Countdown timer for a PhotoBox

One day a colleague from our Laser department (YES we have Lasers!) stood next to my desk and started to ask some suspicious questions about 7-Segment displays and how to control them with an Arduino.

In general its not a problem, I like to share my knowledge about electronics and programming etc., but I hate it to answer questions without knowing the circumstances. So I asked him what the target of the project would be and he said he wanted to program a countdown timer and that he also has not really the time to do it. He also told ne that the project is not for him, its for Murat (Also a guy from the Laser department).

Later that day I went to Murat and asked him about the details of the project and what the main goal would be. He told me that he want to build a PhotoBox for his sisters wedding. I told him that if it is for his sisters wedding it should not only be some 7-Segment displays. He would need something which looks much fancier with more animation and special effects than a 7-Segment display is able to provide, and I showed him my work that I have done with the 3208 lattice clock and that something similar could be done for a countdown display.

He said he would like to have something that does the following:

• Push a button
• Countdown starts
• End of the countdown a Relay is triggered

On that evening I went home and started to dig out my spare 3208 Lattice Clock module and a relay board. I soldered some connectors for power supply, relay output and programming to the board, and started to play with it. After some hours I came up with the following animation:

In the video itself it is not very clear, but the sequence is as follows:

• Press RESET or reconnect the power to start
• Dim up the first number in the middle of the screen
• Shift the number out to the left while the new number comes from the right
• When the 0 is reached then dim it out
• Actuate the relay output for one second
• Wait in idle until RESET or re-power

The complete code including the control for the HT1632C is in one single Arduino sketch with less than 300 lines of code. I apologize for the German comments in the code But it was quick and dirty written. The code can be found in my GitHub repository. 

So far my work was done for this project. Murat also sent me some pictures of the finished project where he put it all together into a nice box with a tablet as picture preview.

Sadly Instagram changed their logo recently.

3208Clock Part 4: Get started with the HT1632C display driver

There are several ways to get started with the HT1632C display driver. One attempt, and this was also my first step, is to check on Google if someone has already written a driver or a library for the HT1632C. And yes, there are a lot of them.

At first I was playing around with the Arduino library from Adafruit. After some modifications I was able to write something to the display.

A Picture from one my first tests with the Adafruit lib

I also figured out that an update of the complete display (from the frame buffer into the HT1632C display driver) took 8.3ms and was only done with a clock speed of approx 35kHz. Please keep in mind that the ATmega8 of the 3208Clock is running at only 8MHz.

Yellow: Chip Select (CS)

Blue: Clock (WR)

A closer look also showed that the duration of the clock cycles is not equal during the transmission. The frequency of the clock is increasing which means that the time for one period of the clock cycle decreases during the transmission of a 16bit value from 33.6µs to 24.8µs.

Yellow: Chip Select (CS)

Blue: Clock (WR)

Yellow: Chip Select (CS)

Blue: Clock (WR)

At this point I wanted to know 2 things.

1. Why is the duration of the clock cycles not constant during the transmission?
2. Why is there only a clock frequency of approx 35 kHz (Datasheet says on page 4 1MHz is ok) which causes a update time for the entire display of 8.3ms?

Investigations and benchmarks

At first I figured out that HT1632::writeScreen() and HT1632::writedata(uint16_t d, uint8_t bits) are the functions of the Adafruit library which take care for the transmission. To have something which makes it easier to investigate I have prepared some small Arduino sketches where I only implemented the corresponding functions.

writedata(uint16_t d, uint8_t bits)

In this function the bit banging except the selection of the chip (CS) is done.

void writedata(uint16_t d, uint8_t bits)
{
  pinMode(_data, OUTPUT);
  for (uint8_t i=bits; i > 0; i--) {
    digitalWrite(_wr, LOW);
    if (d & _BV(i-1)) { digitalWrite(_data, HIGH); }
    else { digitalWrite(_data, LOW); }
    digitalWrite(_wr, HIGH);
  }
  pinMode(_data, INPUT);
}

 Answer for Question No 1

In the line "if (d & _BV(i-1))" happens the magic which causes that the clock cycles are varying during the transmission.

