As the title suggests, the clock is powered via a usb cable. When you disconnect the clock or turn off the computer, it will keep going on its battery but it won´t show the time until you reconnect it or switch you PC on again.
For this project, I made my own PCB´s. I have only made 3 PCB´s yet and all in the last 14 days so I am really proud oft them. Any suggestions about those are very welcome!
A small video (the flickering of the LEDS in due to the camera).
Step 1: What do you need:
- 20 x LEDS: Any color you fancy
- 2 x pushbuttons
- 6 x 100Ohm resistors
- 2 x 1.5K resistors
- 2 x 4.7K resistors
- an Atmega 8
- a DS1307 real time clock IC
- a crystal 32.768K
- a USB connector
- A 3V battery + holder ( I used a CR2032)
- a 13pin male header (L-shaped)
- Soldering Iron and solder
Step 2: Schematics and PCB's
The 2 most important things located on the main board are the microcontroller and the clock. Those 2 are connected to each other via an I2C link (don't forget the 4.7K pull-up resistors on the 2 wires).
The clock also needs a crystal (32.768K) on pins 1 and 2 and a battery on pins 3(+) and 4(-).
The 2 other resistors are 1.5K pull-up resistors for the buttons. They are connected to int0 and int1 on the atmega8 (pins 5 and 6).
The USB connector is only there to provide power, so we only need the VCC (pin 1) and GND (pin 4).
The display contains 20 leds, 6 resistors (100Ohm) and 2 buttons. I arranged the leds in 6 colums instead of 3 so that each digit of the decimal number is represented by its binary equivalent instead of the entiry number in a single binary value. I find it easier to read quickly and it gave me an extra challenge while programming.
The 2 buttons are used to set the time. One button for the hours and one for the minutes. Their pull-up resistors are located on the main board.
I added the schematics of the 2 PCB's here, as well as their layout. I also added 2 pdf-files with the layout for both PCB's, so that you can print them out on a transparent to make your own PCB's.
Step 3: Programming
When power is switched on the microcontroller reads the clock and makes sure that the clock is running by setting bit 7 of register 0 (oscilloscope enable bit) to 0. As the datasheet suggests, this is needed because the initial power-on state of the registers is not defined.
Once the program enters the main loop, it will read the clock again and convert the 3 decimal numbers (hours, minits, seconds) into 6 colums.
I used following calculation(see photo for the colums):
value / 10 (disgard the decimals) gives you colum 2
value - (colum2*10) gives you colum 1
value / 10 (disgard the decimals) gives you colum 4
value - (colum2*10) gives you colum 3
value / 10 (disgard the decimals) gives you colum 6
value - (colum2*10) gives you colum 5
When done, the colum values are passed on to port C according to the colum that is enables on port B. This multiplexing enables us to use less pins and keeps the current lower than 100mA.
After this the loop restarts
When a button is pressed, the corresponding interrupt is called and the hours or minits are increased by 1 accordingly. The clock is set immediately after increasing a value via a set subroutine.
I added a hexfile with the program.