Nuts and Volts - September 2015

with this project, but when we start getting into low power projects this becomes critical.

One of the disadvantages to using the MCP79400 is that it doesn’t come in a nice breadboard-friendly DIP package — meaning that you need a breakout board. This was a compromise I was prepared to make based on a combination of the price and the voltage range — not many other RTCs offer the range at the price (for example, the very popular Maxim DS1307 only operates at 5V). Additionally, a breakout board for an RTC is a pretty useful thing — you’ll see that the RTC needs a number of components which can be nicely wrapped up into a breakout board.

Figure 6: Breadboard build: the EEPROM module.

Onto the connections! Again, the SDA and SCL lines have pull-up resistors; although this time, they’re 2.2K resistors. If you take a look at the datasheet, you’ll see that a range of pull-ups will work, although do affect the possible operating speed. However, this is a longer discussion for another time!

Then Include It

Just as it is good coding practice to separate your code out into modular functions, it is good practice to group similar functions into distinct files. For example, in this article’s project, there are code files containing functions relating to UART communication, accessing the EEPROM, communicating over I2C, etc. These files have been given meaningful names that reflect their purpose.

A watch crystal/tuning fork crystal (i.e., rated at 32.768 kHz) is connected to X1 and X2. This is needed to provide accurate timekeeping, along with a couple of 8. 2 pF capacitors — similar to the configuration for the crystal on your main microcontroller. VBAT is for the backup battery, but here is attached to GND as we don’t have one connected. Finally, the MFP (multi-function pin) is kept high with a 10K pull-up resistor — this pin can be used to indicate when alarms are triggered, to output a square wave (let’s say you want an LED to flash as a second indicator), or even be an additional GPIO.

files.

Separate Your Code,

I’d be doing a poor job if I tried to get into a discussion on the best coding practices for your projects — this is a whole field in itself and not one that I can claim to be an expert in; a search on the Internet will throw up a number of academic and practical books on the subject. However, for our purposes, I’m comfortable that we’ll see the benefits as enthusiasts. Separate files mean that it’s easier to find specific functions without wading through a single lengthy program file. By testing a file completely and making sure it can stand alone, you’re able to use this in other projects without re-testing. You know that your (for example) UART functions are sound and can be included in any future projects for “plug-in” functionality.

Whew! That was a quick overview!

Changing Gears

We’ve covered some of the theory and the physical layout. Now, you can put down your breadboard and fire up Atmel Studio. Let’s get coding!

Project Structure

Download the project from the article link and open it up in

You’ll have noticed that there are two files: a header file (e.g., “UART.h”) and a source file (e.g., “UART.c”). The header file should not include any functioning code — only definitions, macros, function prototypes, etc. The source file needs to “include” the header file — for example, “UART.c” has a statement at the start:

Atmel Studio. Then, take a look at the solution explorer tree; you’ll see something like Figure 7.

You probably noticed that there are more files than before: I2C.c, I2C.h, UART.c, and so on.

#include "uart.h"

As this project is getting large,

This brings the header file and its definitions into the source code file.

Additionally, all the header files need to be included in the main code file. Look at the start of the main “c” file for the project and you’ll see all the “#include” statements there.

We’ll probably touch on header files at a later stage, but this will give you enough background to get started.

I’ve separated out various groupings of functionality into separate files — this makes our lives a great deal easier. Look at the sidebar, “Separate Your