A Real Time Clock IC (DS1307) project using the PIC micro.

Real Time Clock IC : DS1307

Although the PIC16F88 has a built in oscillator for a 32kHz watch crystal a DS1307 is easier to use on a bread board. This is because you can control the layout of the circuit more easily.

The RTC also makes the software easier as it takes care of all calendar functions; accounting for leap years etc.

The DS1307 (RTC) Real Time Clock IC (an I2C RTC) is an 8 pin device using an I2C interface (although the data sheet does not mention I2C to avoid royalty payments!). It has 8 read/write registers that store the following information:

The last address 0x08 is the CONTROL address and it determines what is generated at the SQW/OUT pin. You can control the level directly via I2C or set it to 1Hz, 4096Hz, 8192Hz, or 32768kHz. In this software it is set to 1Hz and used to drive an LED that can be used as a back light for the 4×7-segment module (if you shine a light through the module you’ll see the two central holes (like a colon character) that are between the left and right sets of two 7-segments. This is usually used to flash seconds so placing the LED behind this will achieve that operation.

In the same way as the I2C pins you need to add a pull-up to V+ at the SQW/OUT pin to see any output signal as it is an open drain output! or as in this circuit, an LED and 470R resistor are connected in series and to the +5V power. The other end goes to the SWQ/OUT pin of the DS1307.

Real Time Clock IC : Embedded control bits

There are two specific ‘gotcha’ type controls embedded in the addresses which make using the chip slightly more complicated.

Real Time Clock IC DS1307 : Clock halt / Minutes and Seconds register

The most important is the Clock Halt Bit (CH) which is bit 7 of address 0. This is the register that controls ‘seconds’ and the CH bit has to be preserved otherwise the chip stops the clock. Writing zero to this bit resets the CH bit so that the clock runs.

Note: You have to reset the CH bit to zero to let the chip operate!

Warning: The default state of the DS1307 is undefined
so you must clear the CH bit to start the oscillator.

In general you should leave this bit at zero and only set it if you have to. This bit is contained within register zero which is also the “minutes” and “seconds” register. In general keep this bit at zero unless you are updating the seconds part of the register (you don’t want the seconds changing while you are editing them).

Real Time Clock IC DS1307 : 24/12 Hour control

The second is the 24/12 hour control which is bit 6 of address 2. It is set high for 12 hour mode and low for 24 hour mode. In this project it is set low for 24 hour mode.

The problem with these two bits is that you have to preserve them when accessing the registers to write data and ignore them when reading out values for display. Its not a big problem and you can see how it’s done when you look at the code (see function edit_DS1307() and the 1st 2 case statements for address 0 (CH) and 2 (12H/24H) ).

Real Time Clock IC : 32kHz oscillator

Surprisingly making an accurate 32kHz oscillator is a difficult task (much more than a high speed oscillator e.g. a MHz crystal oscillator). This is because low speed oscillator drivers are designed for low power operation. That means high impedance and therefore low current which makes the driver extremely sensitive to noise (or any nearby signals which can capacitively couple to the crystal wire).

Using the DS1307 lets you put the crystal in the least noisy part of the board. In addition it sets the crystal load capacitance which is critical in making the crystal oscillate at exactly 32kHz – controlling its initial error i.e. for the specified ppm error value the load capacitance must be exact.

The DS1307 loads the crystal with 12.7pF so you need to buy a crystal that is defined to use this load capacitance. Circuit layout also affects the capacitance at the crystal pins so you must keep the crystal as close as possible to the chip and the tracks from crystal to chip must be short.

To ensure the crystal oscillates correctly you must ensure that :

• Crystal uses 12.7pf load capacitance (correct crystal type).
• The crystal is close to the IC.
• The tracks are short.
• The chip supply has lots of decoupling (capacitors from +5V to GND). e.g. A 100n and a 10n
• There are no signal tracks near to the crystal.
• For a pcb: It has a guard ring and a ground plane and away from digital signals.

If you are doing a board layout there is good advice in the PIC 16F88 datasheet (Timer 1 section) on crystal pcb guard rings. Dallas recommends Application note 58 which I have not read yet.

Real Time Clock IC : Power failure.

The DS1307 detects a power failure if its input voltage (Vcc) falls below (VBat) and automatically switches to the Vbat supply input (you should use a lithium 3V battery here as the backup battery). It also inhibits I2C control signals until Vcc is 1.25 x Vbat so you won’t be able to put bad data into the chip as the power is failing!