Nuts and Volts - May 2010

You can find the source code and supplements for this article in Workshop22.zip at www.nutsvolts.com.

SMILEY’S WORKSHOP ☺

could shine a UV lamp onto the memory and erase it.

This operation took a while, the software test procedure generally went like this: fail, pull out the EPROM, stick it in a UV eraser, go get a cup of coffee and watch a soap opera, load the next iteration of the code using an EPROM programmer, fail, pull out the EPROM ... it was actually even more boring that it sounds. Then, some genius figured out a way to erase the EPROM with electricity thereby creating the EEPROM (the additional E stands for Electrical).

EEPROM is a kind of memory that won’t go brain dead if you pull the alligator clip off the lantern battery, and you don’t need a special eraser to erase it. The next innovation was Flash EEPROM, more commonly known as Flash memory. This memory was much cheaper than regular EEPROM because it erased memory in large blocks rather than a byte at a time. It’s the kind of memory you usually find on memory cards and USB Flash drives. This innovation is one of the reasons that we have AVRs. The guys who designed the AVR wanted program memory that could be both non-volatile and easily erasable, so they got with the folks at Atmel (who were making cash on Flash). So, what does this have to do with Baron Munchhausen? Well, the AVR architecture has some special features that allow us to use Flash memory as a bootloader.

All right, that was strange, but the points were: A bootloader allows you to program a microcontroller without an external programmer; and Flash memory is great for a bootloader because it is non-volatile so you can use lantern batteries and alligator clips without cursing and throwing things.

Chicken or Egg

A bootloader is a program that allows you to upload a program without having an external programmer. However, the bootloader IS a program, so how do you program it onto a micro? Well, you use an external programmer. But — you ask — if the reason for a bootloader is so you don’t need an external programmer ... but you need one to put the bootloader on the micro ... Wait a minute! This is confusing! The answer is that if you have an external programmer, then you don’t need a bootloader — but if somebody is kind enough to program the chip with a bootloader for you, then you don’t need an external programmer.

Last month, we relied on SparkFun ( www.sparkfun.com) to put a bootloader on an ATmega328 for our BreadboArduino. This month, we are going to look at how to put a bootloader on an ATmega644. Atmel has several really good programmers that would fit our needs: the AVRISP mkII; the Dragon; and the STK-500 — in order of cost and capability. Since we will be using ISP (In System Programming), any of the three would suffice. I’ll go with the mid-priced (~$50) option to save space.

Building the BeAVR on a Breadboard

To make this truly open source, I thought it best to

Sch. Part Description Mouser Part # IC1 ATmega644 556-ATMEGA644-20PU H1 Right Angle Header 517-500-01-36

Sch. Part Description SparkFun Part # Q1 Crystal 16 MHz COM-00536

S1 Mini Pushbutton COM-00097

C1,C2 22 pF Capacitor COM-08571

C3,C4,C5 100 nF Capacitor COM-08375

R1 10K ohm Resistor COM-08374

R2,R3,R4 1K ohm Resistor COM-08980 LED1 Red LED COM-00533

Breadboard PRT-09567

Hook-up Wire (22AWG) PRT-08025

Table 1. BeAVR bill of materials.

build this first on a breadboard — which is about as open as you can get — then next month, take the design to a PCB. I had all sorts of fun building the prototype on a breadboard. (And by fun I mean real pain and suffering.) Since this is open source, you too get to share in the fun.