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

(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

CIRCUIT BLOCK STATES

OPERATING MODE

SHDNB

TX_ON

RX_ON

RX_PATH

TX_PATH

PLL/VCO/LO GEN.

Shutdown

0

X

X

OFF

OFF

OFF

Standby

1

0

0

OFF

OFF

ON

Receive

1

0

1

ON

OFF

ON

Transmit

1

1

0

OFF

ON

ON

Table 1. Operating Mode Truth Table

CSB

SCLK

DIN

DOUT

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

Figure 1. MAX2820/MAX2821 Serial Interface Timing Diagram