EM250-BBRD-R Ember, EM250-BBRD-R Datasheet - Page 18

• Manufacturer Part Number: EM250-BBRD-R
• Description: EM250 BREAKOUT BOARD
• Manufacturer: Ember
• Type: Transceiver, 802.15.4r

Specifications of EM250-BBRD-R

• Frequency: 2.4GHz
• For Use With/related Products: EM250
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: 636-1024

EM250
4.2
4.3
120-0082-000S
Transmit (TX) Path
Integrated MAC Module
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 oc-
cur 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.
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 differen-
tial signal paths increase noise immunity and provide a common interface for the external balun.
4.2.1
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
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.
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 per-
forms 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 base-
band, 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 CPU from being interrupted. When the re-
ception of a packet begins, the MAC reads 4-bit symbols from the baseband and calculates the CRC. It assem-
bles 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.
TX Baseband
TX_ACTIVE Signal
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