M
2.4GHz 802.11b Zero-IF Transceivers
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15
Operating Modes
The MAX2820/MAX2821 have four primary modes of
operation: shutdown, standby, receive active, and
transmit active. The modes are controlled by the digital
inputs SHDNB, TX_ON, and RX_ON. Table 1 shows the
operating mode vs. the digital mode control input.
Shutdown Mode
Shutdown mode is achieved by driving SHDNB low. In
shutdown mode, all circuit blocks are powered down,
except for the serial interface circuitry. While the device
is in shutdown, the serial interface registers can still be
loaded by applying V
CC
to the digital supply voltage
(VCC_DIG). All previously programmed register values
are preserved during the shutdown mode, as long as
VCC_DIG is applied.
Standby Mode
Standby mode is achieved by driving SHDNB high,
RX_ON, and TX_ON low. In standby mode, the PLL,
VCO, LO generator, LO buffer, LO quadrature, and fil-
ter autotuner are powered on by default. The standby
mode is intended to provide time for the slower-settling
circuitry (PLL and autotuner) to turn on and settle to the
correct frequency before making RX or TX active. The
3-wire serial interface is active and can load register
values at any time. Refer to serial interface specification
for details.
Receive Mode
Receive mode is enabled by driving the digital inputs
SHDNB high, RX_ON high, and TX_ON low. In receive
mode, all receive circuit blocks are powered on and all
VCO, PLL, and autotuner circuits are powered on. None of
the transmit path blocks are active in this mode. Although
the receiver blocks turn on quickly, the DC offset nulling
requires ~10μs to settle. The receiver signal path is ready
~10μs after a low-to-high transition on RX_ON.
Transmit Mode
Transmit mode is achieved by driving the digital inputs
SHDNB high, RX_ON low, and TX_ON high. In transmit
mode, all transmit circuit blocks are powered on and all
VCO, PLL, and autotuner circuits are powered on.
None of the receive path blocks is active in this mode.
Although the transmitter blocks turn on quickly, the
baseband DC offset calibration requires ~2.2μs to
complete. In addition, the TX driver amplifier is ramped
from the low-gain state (minimum RF output) to high-
gain state (peak RF output) over the next 1μs to 2μs.
The transmit signal path is ready ~4μs after a low-to-
high transition on TX_ON.
MODE CONTROL INPUTS
SHDNB
TX_ON
0
1
1
1
CIRCUIT BLOCK STATES
TX_PATH
OFF
OFF
OFF
ON
OPERATING MODE
RX_ON
X
0
1
0
RX_PATH
OFF
OFF
ON
OFF
PLL/VCO/LO GEN.
OFF
ON
ON
ON
Shutdown
Standby
Receive
Transmit
X
0
0
1
Table 1. Operating Mode Truth Table
CSB
SCLK
t
CSS
DIN
DOUT
t
CSO
t
CSW
t
CSH
t
CL
t
CS1
t
TR
BIT 16
BIT 15
BIT 16
BIT 15
BIT 14
BIT 8
BIT 7
BIT 6
BIT 2
BIT 1
BIT 14
BIT 8
BIT 7
BIT 6
BIT 2
BIT 1
t
CH
t
DH
t
DO
t
DV
t
DS
Figure 1. MAX2820/MAX2821 Serial Interface Timing Diagram