_BV(i-1) is the same like (1<<(i-1)), i is the iterator of the for-loop which is initialized with the number of bits. In the worst case the the iterator i is initialized with 16 which causes that in the check for the if-statement a "left shift operation" is executed 15 times. Because that i gets smaller every bit it has to do less and less shift operations to check the if-statement. And this causes that the clock cycle is not equal over the transmission of multiple bits.

I have fixed the problem with the clock speed by the following code modification:

void writedata(uint16_t d, uint8_t bits)
{
  uint16_t compareBit = 0;
  // 0x8000 -> set MSB
  compareBit = bits == 16 ? 0x8000 : compareBit |= (1<<(bits-1)); 
  
  pinMode(_data, OUTPUT);
  for (uint8_t i=bits; i > 0; i--)
  {
    digitalWrite(_wr, LOW);
    if (d & compareBit) { digitalWrite(_data, HIGH); } // 1
    else { digitalWrite(_data, LOW); } // 0
    compareBit = compareBit >> 1;
    digitalWrite(_wr, HIGH);
  }
  pinMode(_data, INPUT);
}

At the beginning of the function I have made a new variable for the comparison if the DATA pin has to be set. These are some lines more code but in this case the bit shift is only executed once in the for-loop. I also set the compareBit variable in a clever way because the most data packages are 16bit in this case the variable is preset with 0x8000 which is nothing else than setting the MSB true. Otherwise we have a repetitive shift operation for the amount of bits that have to be sent.


After this modification the frequency of the clock has been increased to approx 41.5kHz and the duration for a complete screen refresh is now 6.9ms this is 83% of the time it took before the modification.

Yellow: Chip Select (CS)

Blue: Clock (WR)

 

Yellow: Chip Select (CS)

Blue: Clock (WR)

Answer for Question No 2

After the initial improvement of the clock speed to 41.5kHz it still felt to slow for me. So I have removed all the Arduino specific stuff from the corresponding functions and replaced it by native "avr gcc" code.

void writedata(uint16_t d, uint8_t bits)
{
  uint16_t compareBit = 0;
  // 0x8000 -> set MSB
  compareBit = bits == 16 ? 0x8000 : compareBit |= (1<<(bits-1));
  
  DDRB |= (1<<DATA);
  for (uint8_t i=bits; i > 0; i--)
  {
    PORTB &= ~(1<<WR);
    if (d & compareBit) { PORTB |= (1<<DATA); } // 1
    else { PORTB &= ~(1<<DATA); } // 0
    compareBit = compareBit >> 1;
    PORTB |= (1<<WR);
  }
  DDRB &= ~(1<<DATA);
}

I was not aware that the speed improvement would be factor 10!

Yellow: Chip Select (CS)

Blue: Clock (WR)

Yellow: Chip Select (CS)

Blue: Clock (WR)

All three sketches can be downloaded from my GitHub repository.
Slow is the original code, medium is the code with the bugfix for the varying clock cycle and fast is the code for with all improvements.

3208Clock Part 3: Implement the board into the Arduino IDE

General overview

In this post we take a closer look into the configuration of the Arduino IDE and we also upload the first test sketch to the 3208Clock.

UPDATE:

24. Jul.2016:

I figured out that the following descriptions does not work anymore for the actual Arduino IDEs (> 1.5.x). I have no clue how its done now and i assume its a similar way to implement the new board but at the moment i dont have the time to do it.

