I’ve been a bit lax with the blog updating. In Part 1 of this post I promised Part 2 “soon” and here it is, eight weeks later. Oops.

This two-post series, aimed at embedded device beginners, explains some differences between Arduino and Raspberry Pi. In this second part we’re going to focus on one particular issue – “real-time” constraints. We’ll also quickly look over some of the alternative devices available.

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Decisions, Decisions…

“If I can buy a Raspberry Pi so cheaply, why would I ever use an Arduino for an electronics project?”

I often hear this from people who are new to embedded programming and electronics. This post is the first of two, aimed at beginners in the embedded world. We’ll go over some of the differences between a typical Arduino and a Raspberry Pi, and the reasons you might want to use one or the other for a project.

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Too much going on lately:

Still, it’s nice to have things that are (almost) coming to fruition.

On an Arduino or other AVR, EEPROM access is a bit fiddly if you want to store different types of data. In this blog post, I’ll show you a quick trick to use when you have lots of structured data to store in EEPROM.


First, the existing alternatives:

  • Arduino has EEPROM, which is simple but it only reads/writes single bytes.
  • Arduino Playground has two templated functions that let you read/write anything. However, you need to know the offset of whatever you are accessing. Also, I find C++ templates a bit icky for this use case.
  • One level deeper, avr-libc has a more complex API for other kinds of integers, and buffers. However, you still need to remember offsets & sizes.
  • The EEMEM keyword works like PROGMEM to let you flag things as stored in the EEPROM. Which is nice, but you still need to pass size and offset whenever you read a value.


This technique uses a single ‘struct’ to represent the entire contents of your EEPROM, so you can then use macros to read and write fields.

You define a struct called __eeprom_data that describes your EEPROM:

struct __eeprom_data {
  int first;
  int second[64]; 
  boolean third;
  char fourth[buf_len];

Then use macros eeprom_read & eeprom_write to read and write each field:

int x;
eeprom_write(-7, first);
eeprom_read(x, first);

Full Example

 * Copy and paste this block of #include & #defines into your code to use
 * this technique.
 * (Don't worry too much about reading the macros, read through the
 * examples below instead.)

#include <avr/eeprom.h>
#define eeprom_read_to(dst_p, eeprom_field, dst_size) eeprom_read_block(dst_p, (void *)offsetof(__eeprom_data, eeprom_field), MIN(dst_size, sizeof((__eeprom_data*)0)->eeprom_field))
#define eeprom_read(dst, eeprom_field) eeprom_read_to(&dst, eeprom_field, sizeof(dst))
#define eeprom_write_from(src_p, eeprom_field, src_size) eeprom_write_block(src_p, (void *)offsetof(__eeprom_data, eeprom_field), MIN(src_size, sizeof((__eeprom_data*)0)->eeprom_field))
#define eeprom_write(src, eeprom_field) { typeof(src) x = src; eeprom_write_from(&x, eeprom_field, sizeof(x)); }
#define MIN(x,y) ( x > y ? y : x )

const int buflen = 32;

 * __eeprom_data is the magic name that maps all of the data we are 
 * storing in our EEPROM
struct __eeprom_data {
  int first;
  int second; 
  boolean third;
  char fourth[buflen];
  char fifth[buflen];

void setup()

    * Writing simple variables to the EEPROM becomes simple
    * First argument is the value to write, second argument is which field
    * (in __eeprom_data) to write to.
   int q = 132;
   eeprom_write(q, first);
   eeprom_write(5958, second);
   eeprom_write(false, third);
   eeprom_write("Hello from EEPROM!", fourth);

    * You can even write from a pointer address if need be
    * First argument is the pointer to write from.
    * Second argument is the field (in __eeprom_data) 
    * to write to.
    * Third argument is the buffer length
    const char * buf = "Another hello looks like this";
    eeprom_write_from(buf, fifth, strlen(buf)+1);

    int a, b;
    boolean c;
    char d[buflen], e[buflen];
    char *e_p = e;
     * Reading back is just as simple. First argument is the variable to read
     * back to, the second argument is the field (in __eeprom_data) to read
     * from.
    eeprom_read(a, first);
    eeprom_read(b, second);
    eeprom_read(c, third);
    eeprom_read(d, fourth);
     * You can read back to a pointer address, if you need to.
    eeprom_read_to(e_p, fifth, buflen);

