Tag Archives: Atmel

Atmel ICE, JTAG and the ATmega32U4

Having spent a lot of time working with AVR microcontrollers I decided a number of years ago to invest in a dedicated In Circuit Emulator (ICE). At the time Atmel had just released their new Atmel ICE device. This seemed to fit the bill perfectly. atmel-ice_angle1024Not only did it support  traditional JTAG debugging but also debugWIRE. Atmels own propriety debug protocol aimed at low resource devices such as the ATMega88/168/328 family. I purchased the Atmel ICE basic kit. The kit consists of the Atmel ICE and a 10 pin ribbon cable to 10 to pin JTAG (0.05″ pitch) and 6 pin (0.1″ pitch) SPI connector. The full kit available at a much greater cost shipped with a number of different cables and adaptors. As debugWIRE uses the RESET line for debugging I deemed the 6 pin SPI cable would be all I needed. Or so I thought.

Moving on a few years I find myself now working with the ATmega32U4 again (I needed USB functionality) and to my surprise this device does not support debugWIRE only JTAG. What!!! How can this be?

I was wondering if Atmel sold those additional debug cables separately? Turns out they do but they are almost £50. After shelling out just shy of £100 in the first place I wasn’t prepared to spend another £50 on cables.
So I set about building my own adaptor. The main problem I faced was the Atmel have used 0.05″ (1.27mm) pitch connectors rather than the more readily available standard 0.1″ (2.54mm) pitch connectors. What I needed was some form of breakout board or even something to convert from the 0.05″ pitch connector to something more manageable. After trawling the net it turns out Adafruit have a Cortex SWD adaptor which in essence is a 0.05″ pitch to 0.1″ breakout board. The pin mapping is one to one so assuming you ignore the silk screen it should be easy to interface to the ATmega32U4. The best thing about this solution is the price, I managed to get two of these boards from a local supplier for just £3.

The development board I am currently using is an OLIMEXINO-32U4 an Arduino Leonardo type design from Olimex. The table below shows the Cortex SWD to AVR JTAG pin mapping. Also shown is the JTAG connections of the 32U4 (or Arduino Leonardo if you prefer) development board.

# SWD Pin JTAG Pin Olimex 32u4 Pin
1 Vin TCK A3 (PF4)
3 GND TDO A1 (PF6)
5 GND TMS A2 (PF5)
8 N/C N/C N/C
9 GND TDI A0 (PF7)

Below you can see a picture of my setup. I am not sure if they are needed but I did add 10K pull ups to all of the control lines TCK, TDO, TMS and TDI.


One thing that had not dawned on me was the fact JTAG is not enabled by default. The pins designated to the JTAG interface PF4-PF7 are used as analog inputs. So in order for JTAG to work you need to ensure the JTAGEN fuse is programmed. This can be done through the ISP interface in the normal manner. Once I had that programmed I could successfully download and debug my code on target. Result.

Binary Clock

For a while now I have fancied building a binary clock. A clock capable of showing the current time in binary format. Not only do they look great but you have the added bonus that most people probably wont have any idea what an earth they are looking at. For those who have no idea what a binary clock is I would highly recommend this Wikipedia page for more information.

To keep things simple I decided on a Binary Coded Decimal (BCD) format rather than pure binary or grey scale. With BCD the hours, minutes and seconds are all represented by individual binary digits. That way you simply add each digit together to determine the number of hours followed by the number of minutes and finally the number seconds.

I wanted something fairly portable, preferably USB powered and reasonably cheap. I ended up using an Arduino Nano clone, a 0.96″ 128×64 OLED display module and a DS3231 real time clock module. This configuration fitted the bill perfectly.

I love these little OLED displays. They are extremely bright and easy to use. The majority if not all of the ones I found on eBay use the SSD1306 display driver chip. The SSD1306 supports a number of interfaces formats including parallel, SPI  and I²C. The module I purchased uses an I²C bus.

The DS3231 I have used on lots of projects before. These devices are extremely accurate with minimal drift over time. Not a lot to say about these really the device maintains seconds, minutes, hours, day, date, month, and year information. Has two configurable alarms and even has an internal temperature sensor (which is used for temperature compensation of the oscillator) but can also be read via one of the internal registers. Data is transferred serially through an I²C bus.

The OLED display supports two colours. The very top of the display uses yellow on black while the remainder of the display uses blue on black. I decided to take advantage of this and alternate between the current time, date and temperature along the top while showing the current time in binary format below using rectangles to represent each binary bit. I am really pleased with the way it turned out.


The design was implemented on breadboard and all of the code written in C using the Atmel Studio development environment. Rather than continually reading the time from the DS3231 its square wave output pin was configured to output a 1Hz square wave which in turn was connected to one of the external interrupts pins on the ATMega328. Upon interrupt the interrupt handler simply sets a flag to indicate to the main exec to read the time and refresh the display. The clock has been running fine now for around four months with no issues at all. All of the project files and source code will be available in my GitHub account.