Add the new board to the configuration files of the Arduino IDE

There is a configuration file called boards.txt in the program folder of the Arduino IDE. It can be found here .../arduino-1.0.5/hardware/arduino/boards.txt. Open this file in a text editor and add the following lines at the end of the file and save the file. Then start/restart the Arduino IDE. Now there is an entry called "Lattice Clock w/ ATmega8" in the Arduino IDE which can be found in the menu Tools - Boards.

atmega8.name=Lattice Clock w/ ATmega8
atmega8.upload.maximum_size=8192
atmega8.bootloader.low_fuses=0xe4
atmega8.bootloader.high_fuses=0xc9
atmega8.build.mcu=atmega8
atmega8.build.f_cpu=8000000L
atmega8.build.core=arduino
atmega8.build.variant=standard

The entries in the boards.txt are more or less self explaining but we still want to venture a further look on the parameters.

atmega8.name=Lattice Clock w/ ATmega8

With this parameter the name which will be shown in the Arduino IDE under the boards section can be set. In this case its "Lattice Clock w/ ATmega8"

atmega8.upload.maximum_size=8192

This parameter sets the maximum upload size of the hex-file. in this case we can use the complete 8192 bytes (8kB) of the ATmega8L program space. It is not necessary to install a boot loader on the device because the ICSP connection will be used.

atmega8.bootloader.low_fuses=0xe4
atmega8.bootloader.high_fuses=0xc9

With this two parameters the two bytes of the fuses will be set. The fuses are the internal configuration of the Atmel chips.
With this fuse settings the reservation for the boot loader in the flash space is disabled and it is also set to a clock frequency of 8MHz from the internal oscillator.

IMPORTANT! If you don't know exactly what you are doing with the fuse settings then it is very easy to reduce the usage of the device to a paperweight. Changes are at your own risk.

atmega8.build.mcu=atmega8

This parameter tells the compiler of the Arduino IDE which micro controller is used.

atmega8.build.f_cpu=8000000L

This parameter sets the information of the clock speed (in this case 8000000Hz or 8MHz). This parameter is used by the compiler to calculate some constants which will be used by some of the internal hardware of the ATmega e.g. usart / serial connection.

atmega8.build.core=arduino
atmega8.build.variant=standard

I'm not sure what this two parameters are doing. If someone has an idea please leave me a message in the comments.

Burn the fuses

Select the board "Lattice Clock w/ ATmega8" in the boards menu and select the programmer (If you use an Arduino as a programmer then select ArduinoISP). Both can be found in the Tools menu.

IMPORTANT! If the wrong board is selected for example the Arduino NG or older w/ ATmega8 then you will brick the 3208Clock because the fuses for this board configure the clock source to external 16MHz quartz. The 3208Clock does not have an external 16MHz quartz.

After the correct programmer and board has been selected go again to the tool menu and click on Burn Bootloader. In our case it only flashes the fuses and erases the chip.

Build a "Test LED" and flash the test sketch to the device

I guess a picture says more than thousand words. I have used a 1kOhm resistor. Everything between 330 and 1000 Ohm will be fine. Also the selection of the LED is not critical any common LED which emits visible light will fit. It is also not necessary to solder the LED to the board for the first test it only needs to be plugged through the holes of the DS18x20 footprint.

If the LED is prepared so far then please download the test sketch from my github repository or use the blink sketch from the examples and change the ledPin from 13 to 15.

To burn the test sketch to the 3208Clock you must hold the shift key while clicking the upload button. Otherwise the Arduino IDE tries to use the serial connection to upload the sketch which will cause an error message.


In the next posts we will take a closer look on how to get in contact with the Holtec HT1632C display driver to get something shown on the display.

3208Clock Part 2: Prepare the Hardware to get connected with the Arduino IDE

Hardware connection

The 3208Clock has of cause no Arduino boot loader installed and it also has no serial or USB connection on pin headers. For this reason it is only possible to program the device over the ICSP connector with an programmer like an AVR ISPmk2.

Don't be scared. If you have an Arduino (I only tested it with the UNO but it should also work with older versions) then you also have an ISP Programmer. Just flash the ArduinoISP sketch which is delivered with the examples of the Arduino IDE to your Arduino.

Here are the parts that are needed to make the connection.

• 1 Arduino (in this case its an Arduino UNO others should work as well)
• 6 Jumper wires female to male
• 1 2*3 pin header OR I would recommend a boxed header

The pin header / boxed header must be soldered to the 3208Clock board. To make sure that the boxed header is mounted in the correct direction take a closer look on the picture below.

