SN260 datasheet(16/88 Pages) STMICROELECTRONICS

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Functional description

SN260

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5.2

Transmit (TX) path

The SN260 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.

5.2.1

TX baseband

The SN260 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.

5.2.2

TX_ACTIVE signal

Even though the SN260 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 inTable 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 100k Ohm resistor, as shown in the application circuit in Figure 12.

5.3

Integrated MAC module

The SN260 integrates critical portions of the IEEE 802.15.4-2003 MAC requirements in

hardware. This allows the SN260 to provide greater bandwidth to application and network

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

SN260 MAC utilizes a DMA interface to RAM memory to further reduce the overall micro

controller 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.

The MAC resides in RX mode most of the time, and different format and address filters keep

non-intended packets from using excessive RAM buffers, as well as preventing the SN260

CPU from being interrupted. When the reception of a packet begins, the MAC reads 4-bit

symbols from the baseband and calculates the CRC. It assembles the received data for

storage in a RAM buffer. A RX MAC DMA provides direct access to the RAM memory. Once

the packet has been received, additional data is appended to the end of the packet in the

RAM buffer space. The appended data provides statistical information on the packet for the

software stack.

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