Linx Technologies TRM-xxx-NT - Manual (Page 11)

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Using the T/R_SEL Input
The transmit/receive select (T/R_SEL) line is used to switch the transceiver
between transmit and receive mode. If it is pulled low, the transceiver exits
transmit mode and enters receive mode. Alternatively, if the line is pulled
high, the transceiver exits receive mode and enters transmit mode. The
READY output switches high during the change and returns low when the
module is ready to receive or transmit data. None of the other operating
modes are affected by the change. The data rate and channel settings
remain as set.
Using the Low Power Features
The Power Down (POWER_DOWN) line can be used to completely power
down the transceiver module without the need for an external switch.
This line allows easy control of the transceiver power state from external
components, such as a microcontroller. The module is not functional while
in power down mode.
Similar to the POWER_DOWN line, the Standby (STANDBY) line can be
used to put the transceiver into a low-power sleep mode. This line has an
internal pull-up, so when it is held high or left floating, the transceiver enters
a low power (2.6mA) state. When the STANDBY line is pulled to ground,
the module is fully active. During Standby, all operating modes are
deactivated. The READY output is high during standby.
Standby has a higher current consumption than Power Down but a
faster wake-up time. By periodically activating the transceiver, sending
data, then powering down or entering standby, the transceiver’s average
current consumption can be greatly reduced, saving power in
battery-operated applications.
Warning: Pulling any of the module inputs high while in
Power Down can partially activate the module, increasing current
consumption and potentially placing it into an indeterminate state that
could lead to unpredictable operation. Pull all inputs low before pulling
POWER_DOWN low to prevent this issue. Lines that may be hardwired
(for example, the BAUD lines) can be connected to the POWER_DOWN
line so that they are lowered when POWER_DOWN is lowered.
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Using the READY Output
The Ready (READY) line can be used to monitor the status of the module.
It is logic high while the transceiver is busy and logic low when the
transceiver is ready to transmit or receive data. This allows the line to be
used as hardware flow control. It is logic high when in Standby, but is logic
low in Power Down since the entire module is off.
Using the LVL_ADJ Line
The Level Adjust (LVL_ADJ) line allows the transceiver’s output power to be
easily adjusted for range control, lower power consumption or to meet legal
requirements. This is done by placing a resistor to ground on LVL_ADJ to
form a voltage divider with an internal 100k
Ω resistor. When the transceiver
powers up, the voltage on this line is measured and the output power
level is set accordingly. When LVL_ADJ is connected to V
CC
or floating, the
output power and current consumption are the highest. When connected
to ground, the output power and current are the lowest. The power is
digitally controlled in 58 steps providing approximately 0.5dB per step. See
the Typical Performance Graphs section (Figures 5–19) for a graph of the
output power vs. LVL_ADJ resistance.
This line is very useful during regulatory testing to compensate for antenna
gain or other product-specific issues that may cause the output power to
exceed legal limits. A variable resistor to ground can be temporarily used
so that the test lab can precisely adjust the output power to the maximum
level allowed by law. The variable resistor’s value can be noted and a fixed
resistor substituted for final testing. Even in designs where attenuation
is not anticipated, it is a good idea to place resistor pads connected to
LVL_ADJ and ground so that it can be used if needed. Figure 23 on the
following page shows the 1% tolerance resistor value that is needed to
activate each power level.
Warning: The LVL_ADJ line uses a resistor divider to create
a voltage that determines the output power. Any additional current
sourcing or sinking can change this voltage and result in a different
power level. The power level should be checked to confirm that it is set
as expected.
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