M48T128Y-70PM1TR STMicroelectronics, M48T128Y-70PM1TR Datasheet - Page 12

• Manufacturer Part Number: M48T128Y-70PM1TR
• Manufacturer: STMicroelectronics

Specifications of M48T128Y-70PM1TR

• Bus Type: Parallel
• Operating Supply Voltage (typ): 5V
• Operating Supply Voltage (max): 5.5V
• Operating Supply Voltage (min): 4.5V
• Operating Temperature Classification: Commercial
• Operating Temperature (max): 70C
• Operating Temperature (min): 0C
• Pin Count: 32
• Mounting: Through Hole
• Date Format: DW:DM:M:Y
• Time Format: HH:MM:SS
• Lead Free Status / RoHS Status: Compliant

M48T128Y, M48T128V*
Calibrating the Clock
The M48T128Y/V is driven by a quartz controlled
oscillator with a nominal frequency of 32,768Hz.
The devices are factory calibrated at 25°C and
tested for accuracy. Clock accuracy will not ex-
ceed 35 ppm (parts per million) oscillator frequen-
cy error at 25°C, which equates to about ±1.53
minutes per month. When the Calibration circuit is
properly employed, accuracy improves to better
than +1/–2 ppm at 25°C. The oscillation rate of
crystals changes with temperature (see
8., page
periodic counter correction. The calibration circuit
adds or subtracts counts from the oscillator divider
circuit at the divide by 128 stage, as shown in
ure 9., page
The number of times pulses are blanked (subtract-
ed, negative calibration) or split (added, positive
calibration) depends upon the value loaded into
the five Calibration bits found in the Control Regis-
ter. Adding counts speeds the clock up, subtract-
ing counts slows the clock down. The Calibration
bits occupy the five lower order bits (D4-D0) in the
Control Register 1FFF8h. These bits can be set to
represent any value between 0 and 31 in binary
form. Bit D5 is a Sign Bit; '1' indicates positive cal-
ibration, '0' indicates negative calibration. Calibra-
tion occurs within a 64 minute cycle. The first 62
minutes in the cycle may, once per minute, have
one second either shortened by 128 or lengthened
by 256 oscillator cycles. If a binary '1' is loaded into
the register, only the first 2 minutes in the 64
minute cycle will be modified; if a binary 6 is load-
12/22
13). The M48T128Y/V design employs
13.
Figure
Fig-
ed, the first 12 will be affected, and so on. There-
fore, each calibration step has the effect of adding
512 or subtracting 256 oscillator cycles for every
125, 829, 120 actual oscillator cycles, that is
+4.068 or –2.034 ppm of adjustment per calibra-
tion step in the calibration register. Assuming that
the oscillator is running at exactly 32,768Hz, each
of the 31 increments in the Calibration byte would
represent +10.7 or –5.35 seconds per month
which corresponds to a total range of +5.5 or –2.75
minutes per month.
One method is available for ascertaining how
much calibration a given M48T128Y/V may re-
quire. This involves setting the clock, letting it run
for a month and comparing it to a known accurate
reference and recording deviation over a fixed pe-
riod of time.
Calibration values, including the number of sec-
onds lost or gained in a given period, can be found
in the STMicroelectronics Application
“TIMEKEEPER CALIBRATION.”
This allows the designer to give the end user the
ability to calibrate the clock as the environment re-
quires, even if the final product is packaged in a
non-user serviceable enclosure. The designer
could provide a simple utility that accesses the
Calibration byte. For example, a deviation of 21
seconds slow over a period of 30 days would indi-
cate a –8 ppm oscillator frequency error, requiring
a +2(WR100010) to be loaded into the Calibration
Byte for correction.
Note,