MICRF221 datasheet(16/26 Pages) MICREL

16 / 26 page

Micrel Inc.

MICRF221

October 2008

M9999-100108

(408) 955-1690

Figure 10. Data Out Pin with No Squelch (SQ = 1)

The shut down pin (SHDN) can be used to save

energy. If its level is close to VDD (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 1μA.

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

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 Ω resistors) to R5 = 150Ω, R6 = R7 = 33

kΩ. 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 CAGC capacitor

value.

SCLK Pin

Serial interface input clock is a CMOS Schmitt input.

A 25kΩ 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

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

the

device.

MICRF221 Datasheet(PDF) 16 Page - Micrel Semiconductor

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