X1235 – Preliminary Information
DESCRIPTION (continued)
X1, X2
The X1 and X2 pins are the input and output,
respectively, of an inverting amplifier. An external
32.768kHz quartz crystal is used with the X1205 to
supply a timebase for the real time clock. The
recommended crystal is a Citizen CFS206-32.768KDZF.
Internal compensation circuitry is included to form a
complete oscillator circuit. Care should be taken in the
placement of the crystal and the layout of the circuit.
Plenty of ground plane around the device and short
traces to X1 and X2 are highly recommended. See
Application section for more recommendations.
The powerful Dual Alarms can be set to any Clock/
Calendar value for a match. For instance, every
minute, every Tuesday, or 5:23 AM on March 21. The
alarms can be polled in the Status Register or provide
a hardware interrupt (IRQ Pin). There is a repeat
mode for the alarms allowing a periodic interrupt.
The device offers a backup power input pin. This
V
pin allows the device to be backed up by battery
BACK
or SuperCap. The entire X1205 device is fully
operational from 2.7 to 5.5 volts and the clock/calendar
portion of the X1205 device remains fully operational
down to 1.8 volts (Standby Mode).
Figure 1. Recommended Crystal connection
PIN DESCRIPTIONS
X1
X2
X1235
8-Pin SOIC
8-Pin TSSOP
V
SCL
SDA
1
2
V
V
BACK
X1
X2
8
7
6
5
1
2
3
4
CC
8
7
6
5
POWER CONTROL OPERATION
The power control circuit accepts a V
V
CC
BACK
V
and a V
BACK
SS
X1
X2
CC
IRQ
3
4
SCL
SDA
IRQ
input. The power control circuit powers the clock from
when V < V - 0.2V. It will switch back to
V
SS
V
BACK
CC
BACK
power the device from V when V exceeds V
.
CC
CC
BACK
NC = No internal connection
Figure 2. Power Control
Serial Clock (SCL)
V
CC
The SCL input is used to clock all data into and out of
the device. The input buffer on this pin is always active
(not gated).
Voltage
On
V
BACK
In
Off
Serial Data (SDA)
SDA is a bidirectional pin used to transfer data into and
out of the device. It has an open drain output and may
be wire ORed with other open drain or open collector
outputs.The input buffer is always active (not gated).
REAL TIME CLOCK OPERATION
The Real Time Clock (RTC) uses an external
32.768kHz quartz crystal to maintain an accurate inter-
nal representation of second, minute, hour, day, date,
month, and year. The RTC has leap-year correction.
The clock also corrects for months having fewer than
31 days and has a bit that controls 24 hour or AM/PM
format. When the X1205 powers up after the loss of
An open drain output requires the use of a pull-up
resistor. The output circuitry controls the fall time of the
output signal with the use of a slope controlled pull-
down. The circuit is designed for 400kHz 2-wire inter-
face speeds.
V
BACK
both V
and V
, the clock will not operate until at
CC
BACK
This input provides a backup supply voltage to the
least one byte is written to the clock register.
device. V
event the V
supplies power to the device in the
supply fails. This pin can be connected
BACK
CC
Reading the Real Time Clock
to a battery, a Supercap or tied to ground if not used.
The RTC is read by initiating a Read command and
specifying the address corresponding to the register of
the Real Time Clock. The RTC Registers can then be
read in a Sequential Read Mode. Since the clock runs
continuously and a read takes a finite amount of time,
there is the possibility that the clock could change during
Interrupt Output – IRQ
This is an interrupt signal output. This signal notifies a
host processor that an alarm has occurred and
requests action. It is an open drain active low output.
Characteristics subject to change without notice. 2 of 22
REV 1.0.9 8/29/02
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