Real-Time Clock - Experimenting with Raspberry Pi - page 76

Pins X1 and X2
These pins are for connection to a 32.768 kHz crystal. The datasheet states that the
“internal oscillator circuitry is designed for operation with a crystal having a specified
load capacitance ( C L ) of 12.5 pF.”

Pin SQW/OUT
This is an open-drain output from the DS1307 chip that you can choose to ignore if you
like. It can be used as follows:

• A GPIO-like output. If you plan on wiring this to a Pi GPIO pin, be

sure to remove the pull-up resistor (on PCB) first. The +5 V from
the pull-up will damage the Pi.

• A square wave clock output, at one of the following programmable

frequencies:

· 1 Hz

· 4.096 kHz

· 8.192 kHz

· 32.768 kHz

The datasheet lists a current rating for output when low:

Parameter Symbol Min Typ Max Units

Logic 0 Output ( I OL = 5 mA ) V OL 0.4 Volts

Without more-specific information, we arrive at the conclusion that a given logic pin
is capable of sinking a maximum of 5 mA (SDA). While doing so, the maximum voltage
appearing at the pin is 0.4 V. This is well under the 0.8 V maximum, for the V IL voltage
level of the Raspberry Pi.
The datasheet indicates that the SQW/OUT pin is an open-drain output. As such,
you can use a pull-up to +3.3 V or +5 V as your interface requires. It could be used to drive
a small LED at 1 Hz, if the current is limited to less than 5 mA (although the datasheet
doesn't indicate the current capability of this open-drain transistor). Alternatively, it can
be used to drive other external logic with a pull-up resistor.
The datasheet indicates that the SQW/OUT pin can pulled as high as 5.5 V, even
when V CC is lower in voltage (such as +3.3 V.) This is safe, provided that these higher
voltages never connect to the Pi's GPIO pins.