The timekeeping is done by a ds1307 I2C realtime clock and the temperature is measured with a DS18B20 1wire temperaturesensor.
An Atmega48 is used to proces the data and to drive the segments in the numitrons. I used Bascom to write the code and a MyAvr MK2 Programmer to stuff it into the microcontroller. You can find a free Bascom demo here. The only limitation of the demo version is that you can only compile 4Kbytes of code (but the atmega48 has only 4Kbytes so that will work fine.)
I added this instructable to the microcontroller contest, so give it a vote if you like it.
Step 1: The DS1307 realtime clock
For this little project we'll only use it to keep track of the time. Therefore it needs a 32.768kHz quartz crystal connected between pins 1 and 2. We can also add a battery with + to pin 3 and - to pin 4. This enables the IC to keep working when the mainpower is switched off. If you don't want to use this feature, you can just connect pin 3 to pin 4 and everything will work fine.
Pin 5 and pin 6 will be used to transfer the data to our microprocessor. They should be connected to the SCL and SDA pins on your microprocessor. These lines need to be pulled high by a 4K7 pullup resistor.
Bascom makes working with I2C devices easy. You only need to know 4 commands:
- I2cstart: This commant will startup I2c communications
- I2cstop: This command will stop I2c communications
- I2crbyte var: This command reads a byte from the device and stores it in 'var'
- I2cwbyte var: This command writes the variable 'var' to the device
The DS1307 sends and wants to receive data in BCD format. This is a variation on binary for diplays where every digit is represented by four bits. More about that here. Luckily converting from BCD to decimal and visa versa is very easy in Bascom.
- var = Makebcd(var) will convert decimal, hex and binary into BCD
- var = Makedec(var) will convert hex, binary and BCD into decimal
The data is stored on the IC in register. You can imagine them as those oldfashioned filingcabinets. Each drawer has its number and contains some info:
04H Date The H tell us that these are hexadecimal figures.
08H to 3FH Ram
If we want to read or store some data we'll first have to tell the device in which drawer we want to be. We can do this by writing the hex code for that drawer to the device. The device then will grant us acces to that drawer. After you write or read something from or to this register the device will automatically jump to the next one. So there is no need to send the location every time
Now lets put this in code:
For this code you will need to dim hours as byte, minutes as byte and seconds as byte.
First, we will set the clock:
Seconds = Makebcd(Seconds) We convert our variables into BCD format
Minutes = Makebcd(Minutes)
Hours = Makebcd(Hours)
reset hours.6 We reset bit 6 of the hoursbyte to make sure that
our Clock runs in 24h modus. If bit 6 is 1 then the
clock runs in 12h modus and bit 5 will then
contain the AM/PM data.
I2cwbyte &HD0 We tell the device that we want to write a byte
2cwbyte &H00 We start at the register for seconds hex 00
I2cwbyte Seconds Adding seconds
I2cwbyte Minutes Adding minutes
I2cwbyte Hours Adding hours
Now our clock is set! Lets read from it now.
I2cwbyte &HD0 We tell the device that we want to write a byte.
I2cwbyte &H00 We ask the device to go to the seconds register.
I2cwbyte &HD1 We tell the device that we want to read bytes.
I2crbyte Seconds , Ack We read the data and acknowledge that we want
to read the next byte too.
I2crbyte Minutes , Ack
I2crbyte Hours , Nack We don't ackowledge here so the device knows
that we are done reading.
Hours = Hours And &B00111111 We remove bits 6 and 7 as they contain other
data. If you are in 12h modus, then you need to
remove bit 5 too
Hours = Makedec(Hours) We convert back to decimal format.
Minutes = Makedec(minutes)
Seconds = Makedec(seconds)
Now we know what time it is.
In the next step we will take a closer look at the DS18B20.