m48t512y STMicroelectronics, m48t512y Datasheet - Page 12

• Manufacturer Part Number: m48t512y
• Description: 5.0 Or 3.3v, 4 Mbit 512 Kbit X 8 Timekeeper Sram
• Manufacturer: STMicroelectronics

M48T512Y, M48T512V*
Calibrating the Clock
The M48T512Y/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
M48T512Y/V design employs periodic counter
correction. The calibration circuit adds or subtracts
counts from the oscillator divider circuit at the di-
vide by 256 stage (see
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 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 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
Figure 8. Crystal Accuracy Across Temperature
12/21
Frequency (ppm)
–100
–120
–140
–160
changes
–20
–40
–60
–80
20
0
–40
–30
with
Figure 9., page
–20
temperature.
–10
13).
0
The
10
Temperature C
F
F = -0.038
20
minute cycle will be modified; if a binary 6 is load-
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 Cali-
bration byte would represent +10.7 or –5.35 sec-
onds per month which corresponds to a total range
of +5.5 or –2.75 minutes per month.
One method for ascertaining how much calibration
a given M48T512Y/V may require involves setting
the clock, letting it run for a month and comparing
it to a known accurate reference and recording de-
viation over a fixed period of time.
Calibration values, including the number of sec-
onds lost or gained in a given period, can be found
in STMicroelectronics Application Note AN934,
TIMEKEEPER Calibration.” This allows the de-
signer to give the end user the ability to calibrate
the clock as the environment requires, even if the
final product is packaged in a non-user service-
able enclosure. The designer could provide a sim-
ple utility that accesses the Calibration bits. For
more information on calibration (see the Applica-
tion Note AN934, “TIMEKEEPER
the ST Web Site).
T
30
0
ppm
= 25 C
C
2
(T - T
40
0
)
2
50
10%
60
70
®
80
AI00999
Calibration” on