And here is a picture with the finished setup. Please double check the connections before you power on the Arduino programmer just to make sure that the Arduino and the 3208Clock is still alive after our first try.

If there is already an ISP programmer by hand then of cause this can be used. I personally use a USBprog which works as an AVR ISPmkII clone.

In the next post I will show a closer look into the configuration of the Arduino IDE. Especially the way how a new device can be added so that it can be selected under Tools - Board in the Arduino IDE.

The modified 3208Clock Part 1 the beginning

I stumbled upon this lattice clock while I was surfing through the products of one of this Chinese Internet sellers with worldwide free shipping. I thought "OK, 12$ (~9,0 EUR) what can I loose?" So I've ordered one of this devices.

I have stolen this Picture from www.FastTech.com

After only four weeks the package has arrived. My first thought was to quickly unwrap it and connect it to power to see what it is doing...

...BÄÄM! INSTANT KILL!
It took me $12.30 and a lot of patience (waiting for delivery) to learn that this device can not be powered directly by 12V DC. After the magic smoke has disappeared and I have turned the PCB I have seen that this device is labeled with DC 5V

From this setback, I did not let me discourage and I ordered a new unit. During the delivery time for the new device I had some time to do some reverse engineering. So I unmounted all the parts which hide the direct view to the PCB.

The contents of the 3208Clock

This device consists of an ATmega8L which is the brain of the Clock and a Holtec HT1632C Display controller paired with four 8x8 LED segments.

Optional components

During my investigation I have discovered that the PCB was in general developed in a very clever way. The following parts can be retrofitted to the board.

• Mini USB port
• TSOP infrared receiver
• DS18x20 1-Wire temperature sensor / LM35 temperature sensor
• Bi-color LED (color changing by reversing polarity)
• 6-Pin ICSP header to reprogram the ATmega8L
• Buzzer/Speaker (can not be mounted if ICSP header is mounted)
• On the backside is a place to mount a RTC IC and a coin cell battery to power the clock IC if the main power is not connected.

But due to the Problem that the ATmega8L has only 8KB of program space I'm not sure if all of the above components can be implemented into the software.

PCB Overview

The most interesting part seems to be the right part of the front side. Here are a lot of footprints of the unpopulated optional parts which can be used to interact with the real world.

I have prepared a picture which shows which pins of the ATmega8L are connected to the footprints. The numbers are the Arduino pin numbers. I was to lazy to also write the original port pin numbers like "PB3" to the Picture. But here you can see a very good pinout diagram made by Pighixxx. I know its for a ATmega168 but the Pinout is more or less compatible.

Here is a picture of the backside of the PCB. We see here that three ICs can be mounted to the board. From left to right we have at first the footprint for the ATmega8L with its TQFP32 package. The next ones are the footprint for a battery and a SO16 IC where a RTC can be mounted.

The last one is the Footprint 52 pin QFP footprint for the Holtec HT1632C driver IC for the 8x8 LED displays.

I also have made a picture with some additional descriptions for some pins. During my reverse engineering I also painted some pads red to make it easier for me to track the +5V line. Also take a look at the top left of the picture there is a very important information written which I have ignored at my first try.

My actual setup

The only parts from the optional components which I have installed are the ICSP header for programming the device and a bi-color LED which was laying around for debugging purposes.

With the Setup in the picture above I'm now able to send a zero terminated string to the device over Bluetooth which will then scroll through the display. Here is a short video which I have made.

The next steps

In the next posts I will show a more insight view about the single steps which I have made to get this device up and running with the Arduino IDE and a couple of libraries. I also want to upload a well commented version the code to GitHub so that everyone can get started with playing around with this cheap and versatile device.

I'm not sure which topic the next posts will have but they will be more or less related to the 3208Clock device. It is also possible to post requests for additional informations of specific parts of this device or the procedure of hacking it to the comments.