SN250 datasheet(28/130 Pages) STMICROELECTRONICS

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Functional description—system modules

SN250

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6.10.3

Internal RC oscillator

The SN250 has a low-power, low-frequency RC oscillator that runs all the time. Its nominal

frequency is 10kHz.

The RC oscillator has a coarse analog trim control, which is first adjusted to get the

frequency as close to 10kHz as possible. This raw clock is used by the chip management

block. It is also divided down to 1kHz using a variable divider to allow software to accurately

calibrate it. This calibrated clock is available to the sleep timer.

Timekeeping accuracy depends on temperature fluctuations the chip is exposed to, power

supply impedance, and the calibration interval, but in general it will be better than 150ppm

(including crystal error of 40ppm).

Table 14 lists the specifications of the RC oscillator.

6.11

Random number generator

The SN250 allows for the generation of random numbers by exposing a randomly

generated bit from the RX ADC. Analog noise current is passed through the RX path,

sampled by the receive ADC, and stored in a register. The value contained in this register

could be used to seed a software-generated random number. The ZNet stack utilizes these

random numbers to seed the Random MAC Backoff and Encryption Key Generators.

6.12

Watchdog timer

The SN250 contains a watchdog timer clocked from the internal oscillator. The watchdog is

disabled by default, but can be enabled or disabled by software.

If the timer reaches its time-out value of approximately 2 seconds, it will generate a reset

signal to the chip.

When software is running properly, the application can periodically restart this timer to

prevent the reset signal from being generated.

The watchdog will generate a low watermark interrupt in advance of actually resetting the

chip. This low watermark interrupt occurs approximately 1.75 seconds after the timer has

been restarted. This interrupt can be used to assist during application debug.

Table 14.

RC Oscillator Specifications

Parameter

Test Conditions

Min.

Typ.

Max.

Unit

Frequency

kHz

Analog trim steps

kHz

Frequency variation with

supply

For a voltage drop from 3.6V

to 3.1V or 2.6V to 2.1V

0.5

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