MAX6902ETA+T【PDF 6/20 页】IC RTC SPI COMPAT 8-TDFN

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MAX6902

SPI-Compatible RTC in a TDFN

______________________________________________________________________________________

k = parabolic curvature constant (-0.035 ±0.005ppm/°C2

for 32.768kHz watch crystals)

T0 = turnover temperature (+25°C ±5°C for 32.768kHz

watch crystals)

T = temperature of interest (°C)

For example: What is the worst-case change in oscilla-

tor frequency from +25°C ambient to +45°C ambient?

What is the worst-case timekeeping error per second?

Error due to temperature drift:

Error due to 25°C initial crystal tolerance of ±20ppm:

Total timekeeping error per second:

After 1 month that translates to:

Total worst-case timekeeping error at the end of 1

month at 45°C is about 120s or 2min (assumes negligi-

ble parasitic layout capacitance).

Oscillator Start Time

The MAX6902 oscillator typically takes 5s to 10s to

begin oscillating. To ensure the oscillator is operating

correctly, the software should validate proper time-

keeping. This is accomplished by reading the Seconds

of zero seconds is a validation of proper startup.

Δt

day

()

min

(0.000045s/s)

=120.528s

ΔΔ

ts s

s s

initial

total

drift

initial

total

()

[]

{}

=+=

1 32 768 0 6

32 768

1 1

0 000020

0 000025

0 000020

0 000045

- . 5536

ΔΔ

fHz

initial

()

[]

{}

=×

× ×

()=

32 768

1 10

0 65536

)

-s

-20ppm

-6

ΔΔ

tHz

drift

()

[]

{}

()

[]

{}

32 768

1 32 768

32 768

0 000025

-s

- .8192

- s

ΔfHz

drift =×

× ×

32 768

1 10

0 8192

)

()

-0.04ppm/ C

C - 45 C)

2-6

Table 3. 32.768kHz Surface-Mount Watch Crystals

MANUFACTURER

PART NO.

TEMP.

RANGE

CL (pF)

+25

°C

FREQUENCY

TOLERANCE

(ppm)

Abracon Corporation

ABS25-32.768-12.5-B-2-T

-40

°C to +85°C

12.5

±20

Caliber Electronics

AWS2A-32.768KHz, AWS2B-32.768KHz

-20

°C to +70°C

12.5

±20

ECS INC International

ECS-.327-12.5-17

-10

°C to +60°C

12.5

±20

Fox Electronics

FSM327

-40

°C to +85°C

12.5

±20

M-tron

SX2010/ SX2020

-20

°C to +75°C

12.5

±20

Raltron

RSE-32.768-12.5-C-T

-10

°C to +60°C

12.5

±20

SaRonix

32S12A

-40

°C to +85°C

12.5

±20

-200

-250

-100

-150

-50

-40

-20

100

TYPICAL TEMPERATURE CHARACTERISTICS

(k = -0.035 ppm/

°C2; TO = +25°C)

TEMPERATURE (

°C)

f(ppm)

Figure 6. Typical Temperature Curve for 32.768kHz Watch

Crystal

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