEAR);
}

/* Return Home */
void lcdReturnHome(struct LCD *l) {
	l->curs_row = l->curs_col = 0;
	lcdSendCmd(l, LCD_CMDMASK_RETHOME);
}

/* Set LCD Entry Mode (RE=1) */
void lcdEntryModeSet(struct LCD *l, unsigned char args) {
	if (!(l->mode & LCD_EXTENDED_MODE)) {
		lcdTossError(l, LCD_MODERR);
		return;
	}

	lcdSendCmd(l, LCD_CMDMASK_EXT_ENTRYSET | args);
}

/* Augment LCD functionality (RE=1) */
void lcdExtendedFunctionSet(struct LCD *l, unsigned char args) {
	if (!(l->mode & LCD_EXTENDED_MODE)) {
		lcdTossError(l, LCD_MODERR);
		return;
	}
	// Update LCD state if we're changing display modes
	if ((args & LCD_ARGMASK_NW)
	&& !(l->mode & LCD_MODE_NW)) {	// NW=0 -> NW=1
		l->mode |= LCD_MODE_NW;
	}
	else if (!(args & LCD_ARGMASK_NW)
	&& (l->mode & LCD_MODE_NW)) {	// NW=1 -> NW=0
		l->mode &= ~LCD_MODE_NW;
	}

	lcdSendCmd(l, LCD_CMDMASK_EXTFUNCSET | args);
}

void lcdContrastSet(struct LCD *l, unsigned int contrast) {
	if (l->mode & LCD_EXTENDED_MODE
	|| !(l->mode & LCD_SPECIAL_MODE)) {
		lcdTossError(l, LCD_MODERR);
		return;
	}

	if (contrast > LCD_MAX_CONTRAST) contrast = LCD_MAX_CONTRAST;
	lcdSendCmd(l, LCD_CMDMASK_CONTRASTSET | contrast);
}
void lcdSetDDRAMAddress(struct LCD *l, unsigned int a) {
	if (l->mode & LCD_EXTENDED_MODE) {
		lcdTossError(l, LCD_MODERR);
		return;
	}

	// Restrict address space according to the display mode
	if (!(l->mode & LCD_DISPLAY_MODE - LCD_MODE_1)	// One-line
	&& (LCD_DDRAMLEN_ONELINE < a))
		a = LCD_DDRAMLEN_ONELINE - 1;
	else if (!(l->mode & LCD_DISPLAY_MODE - LCD_MODE_2)	// Two-line
	&& (LCD_DDRAMLEN_TWOLINE < a))
		a = LCD_DDRAMLEN_TWOLINE - 1;
	else if (!(l->mode & LCD_DISPLAY_MODE - LCD_MODE_3)	// Three-line
	&& (LCD_DDRAMLEN_THREELINE < a))
		a = LCD_DDRAMLEN_THREELINE - 1;
	else if (!(l->mode & LCD_DISPLAY_MODE - LCD_MODE_4)	// Four-line
	&& (LCD_DDRAMLEN_FOURLINE < a))
		a = LCD_DDRAMLEN_FOURLINE - 1;
	unsigned char addr = (unsigned char) a;

	lcdSendCmd(l, LCD_CMDMASK_SETDDRAMADDR | addr);

	unsigned char col = addr; unsigned char row;
	for (row = 0; l->screen[row].addr < addr && row < l->ROWS; row++) { }

	l->curs_row = row;
	l->curs_col = addr - l->screen[row].addr;
}

void lcdTossError(struct LCD *l, unsigned char error) { l->error |= error; }

… I absolutely love using struct (as you can tell above); almost all of the LCD’s functionality is managed via this paradigm because (1) struct naturally organizes data in a coherent way and (2) to add another LCD, one only needs to instantiate another struct, not to rewrite the whole library; notice that each function above takes struct LCD *l as its first parameter; this provides one more layer of abstraction to the library, permitting the OO approach which I explained before.

Interesting to note also is that I regularly use error-handling code, something which many embedded programmers scoff at; I believe that this approach (as opposed to taking a “halt and catch fire” approach to errors) allows sub-fatal errors to be handled appropriately and avoid a total system freeze (especially for non-critical errors).

Finally, my LCD.c file concludes with code for handling an interrupt vector for writing to the SPI buffer; the LCD communication (and all serial communications on my board) are handled by a number of ring buffers declared in heap memory. The interrupt service routine (ISR) looks like this:

/* #pragma vector = EUSCI_B1_VECTOR
__interrupt void eUSCI_B1_ISR(void) { */
void __attribute__ ((interrupt(EUSCI_B1_VECTOR))) eUSCI_B1_ISR() {
	struct LCD *l = &framLCD;
	struct serial_buf *s = l->sbuf;
	switch(__even_in_range(UCB1IV, 4)) {
	case 2:	// RX ready
		break;
	case 4: // TX ready
		if (!(s->wi - s->ri)) {
			UCB1IE &= ~UCTXIE0;	// Nothing more to transmit
			return;
		}
		// Put one more character on the wire
		UCB1TXBUF = readSBuf(s);
	}
}

… this ISR is slim and fast, meaning that the time spent in this ISR is minimized so that the MSP430 has time to do the other necessary things (e.g. IoT communication, starting / stopping motors, etc.); this is a deliberate design choice on my part because ISRs block the execution of other ISRs on the MSP430 (at least by default); spending too much time handling an interrupt may mean that some other interrupts are delayed or maybe even never called! This is a scenario one wishes to avoid, especially when operating within the millisecond-by-millisecond world of IoT controllers.