    Serial.println(c ? "TRUE" : "FALSE");
     * The eeprom_write macros do bounds checking, 
     * so you can't overrun a buffer.
     * In __eeprom_data, 'third' is a one-byte boolean, but 
     * eeprom_write knows this so only the first char 'T' is written
     * to EEPROM
    eeprom_write("This is a buffer overflow", third);
     * If you have an array, like char[], you can write & read a single
     * array entry from a particular constant index
     * Unfortunately, it only works for constant indexes not variables.
     * eeprom_write('X', fourth[x]) does not work with these macros.
    eeprom_write('X', fourth[3]);
    eeprom_read(d, fourth);
    char x;
    eeprom_read(x, fourth[3]);

void loop() { }

(This is Arduino code, obviously if you’re using avr-libc directly then you can rewrite it for that.)


The downsides of this technique (as I see them) are:

  • Uses macro magic (so a bit icky.)
  • Overkill if you only need to store one type of data in EEPROM, but useful if you have lots of different types.

Initial Values

When you start up, you need to know if the data in EEPROM is data that your program saved, or something else. There are a few different ways to deal with this.


EEMEM provides a way for you to set default values easily in your code:

EEMEM struct __eeprom_data initial_data EEMEM = { 
  1, // first
  2, // second
  false, // third
  "Initial fourth", // fourth
   "Initial fifth" // fifth

Via avr-objcopy & avrdude you can generate a .eep file and flash it to the AVR. This is nice, but it won’t work if you’re using the Arduino IDE because (as of version 0018) it doesn’t generate .eep files properly, and it also doesn’t support flashing them.

It’s a good option if you’re using your own Makefile, though.

“Magic” number

The other way is to check for and expect a “magic” value somewhere in the EEPROM data. Something like:

// Change this any time the EEPROM content changes
const long magic_number = 0x5432;

struct __eeprom_data {
  long magic; // should be set to magic_number
  int first;
  int second; 

void setup() {
  long magic;
  eeprom_read(magic, magic);
  if(magic != magic_number)

void initialise_eeprom() {
  eeprom_write(0, first);
  eeprom_write(0, second);
  eeprom_write(magic_number, magic);

I recently bought a couple of cheap MG996R high-torque servos on ebay, and I want to use them with my Arduino.

Arduino + MG996 + Dusty Breadboard

These have the “JR” servo pinout, so orange = signal, red = V+, brown = V-.

You control these servos by sending a 50Hz pulse width modulated signal. The pulse width determines the position of the servo. Arduino wraps this in a nice Servo library. So you can just use servo.write(<angle>) to set the servo to a certain angle. Cool!

The servo library defaults to pulsing 544ms to 2400ms for angles zero to 180. This is too wide for the MG996R, the servo only moves when you write angles through 20-150 or so. Setting the value outside this range stresses the servo and can wear it out!

I wrote a quick sketch to interactively find the real minimum and maximum values: . You just watch the serial port and follow simple prompts:

Download the sketch here.

Servo Range Sketch (running)

Using that sketch, I found my MG996R servos to have minimum pulse width around 771 and maximum around 2193 when running off 5v1. The full sweep is approximately 0-130 degrees.

So, to use the servo library with correct angles the sweep will be 771 to 17982. So I can call:

servo.attach(servopin, 771, 1798);
servo.write(0); // Min
servo.write(65); // Midpoint
servo.write(130); // Max

Yay Arduino!

  1. While the servo is unloaded I’m powering it from the Arduino via USB, but this won’t supply enough current for a loaded servo. []
  2. The adjusted max of 1798 is calculated as ( (MaxPW – MinPW) * (MaxAngle / 180) ) + MinPW, ie ( (2193 – 771) * (130 / 180) ) + 771 []