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Micrel Inc.
Figure 10. Data Out Pin with No Squelch (SQ = 1)
Figure 11. Data Out Pin with Squelch (SQ = 0)
Other components used include:
C5 is a decoupling capacitor for the Vdd line.
R4 should be referenced to ground when a
microcontroller connection is not made and
kept low by the microcontroller when not
programming the device.
R3 is the reference for the shutdown pin
(SHDN = 0, device is operation), which can
be removed if that pin is connected to a
microcontroller or an external switch.
R1 and R2 form a voltage divider for the AGC pin.
One can purposely decrease the device sensitivity
by forcing a voltage to this AGC pin. Special care
is needed when doing this operation, as an
MICRF221
external control of the AGC voltage may vary from
lot-to-lot and may not work the same for several
devices.
5V Operation
5-volt operation can be obtained by replacing R5, R6,
and R7 (0 ? resistors) to R5 = 150 ? , R6 = R7 = 33
k ? . The 5-volt source must be regulated and
guaranteed never to exceed 5V. DO is equal to VDD
levels.
Four other pins are worthy of comment. They are the
DO, RSSI, SHDN, and SCLK pins.
DO Pin
The DO pin has a driving capability of 0.4mA. This is
good enough for most of the logic families ICs on the
market today. It also works as an input when
programming the device for the serial register control
RSSI Pin
The RSSI pin provides a transfer function of RF signal
intensity versus voltage. It is useful to determine the
signal-to-noise ratio of the RF link, crude range
estimate from the transmitter source and AM
demodulation, which requires a low C AGC capacitor
value.
SCLK Pin
Serial interface input clock is a CMOS Schmitt input.
A 25k ? pull-down is present when device is in
shutdown mode. See “Programming the Device”
section for timing diagram and functional operation
SHDN (Shut Down) Pin
The shut down pin (SHDN) can be used to save
energy. If its level is close to V DD (SHDN = 1), the
device will not be in operation. Its DC current
consumption is less than 1μA (R3 must be removed).
This input pin is designed with a weak pull-up. The
pull-up current is decreased once the input has
switched above the threshold level, that is, the device
is shut down and progressively decreases to levels
below 1 μ A.
When shut down pin is toggling from high to low
(getting out of shut down mode), there is some time
required for the device to come to steady-state mode
and some time needed for data to appear at the DO
pin. The actual time required is dependent upon
several factors, such as temperature, the crystal used
and if the there is an external oscillator coupled
through C2 with faster startup time. Normally
(assuming the suggested crystal vendors are used),
the preamble data will appear at the DO pin at
approximately 1msec time, and 2msec over the
temperature range of the device.
October 2008
16
M9999-100108
(408) 955-1690
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