One intersting thing to note in the above ISR is that: I never handle the case where I am receiving data from the LCD (i.e. UCB1RXBUF); I can get away with this because the LCD never sends any interesting signals back in typical operation; the RX case above simply ensures that the interrupt in deasserted. The circular nature of the buffer is evident by the line !(s->wi - s->ri) which simply tests if the write-index of the serial buffer (l->sbuf->ri) is equal to the read-index of the serial buffer (l->sbuf->wi); this is the typical implementation of buffers because the read and write indices are advanced incrementally according to each read and write operation on the buffer such that the indices will be equal once all data has been read which has been written.

These RC-Cars are entirely driven by batteries, making low-power operation an important thing to consider. I actually achieved a relatively low-power design (a pair of 4 AAs would let me drive my car for ~6 hours continuously) by:

  1. Slowing the clock periods on the MSP430
  2. Toggling the LCD backlight when necessary
  3. Taking advantage of low-power states on the wireless chip

All this fine-tuning is detailed in my lab notebook (which was not a requisite part of the course but which I found useful to have), a page of which is provided below:

P10 Dev
A page from my lab notebook detailing clock propagation to various subsystems in the RC-Car

As you can see VLOCLK drives low-frequency subsystems (button debouncing, motors, obstacle detection) and DCOCLK drives the subsystems which demand high-frequency bands for communication (the wireless module and the LCD module). When the car is in a software-defined “standby mode” I can easily cut the timer to high-frequency subsystems using a special function on the MSP430; this allows me to keep the car in a state which boots far, far faster than if I had toggled the power switch (because the car is still “on”) while conserving power by clock-gating the important modules and using my power budget of 4 AAs more effectively.

Driving the RC-Car with a NES Gamepad

As discussed in the introduction, one of the requirements is to be able to control the car manually; you must be able to navigate a 10-part obstacle course for a (significant) part of grade in the course; this course is set up on the last day of class and you can have any (reasonable) amount of time to complete it and the only requirements are that:

  1. You may not touch the car upon beginning the course
  2. The RC-Car must drive within 3 inches of the checkpoint for each section of the course
  3. Your car should not interfere with other students’ while navigating the course
  4. Failure to comply with the above necessitates you starting the entire obstacle course over from the beginning

That last day of class (the day we drove the obstacle course) was particularly hectic; since this counted as a major part of our grade, many people wanted to do the course as fast as possible to mitigate (1) the risk of collision with other students running the course as well and (2) maximize the time spent on the black-line interception portion of the course (also tested on that day).

To facilitate the speedy completion of the obstacle course I decided early-on to use a dedicated controller to drive my car around the course; it seems that many people decide to drive the car around by issuing “forward”, “backward”, and “turn” commands over the wireless interface; however this routine is extremely slow in practice, allowing students who actually drive their cars to speed ahead in the course.

P10 Dev
My typical development environment (note the coffee)

My NES controller in the above picture is heavily labelled with the mappings which I designed for each button: the DPad causes the car to move in the particular direction, while the L and R buttons toggle some features on the LCD; furthermore the two middle buttons calibrate the on-board A2D converters for intercepting the black line and the remaining four buttons cause the car to follow an according arc-and-intercept routine for the final part of the obstacle course.

I finished the course in under 10 minutes because the precision of my controller outpaced every other car. Because the wireless access points were border-line overloaded by the mass of students present, many people were forcibly disassociated from the AP; the lack of error handling in many students’ firmwares left their car immobile on the course, forcing them to retrieve it (by hand) and retry the course from the beginning; my firmware includes the necessary error-handling routines which (aggressively) attempt to re-associate with the AP it was connected to; this ensured that my car never stalled on the course for more than the time it took to reassociate with the AP (normally less than one second).

Migrating from IAR to a GCC-based toolchain

One prolonged antagonist in ECE306 is the 8KiB code-size limit in IAR; with the free version of IAR Embedded Workbench, the size of the firmware may not exceed 8KiB in total (Kickstart IAR refuses to flash firmware images larger than this with the error “size limit exceeded”). Of course the MSP430 has much, much more memory than 8KiB; I wanted to break into this extra memory to store things like wireless profiles and motor configurations which would allow me to fine-tune the motors en-route without ever laying a finger on the RC car.

I knew IAR could not accomplish this (and I was not about to shell out the several hundred dollars for a license); additionally I cannot run IAR on my laptop; a GCC-based toolchain suits my development environment far, far better. Unfortunately nobody had ever done this in the history of the course either (two firsts!) so I set off on this journey alone.

The details of my development environment are boring but involve a TI-provided version of GCC called MSP430-GCC and a flashing / debugging tool called mspdebug; these two softwares form the core of my development environment; all compilation was done on my personal (Gentoo/Linux) laptop pictured above and the source-code is edited using my favorite text-editor JoE (Joe’s Own Editor).

Moving away from IAR has the following benefits:

Replacing the pre-compiled LCD.c code as above was the final barier preventing me from jumping straight in to a GCC-based toolchain; after reverse-engineering the LCD driver and (re)writing the interface I was finally able to develop 247 on my Linux computer.

P10 Run
The final obstacle course; no wonder the access points were overloaded!... each waypoint of the course is an orange mat barely bigger than the car itself (and notice the line-interception portion of the course)

Results

I am proud of my final project: I wrote every last line of code myself and I’ll own all the paradigms, quirks, and data-structures. The extra work I invested in this course has taught me more about embedded systems than I would ever have learned just by sitting in the lecture hall; if I had to do it all again I’d do exactly the same things; I only wish I had access to the knowledge which I’ve dumped here and in my local git repository for this project.