M48T201Y datasheet(22/35 Pages) STMICROELECTRONICS

22 / 35 page

Clock operation

M48T201Y, M48T201V

22/35

The oscillation rate of crystals changes with temperature (see Figure 10 on page 23). The

M48T201Y/V design employs periodic counter correction. The calibration circuit adds or

subtracts counts from the oscillator divider circuit at the divide by 256 stage, as shown in

Figure 11 on page 23.

The number of times pulses which 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 (see Figure 11 on

page 23). 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 32,768Hz, 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.

Two methods are available for ascertaining how much calibration a given M48T201Y/V may

require. 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. Calibration

values, including the number of seconds lost or gained in a given period, can be found in the

STMicroelectronics Application Note AN934, “TIMEKEEPER® CALIBRATION.” 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 the Calibration byte.

The second approach is better suited to a manufacturing environment, and involves the use

of the IRQ/FT pin. The pin will toggle at 512Hz, when the Stop Bit (ST, D7 of 7FFF9h) is '0,'

the Frequency Test Bit (FT, D6 of 7FFFCh) is '1,' the Alarm Flag Enable Bit (AFE, D7 of

7FFF6h) is '0,' and the Watchdog Steering Bit (WDS, D7 of 7FFF7h) is '1' or the Watchdog

Note:

A 4-second settling time must be allowed before reading the 512Hz output.

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 a –10 (WR001010) to be loaded into the Calibration

Byte for correction. Note that setting or changing the Calibration Byte does not affect the

Frequency Test output frequency.

The IRQ/FT pin is an open drain output which requires a pull-up resistor to VCC for proper

operation. A 500-10k

Ω resistor is recommended in order to control the rise time. The FT Bit

is cleared on power-down.

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