MICRF221 datasheet(17/26 Pages) MICREL

17 / 26 page

Micrel Inc.

MICRF221

October 2008

M9999-100108

(408) 955-1690

BIT TIME 1

BIT TIME 0

DO AS

OUTPUT

DO INPUT BITS:

SCLK

D19

“19”

“0”

“1”

D18

D17

BIT TIME 2

Figure 12. Serial Interface Start Sequence

When using an external oscillator or reference

oscillator signal, the maximum level should not

exceed 1.5VPP. See Figure 13. Channel 4 is the

transmitted data, which is synchronized with the

shutdown shown in the oscilloscope (channel 2). Data

out is shown on channel 1 and, as seen below, the

preamble data starts to appear just over 1msec after

the shutdown pin cycle low to high. Also note that

when squelch is used time for data to appear is

increased.

Figure 13. Time-to-Preamble Data after Shut Down

Cycle, Room Temperature

Programming the Device

Several additional functions are available by the serial

interface. They are:

•

Desense,

reduce

the

device

sensitivity

•

Slice Level, to further optimize data

profile demodulation

•

Autopoll Mode, to wake an external

device through the DO pin toggling

from low to data

•

High Demodulator Bandwidth, for

faster baud rates

•

Parallel input pins SEL0, SEL1, and

SQ can be programmed using the

serial interface

Programming the device is accomplished by the use

of pins DO and SCLK. Normally, pin 10 (DO) is

outputting data and needs to switch to an input pin

made by the start sequence, as shown at Figure 13. A

high at the SCLK pin tri-states the DO pin, enabling

the external drive into the DO pin with an initial low

level. The start sequence is completed by taking

SCLK low, then high while DO is low, followed by

taking DO high, then low while SCLK is high. The

serial interface is initialized and ready to receive the

programming data.

Bits are serially programmed starting with the most

significant bit (MSB = D19) if all bits are being

programmed until the least significant bit (LSB =D0)

For instance, if only the desense bits D0, D1, and D2

are being programmed, then these are the only bits

that need to be programmed with the start sequence

D2, D1, D0, plus the stop sequence. Or, if only the

squelch bit D17 is needed, then the sequence must

be from start sequence, D17 through D0 plus the stop

sequence, making sure the other bits (besides D17)

are programmed as needed. It is recommended that

all parallel input pins (SEL0, SEL1, and SQ) be kept

high when using the serial interface. After the

programming bits are finished, a stop sequence (as

shown in Figure 14) is required to end the mode and

make the DO pin as an output again. To do so, the

SCLK pin is kept high while the DO pin changes from

low to high, then low again, followed by the SCLK pin

made low. Timing of the programming bits are not

critical, but should be kept as shown below:

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