EM260-RTR datasheet(16/50 Pages) SILICONIMAGE

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EM260

120-0260-000M Rev 1.1

Page 16

seen in range testing measurements when nodes are closely positioned and transmitting at high power or when

receiving from test equipment. There is no damage to the EM260 from this problem. This issue will rarely

occur in the field as ZigBee Nodes will be spaced far enough apart. If nodes are close enough for it to occur in

the field, the MAC and networking software treat the packet as not having been received and therefore the

MAC level and network level retries resolve the problem without needing to notify the upper level application.

4.2

Transmit (TX) Path

The EM260 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.

4.2.1

TX Baseband

The EM260 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.

4.2.2

TX_ACTIVE Signal

Even though the EM260 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 EM260

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 100k

Ohm resistor, as shown in the application circuit in Figure 14.

The TX_ACTIVE signal can only source 1mA of current, and it is based upon the 1.8V signal swing. If the PA

Control logic requires greater current or voltage potential, then TX_ACTIVE should be buffered externally to

the EM260.

4.3

Integrated MAC Module

The EM260 integrates critical portions of the IEEE 802.15.4-2003 MAC requirements in hardware. This allows

the EM260 to provide greater bandwidth to application and network operations. In addition, the hardware acts

as a first-line filter for non-intended packets. The EM260 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.

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 EM260 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.

The primary features of the MAC are:



CRC generation, appending, and checking



Hardware timers and interrupts to achieve the MAC symbol timing



Automatic preamble, and SFD pre-pended to a TX packet



Address recognition and packet filtering on received packets

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