M48T512Y-85PM1 STMICROELECTRONICS [STMicroelectronics], M48T512Y-85PM1 Datasheet - Page 12

• Manufacturer Part Number: M48T512Y-85PM1
• Manufacturer: STMICROELECTRONICS [STMicroelectronics]

Clock operations
3.4
12/23
Table 5.
Keys:
Calibrating the clock
The M48T512Y/V is driven by a quartz controlled oscillator with a nominal frequency of
32,768 Hz. The devices are factory calibrated at 25 °C and tested for accuracy. Clock
accuracy will not exceed 35 ppm (parts per million) oscillator frequency 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. The M48T512Y/V design employs periodic counter
correction. The calibration circuit adds or subtracts counts from the oscillator divider circuit
at the divide by 256 stage (see
The number of times pulses are blanked (subtracted, negative calibration) or split (added,
positive calibration) depends upon the value loaded into the five calibration bits found in the
control register. Adding counts speeds the clock up, subtracting counts slows the clock
down. The calibration bits occupy the five lower order bits (D4-D0) in the control register
7FFF8h. 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 calibration, '0' indicates negative calibration. Calibration
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 loaded, the first 12 will be affected, and so on. Therefore, 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 calibration step in
the calibration register.
Assuming that the oscillator is running at exactly 32,768 Hz, 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.
Address
7FFFEh
7FFFDh
7FFFCh
7FFFBh
7FFFFh
7FFFAh
7FFF9h
7FFF8h
S = SIGN bit
R = READ bit
W = WRITE bit
ST = STOP bit
0 = Must be set to '0'
D7
ST
Register map
W
0
0
0
0
0
D6
R
0
0
0
10 years
0
10 seconds
10 minutes
D5
S
0
0
10 hours
10 date
Doc ID 5747 Rev 6
Figure 8 on page
10 M
D4
0
Data
D3
0
Calibration
D2
Seconds
Minutes
Month
Hours
13).
Date
Year
Day
D1
D0
Seconds
Minutes
M48T512Y, M48T512V
Control
Month
Hours
Date
Year
Day
Function/range
BCD format
00-99
01-12
01-31
01-07
00-23
00-59
00-59