M48T513VPM datasheet(12/23 Pages) STMICROELECTRONICS

12 / 23 page

M48T513Y, M48T513V

12/23

RESET INPUT (RSTIN)

The M48T513Y/V provides an independent input

which can generate an output reset. The duration

and function of this reset is identical to a reset gen-

erated by a power cycle. Table 13 and Figure 14

illustrate the AC reset characteristics of this func-

tion. Pulses shorter than tR will not generate a re-

set condition. RSTIN is internally pulled up to VCC

through a 100K

Ω resistor.

CALIBRATING THE CLOCK

The M48T513Y/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 4 ppm at 25

°C. The oscillation rate of crystals

changes with temperature. The M48T513Y/V de-

sign employs periodic counter correction. The cal-

ibration circuit adds or subtracts counts from the

oscillator divider circuit at the divide by 256 stage,

as shown in Figure 11.

The number of times pulses which are blanked

(subtracted, negative calibration) or split (added,

positive calibration) depends upon the value load-

ed into the five Calibration bits found in the Control

tracting counts slows the clock down. The Calibra-

tion 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 bi-

nary form. Bit D5 is a Sign bit; ’1’ indicates positive

calibration, ’0’ indicates negative calibration. Cali-

bration 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 modi-

fied; if a binary 6 is loaded, the first 12 will be af-

fected, and so on. Therefore, each calibration step

has the effect of adding 512 or subtracting 256 os-

cillator cycles for every 125, 829, 120 actual oscil-

lator cycles, that is +4.068 or –2.034 ppm of

adjustment per calibration step in the calibration

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. Figure

11 illustrates a TIMEKEEPER calibration wave-

form.

Two methods are available for ascertaining how

much calibration a given M48T513Y/V may re-

quire. The first involves setting the clock, letting it

run for a month and comparing it to a known accu-

rate reference and recording deviation 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 Application Note: TIMEKEEPER CALIBRA-

TION.

Figure 10. Chip Enable Controlled, Write AC Waveforms

AI02582

tAVAV

tEHAX

tDVWH

A0-A16

DQ0-DQ7

VALID

tAVEL

tAVWL

tELEH

tWHDX

DATA INPUT

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