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EM250
Table 10. CCA Mode Behavior
CCA Mode
0
1
2
3
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.
The EmberZNet PRO Software Stack sets the CCA Mode, and it is not configurable by the Application Layer. For
software versions beginning with EmberZNet 2.5.4, CCA Mode 1 is used, and a busy channel is reported if the
RSSI exceeds its threshold. For software versions prior to 2.5.4, the CCA Mode was set to 0.
At RX input powers higher than –25dBm, there is some compression in the receive chain where the gain is not
properly adjusted. In the worst case, this has resulted in packet loss of up to 0.1%. This packet loss can be
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 EM250 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 upper level application being notified.
4.2
Transmit (TX) Path
The EM250 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 EM250 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 EM250 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 EM250
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 16.
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 EM250.
4.3
Integrated MAC Module
The EM250 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 EM250 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
120-0082-000V Rev 1.1
Page 18
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