MICRF219A datasheet(12/23 Pages) MICREL

12 / 23 page

Micrel, Inc.

MICRF219A

July 2011

M9999-071811-A

RadioTech@micrel.com

Reference Oscillator

The reference oscillator in the MICRF219A (Figure 5)

uses a basic Pierce crystal oscillator configuration with

MOS transconductor. Though the MICRF219A has built-

in load capacitors for the crystal oscillator, the external

load capacitors are still required for tuning it to the right

frequency. RO1 and RO2 are external pins of the

MICRF219A to connect the crystal to the reference

oscillator.

Figure 5. Reference Oscillator Circuit

Reference oscillator crystal frequency can be calculated

using Equation 3. For example, if fRF = 433.92MHz, fREF

= 13.52313MHz. Table 4 lists the values of reference

frequencies at different popular RF frequencies. To

operate the MICRF219A with minimum offset, use

proper loading capacitance recommended by the crystal

manufacturer.

RF Input Frequency (MHz)

Reference Frequency (MHz)

315.0

9.81713*

390.0

12.15446

418.0

13.02708

433.92

13.52313*

or (408) 944-0800

*Empirically derived, slightly different from Equation 3.

Table 4. Reference Frequency Examples

Auto-Polling

The MICRF219A can be programmed into an auto-

polling mode by setting register bit D[15] to 1, where it

monitors if there is a valid incoming RF signal while

holding DO low. In this mode, the chip goes between

sleep state and polling state. In sleep state, only a low

power sleep clock is on, resulting in very low current

consumption of 13μA typical. The sleep time is

programmable from 10ms to 1.28s. In a polling state,

every block in the MICRF219A is on, and the chip looks

for signal with bit durations greater than a user-

programmed value.

This operation is subsequently

called “bit checking” in this datasheet. A “valid bit” is a

mark or space with duration that is longer than the bit

check window. A “bad bit” is a mark or space with

duration that is shorter than the bit check window. The

user can set different bit check window time to suit a

particular signal by programming register bits D[11:9] as

listed in the register programming section. The number

of consecutive valid bits before releasing DO and exiting

polling mode can also be set by register bits D[8:7].

Figure 6. One Bad Bit Followed by Two Valid Bits

During the bit checking operation, DO is held low while

the bit checker examines the pulse widths at the node

labeled DO’ in Figure 1. If there is no signal present and

DO’ randomly chatters, the MICRF219A returns to sleep

after seeing 4 consecutive bad bits.

Note that since DO’ randomly chatters with no signal

present, the amount of time it takes for 4 consecutive

bad bits to happen is random. Therefore, the duration of

polling time is random without signal.

If enough consecutive valid bits are found, DO is

released and the MICRF219A stays on in the continuous

receive mode. Once the chip is in continuous receive

mode, it will not go back to sleep automatically when RF

signal is removed. The register bits must be

programmed again to put the MICRF219A back into

auto-polling mode.

Squelch Operation

Squelch operation can be used to limit the amount of

activity on the DO pin during normal operation, which is

particularly useful when interrupts generated on DO can

interfere with correct operation. Using squelch is not

recommended for systems that can program the chip

into auto-polling mode, since it suppresses more activity

on DO, and reduces the average current consumption.

When squelch is enabled (SQ pin tied low while register

bit D17 is 0), DO pin goes high once about every 100ms

instead of chattering all the time when squelch is

disabled.

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