m41t93 STMicroelectronics, m41t93 Datasheet - Page 26

• Manufacturer Part Number: m41t93
• Description: Serial Spi Bus Rtc With Battery Switchover
• Manufacturer: STMicroelectronics

Note:
Figure 12. Clock divider chain and calibration circuits
26/51
32kHz
Two methods are available for ascertaining how much calibration a given M41T93 may
require:
●
●
Setting or changing the Digital Calibration Byte does not affect the Frequency Test, Square
Wave, or Watchdog Timer frequency, but changing the Analog Calibration byte DOES affect
all functions derived from the low current oscillator (see
The first involves setting the clock, letting it run for a month and comparing it to a known
accurate reference and recording deviation over a fixed period of time. This allows the
designer 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 serviceable enclosure. The
designer could provide a simple utility that accesses either or both of the Calibration
bytes.
The second approach is better suited to a manufacturing environment, and involves the
use of the IRQ/FT/OUT pin. The IRQ/FT/ OUT pin will toggle at 512Hz when FT and
OUT Bits = '1' and ST = '0.' Any deviation from 512Hz indicates the degree and
direction of oscillator frequency shift at the test temperature. For example, a reading of
512.010124Hz would indicate a +20 ppm oscillator frequency error, requiring either a –
10 (xx001010) to be loaded into the Digital Calibration byte, or +6pF (00011000) into
the Analog Calibration byte for correction.
C XO
C XI
Low Current
Oscillator
Analog Calibration
Circuitry
2
8
2
Digital Calibration Circuitry
2
(divide by 511/512/513)
Frequency Test
512Hz Output
2
Figure
1Hz Signal
2
12).
Watchdog Timer
Registers
Remainder of
Divider Circuit
Square Wave
Clock
8-Bit Timer
AI11806a