EM250-BBRD-R Ember, EM250-BBRD-R Datasheet - Page 25

• Manufacturer Part Number: EM250-BBRD-R
• Description: EM250 BREAKOUT BOARD
• Manufacturer: Ember
• Type: Transceiver, 802.15.4r

Specifications of EM250-BBRD-R

• Frequency: 2.4GHz
• For Use With/related Products: EM250
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: 636-1024

4.12 Random Number Generator
4.13 Watchdog Timer
Parameter
Frequency
Analog trim steps
Frequency variation with supply
4.11.3 Internal RC Oscillator
The EM250 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). If
this tolerance is accurate enough for the application, then there is no need to use an external 32.768kHz crys-
tal oscillator. By removing the 32.768kHz oscillator, the external component count further decreases as does
the Bill of Material cost.
Note: If the 32.768kHz crystal is not needed, then OSC32A and OSC32B pins should be left open or not con-
Table 14 lists the specifications of the RC oscillator.
The EM250 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 regis-
ter. The value contained in this register could be used to seed a software-generated random number. The Em-
berZNet stack utilizes these random numbers to seed the Random MAC Backoff and Encryption Key Genera-
tors.
The EM250 contains a watchdog timer clocked from the internal oscillator. The watchdog is disabled by de-
fault, 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 sig-
nal from being generated.
The watchdog will generate a low watermark interrupt in advance of actually resetting the chip. This low wa-
termark interrupt occurs approximately 1.75 seconds after the timer has been restarted. This interrupt can be
used to assist during application debug.
nected.
For a voltage drop from 3.6V to 3.1V or 2.6V
Test Conditions
to 2.1V
Table 14. RC Oscillator Specifications
Page 25
Min.
Typ.
0.75
10
1
Max.
1.5
EM250
120-0082-000S
Unit
kHz
kHz
%