After a few months of development, it’s finally here! The QR clock!

Hey Hef, ask me what time it is.

What the hell?

Right?  This is an idea I had at some point along my four day drive across the country.  QR codes are very popular for some reason.  Marketers insist that they’re useful and demand to put them in everything despite the fact that nobody uses them.

The real problem with QR codes is that they force people to use a machine to translate what could easily be human-readable information.  Rather than a QR code, why not just show the text “Chevy.com” or even better just the word “Chevy” because everyone knows how to google stuff?  Seriously, I’ve seen QR codes on highway billboard signs as if anyone is going to whip out their smartphone while they’re pulling 65.

So why a clock?  As I’ve said before, clocks have already sort of been perfected.  A digital wristwatch is probably the most convenient, accurate, and durable way to tell the time possible.  Any attempt to change the typical blue-light special wristwatch will only make it more expensive or harder to read.  This clock attempts to do both.

Also, this clock points out the ultimate irony of QR codes which is that they are a technological convenience that really isn’t convenient.  In order to read this clock, the user will require some sort of QR scanning device which is guaranteed to have a time-telling function built into it already.

So, that’s why I made it.  Hilarious, right?

Overview

The goal here was to build an attractive self-contained desk or wall clock that can display the current time as a QR code.  This presented a challenge as there are 1440 minutes in a day, and it would take a substantial amount of memory to store that many QR codes.  Because of this, my device had to generate its own QR codes on the fly.

Hardware

Despite this project being the most intricate circuit board I’ve made so far, it probably has one of the most straight forward schematics.

I know it looks pretty complicated, but it’s really just a bunch of repitition.

PCBs

This project was my largest PCB layout to date (unprofessionally, of course).  It was so large in fact that it exceeded the limitations of the free version of Eagle.  I ended up purchasing a license for the full version to finish the job.

Given how incredibly busy and large this board is, I decided it was an appropriate occasion to get the board properly manufactured.  In the past, I’ve used Advanced Circuits which is a fab house based in the ‘States.  Their turnaround is fast and reliable, but their prices are a little high.

This time I chose Myro, a Chinese-based fab house.  Their customer service was quick and competent, and they even managed to squeeze in an update to the silkscreen labeling that I sent them a day after the original order.

The setup fee ran me 4.11, so I ordered 5 to be safe.  They are beautiful:

The whole bill with shipping ran me

   

225.  If you are really interested, you can do what I did and find access to it at a local university’s engineering library.

Thumbing through this spec, I found diagrams such as this one:

and decided that it was a waste of everyone’s time to continue.  If you are really interested in writing a QR code generator, I will give you the source code for the QR generation portion of my firmware:

main.c

When compiled and run in a terminal, this program will take up to a 16 character input (in all caps, that’s all this type of QR code supports) and output a valid QR code along with a few of the steps along the way.  Read the Thonky tutorial for details.

I will warn you that I am not a coder, and this code is incredibly ugly.  A large portion of the QR generation process involves using a left-justified binary string and I found it easier to code a way to make that happen than to do the mental gymnastics of turning LSB into MSB.  I’m sure it can be improved and optimized.  I’m really not concerned with that.

It’s still a good resource though as it will let you check each step of your QR code generator against a working one.  One of the challenges I came across in making my generator is that it was difficult to see where a problem was as just about every QR code generator only shows you the final result.  Thonky’s tutorial has the steps laid out, but only for the input “HELLO WORLD”.  I found that for a while, that particular input worked with my generator, but a number of bugs were messing up other inputs.

For example, one bug made it so that the phrase “HI LAUREN ” would encode a valid QR, but “HI LAUREN” would not.  QR does a good job of scrambling the input, so two very similar inputs can produce very different outputs.

If you find one of these types of errors in my code, please let me know.

This (very unoptimized) QR generator takes approximately seven seconds to complete on the ATMega328 running at 8MHz (10 seconds if I’m refreshing the display at the same time).  Fortunately, that’s less than a minute, so I’m okay, but damn that is a long time for an AVR.

Everything that isn’t QR related

If you read this blog often, you’ll note that I try to do as few things in software as possible.  Because the QR code generator sort of required a large chunk of software, I made sure to do just about everything else in hardware.

Dimming

To keep my clock from being a distraction, I incorporated an automatic dimming feature.  A phototransistor is used as a pull down on one of the ADC lines.  This ADC value is read in on regular intervals (whenever the display is refreshed) and an optimal brightness is determined using a simple lookup table.  To save time, I set the ADC to start its measurement at the end of a refresh operation, and collect the values at the beginning.  In this way, it can do its busy work without taking valuable time away from the QR code generation in the main loop.

The brightness is controlled by flicking the ~OE pin high and low quickly and varying the duty cycle.  The ATMega328 has an hardware PWM generator function which can be made to directly control an output pin (OC0A).  I made sure to connect ~OE to this pin.

