SN250 datasheet(20/130 Pages) STMICROELECTRONICS

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Functional description—system modules

SN250

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Functional description—system modules

The SN250 contains a dual-thread mode of operation—System Mode and Application

Mode—to guarantee microcontroller bandwidth to the application developer and protect the

developer from errant software access.

During System Mode, all areas including the RF Transceiver, MAC, Packet Trace Interface,

Sleep Timer, Power Management Module, Watchdog Timer, and Power on Reset Module

are accessible.

Since the SN250 comes with a license to ZNet, a ZigBee-compliant software stack

developed by Ember Corporation, these areas are not available to the application developer

in Application Mode. The following brief description of these modules provides the

necessary background on the operation of the SN250. For more information, please contact

your nearest STMicroelectronics sales office.

6.1

Receive (RX) path

The SN250 RX path spans the analog and digital domains. The RX architecture is based on

a low-IF, super-heterodyne receiver. It utilizes differential signal paths to minimize noise

interference. The input RF signal is mixed down to the IF frequency of 4MHz by I and Q

mixers. The output of the mixers is filtered and combined prior to being sampled by a

12Msps ADC. The RX filtering within the RX path has been designed to optimize the co-

existence of the SN250 with other 2.4GHz transceivers, such as the IEEE 802.11g and

Bluetooth.

6.1.1

RX baseband

The SN250 RX baseband (within the digital domain) implements a coherent demodulator for

optimal performance. The baseband demodulates the O-QPSK signal at the chip level and

synchronizes with the IEEE 802.15.4-2003 preamble. Once a packet preamble is detected,

it de-spreads the demodulated data into 4-bit symbols. These symbols are buffered and

passed to the hardware-based MAC module for filtering.

In addition, the RX baseband provides the calibration and control interface to the analog RX

modules, including the LNA, RX Baseband Filter, and modulation modules. The ZNet

software includes calibration algorithms which use this interface to reduce the effects of

process and temperature variation.

6.1.2

RSSI and CCA

The SN250 calculates the RSSI over an 8-symbol period as well as at the end of a received

packet. It utilizes the RX gain settings and the output level of the ADC within its algorithm.

The SN250 RX baseband provides support for the IEEE 802.15.4-2003 required CCA

methods summarized in Table 10. Modes 1, 2, and 3 are defined by the 802.15.4-2003

standard; Mode 0 is a proprietary mode.

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