SN250 datasheet(27/130 Pages) STMICROELECTRONICS

27 / 130 page

SN250

Functional description—system modules

27/130

for higher ESR, the cost decreases but the current consumption increases. Therefore, the

designer can choose a crystal to fit the needs of the application.

Table 12 lists the specifications for the high-frequency crystal.

6.10.2

Low-frequency oscillator

The optional low-frequency crystal source for the SN250 is a 32.768kHz crystal. Table 13

lists the requirements for the low-frequency crystal. The low-frequency crystal may be used

for applications that require greater accuracy than can be provided by the internal RC

oscillator. The crystal oscillator has been designed to accept any standard watch crystal

with an ESR of 100 k

Ω.

Table 12.

High-frequency crystal specifications

Parameter

Test Conditions

Min.

Typ.

Max.

Unit

Frequency

MHz

Duty cycle

Phase noise from 1kHz to

100kHz

− 120

dBc/Hz

Accuracy

Initial, temperature, and aging

− 40

+ 40

ppm

Crystal ESR

Load capacitance of 10pF

100

Crystal ESR

Load capacitance of 18pF

Start-up time to stable clock

(max. bias)

1ms

Start-up time to stable clock

(optimum bias)

2ms

Current consumption

Good crystal: 20

Ω ESR, 10pF

load

0.2

0.3

Current consumption

Worst-case crystals (60

Ω, 18pF

or 100

Ω, 10pF)

0.5

Current consumption

At maximum bias

Table 13.

Low-Frequency Crystal Specifications

Parameter

Test Conditions

Min.

Typ.

Max.

Unit

Frequency

32.768

kHz

Accuracy

Initial, temperature, and aging

− 100

+ 100

ppm

Load capacitance (double

this each side to ground)

12.5

Crystal ESR

100

Start-up time

Current consumption

0.5

μA

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