SN250 datasheet(21/130 Pages) STMICROELECTRONICS

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SN250

Functional description—system modules

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6.2

Transmit (TX) path

The SN250 transmitter utilizes both analog circuitry and digital logic to produce the O-QPSK

modulated signal. The area-efficient TX architecture directly modulates the spread symbols

prior to transmission. The differential signal paths increase noise immunity and provide a

common interface for the external balun.

6.2.1

TX baseband

The SN250 TX baseband (within the digital domain) performs the spreading of the 4-bit

symbol into its IEEE 802.15.4-2003-defined 32-chip I and Q sequence. In addition, it

provides the interface for software to perform the calibration of the TX module in order to

reduce process, temperature, and voltage variations.

6.2.2

TX_ACTIVE signal

Even though the SN250 provides an output power suitable for most ZigBee applications,

some applications will require an external power amplifier (PA). Due to the timing

requirements of IEEE 802.15.4-2003, the SN250 provides a signal, TX_ACTIVE, to be used

for external PA power management and RF Switching logic. When in TX, the TX Baseband

drives TX_ACTIVE high (as described in Table 6). When in RX, the TX_ACTIVE signal is

low. If an external PA is not required, then the TX_ACTIVE signal should be connected to

GND through a 100 k

Ω resistor, as shown in the application circuit in Figure 16.

6.3

Integrated MAC module

The SN250 integrates critical portions of the IEEE 802.15.4-2003 MAC requirements in

hardware. This allows the microcontroller to provide greater bandwidth to application and

network operations. In addition, the hardware acts as a first-line filter for non-intended

packets. The SN250 MAC utilizes a DMA interface to RAM memory to further reduce the

overall microcontroller interaction when transmitting or receiving packets.

When a packet is ready for transmission, the software configures the TX MAC DMA by

indicating the packet buffer RAM location. The MAC waits for the backoff period, then

transitions the baseband to TX mode and performs channel assessment. When the channel

is clear, the MAC reads data from the RAM buffer, calculates the CRC, and provides 4-bit

symbols to the baseband. When the final byte has been read and sent to the baseband, the

CRC remainder is read and transmitted.

Table 10.

CCA Mode Behavior

CCA Mode

Mode Behavior

Clear channel reports busy medium if either carrier sense OR RSSI exceeds their

thresholds.

Clear channel reports busy medium if RSSI exceeds its threshold.

Clear channel reports busy medium if carrier sense exceeds its threshold.

Clear channel reports busy medium if both RSSI AND carrier sense exceed their

thresholds.

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