atr2434 ATMEL Corporation, atr2434 Datasheet - Page 6

• Manufacturer Part Number: atr2434
• Description: Wirelessusb 2.4-ghz Dsss Radio Soc - Atmel Corporation
• Manufacturer: ATMEL Corporation

Receive Signal Strength
Indicator (RSSI)
Application Interfaces
SPI Interface
6
ATR2434 [Preliminary]
The RSSI register (Reg 0x22) returns the relative signal strength of the ON-channel
signal power and can be used to:
1. determine the connection quality,
2. determine the value of the noise floor, and
3. check for a quiet channel before transmitting.
The internal RSSI voltage is sampled through a 5-bit Analog-to-Digital Converter (ADC).
A state machine controls the conversion process. Under normal conditions, the RSSI
state machine initiates a conversion when an ON-channel carrier is detected and
remains above the noise floor for over 50 µs. The conversion produces a 5-bit value in
the RSSI register (Reg 0x22, bits 4:0) along with a valid bit, RSSI register
(Reg 0x22, bit 5). The state machine then remains in HALT mode and does not reset for
a new conversion until the receive mode is toggled off and on. Once a connection has
been established, the RSSI register can be read to determine the relative connection
quality of the channel. A RSSI register value lower than 10 indicates that the received
signal strength is low, a value greater than 28 indicates a strong signal level.
To check for a quiet channel before transmitting, first set up the receive mode properly
and read the RSSI register (Reg 0x22). If the valid bit is zero, then force the Carrier
Detect register (Reg 0x2F, bit 7 = 1) to initiate an ADC conversion. Then, wait a mini-
mum of 50 µs and read the RSSI register again. Next, clear the Carrier Detect register
(Reg 0x2F, bit 7 = 0) and turn the receiver OFF. Measuring the noise floor of a quiet
channel is inherently a noisy process so, for best results, this procedure should be
repeated several times (~20) to compute an average noise floor level. A RSSI register
value of 0-10 indicates a channel that is relatively quiet. A RSSI register value greater
than 10 indicates the channel is probably being used. A RSSI register value greater than
28 indicates the presence of a strong signal.
The ATR2434 has a four-wire SPI communication interface between an application
MCU and one or more slave devices. The SPI interface supports single-byte and multi-
byte serial transfers. The four-wire SPI communications interface consists of Master
Out-Slave In (MOSI), Master In-Slave Out (MISO), Serial Clock (SCK), and Slave Select
(SS).
The SPI receives SCK from an application MCU on the SCK pin. Data from the applica-
tion MCU is shifted in on the MOSI pin. Data to the application MCU is shifted out on the
MISO pin. The active-low Slave Select (SS) pin must be asserted to initiate an SPI
transfer.
The application MCU can initiate an SPI data transfer via a multi-byte transaction. The
first byte is the Command/Address byte, and the following bytes are the data bytes as
shown in Table 1 on page 7 and Figure 3 through Figure 5 on page 7. The SS signal
should not be deasserted between bytes. The SPI communications is as follows:
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Command direction (bit 7) = 0 enables SPI read transaction. A 1 enables SPI write
transactions.
Command increment (bit 6) = 1 enables SPI auto address increment. When set, the
address field automatically increments at the end of each data byte in a burst
access, otherwise the same address is accessed.
Six bits of address.
Eight bits of data.
4822C–ISM–09/04