In my configuration, all I had to do was start up Timer0 in the appropriate mode and let it go.  Timer0 starts from zero with the ~OE pin pulled low (LEDs on) and counts up.  When its value equals the value stored in the OCR0A register, it pulls ~OE high and turns the LEDs off.  When it gets to 255, it restarts.   Without another single line of code, it maintains the duty cycle on the ~OE pin and maintains the display brightness.  To change the brightness, I just had to change the value in the OCR0A register.

Refreshing

Timer1 handles refreshing the display.  Originally, I was just going to run Timer1 in an overflow mode so that it operates on regular intervals.  Unfortunately, TImer1 is a 16 bit timer, and a 16 bit timer takes too long to count up and overflow.

I ended up using an output compare mode.  Every time Timer1 reaches the value stored in OCR1A, it refreshes the display.  I can change the framerate by simply changing this register value.

Refreshing the display is a fairly complicated process as a function has to grab some values from a special frame buffer array, punch them out the shift registers, and switch PB0-2 to the appropriate values to select the right row.

I had an issue at one point where it turned out that there was a very brief period in time when the shift registers were updated, but PB0-2 were not.  The results was a faint “ghosting” effect on the display. I ended up coding it to turn off the LEDs while this update is occurring so that they can all be turned on once the change is complete.

The 16 bit overflow delay thing did give me a funny idea though. If I really wanted to mess with people, I could lower the refresh rate to the point where digital cameras can’t read it. That way, humans can see the detail of the clock, but can’t translate it, and computers can translate it, but can’t see it.

I crack me up.

Note: Despite what Hack-a-Day said, the final version of the firmware makes the clock perfectly readable by most QR code readers as demonstrated in the video below.

Timing

Timer2 on the ATMega328 has a special feature that allows it to be run asynchronously which means that it’s actually run off a different clock from the rest of the micro controller’s operations.  The designer has the option to use an external square wave or an external crystal.  In the latter mode, the pins are specifically tuned for operation with a 32.768kHz crystal.  This frequency is very convenient for use in digital timekeeping systems as 2^15 ticks equals one second.

I set up Timer2 in an overflow mode which means that my interrupt code will run every 256 ticks.  In addition to this, I set up Timer2 with a clk/256 prescaler which combined with the overflow means that it will only run its interrupt code every 65,535 ticks which at 32.768kHz works out to once every two seconds.  Seeing how I don’t need any granularity under 60 seconds, having the interrupt run only every two frees up a bunch of time for the all-important QR generator to do its magic.

Working with Timer2 in asynchronous mode is odd.  Any attempt to write to a Timer2 register only takes effect at the next clock cycle which given how slow 32kHz is compared to the 8MHz core clock can take a “while”.  Because of this, the firmware provides you with flags that let you know when the registers have been written.  You also have to be very careful as Timer2 registers can easily become corrupted if you do things out of order or try to put the micro controller to sleep.

When enough ticks have gone by to add up to a full minute, I have a small snippet of interrupt code that sets a flag.  This flag gets picked up in the main loop and tells the program to output the currently stored QR code.  Because it takes 7-10 seconds to generate each code, I have it generate it for the next minute at the start of the current minute and then hold on to it until the time is right.

Setting the Time

Because QR codes are unreadable, and the display takes 7-10 seconds to create a new QR code, I quickly realized that setting the time on this thing was going to be a real hassle.

My solution was to start up with a simple time setting mode.  The time is displayed in a simple human-readable format on the screen and modified by hitting the the buttons.  One increments minutes and the other hours.

Every time a button is pressed, Timer2 (and the Timer2 prescaler) is reset.  This makes it so that the following minute starts at the exact moment the user presses a button.  With some coordination, this allows the user to easily synchronize this clock with another one.

After 10 seconds of non-interaction, the QR time setting mode disappears, and the QR clock takes 10 seconds to generate its first code before displaying it.  This time is only displayed for about 30 seconds though after which time it’s been a minute since the last button was pressed and the clock updates.

I wanted to find a way to have it get a head start on the code while still in the time-setting mode, but I found it impossible (read: kind of hard) to abort the QR code generating process once it started.

Demonstration

Here’s a video that hilights the QR clock’s operation:

Conclusion

I’ve put a lot of time into this project and I am extremely pleased with how it turned out.  I have a few ideas for the clock for the future.  I made sure to include RX and TX pins on the PCB that will allow me to connect a QR clock to a PC and use it as an external QR display for a number of applications.

Given that it can already generate its own codes, a simple firmware update could also add a simple calendar function.  “12:00AM 10/31/12” adds up to 16 characters which is exactly what this thing can display.

I had another idea for a more human-readable version that includes a pixelated analog clock in the center of a larger QR.  This would take advantage of the error-correction capabilities of QR as a scanner wouldn’t be bothered by the clock covering up a portion of its data (as if you would scan it after seeing the analog portion).

If you are interested in the QR clock let me know.  Unlike the bullet counter, this clock doesn’t require any specially machined parts, so mass producing them is easy.  I might try to see if I can sell a bunch to raise money for a charity.

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Project files can be found here: QR Clock v1.1

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