P4022 datasheet(11/13 Pages) EMMICRO

11 / 13 page

PRELIMINARY

EMMICROELECTRONIC-MARINSA

P4022

WAKE-UP

An ACK sent after correct receipt of a code is

interpreted by the other transponders in the field

as a WAKE-UP. The ACK arrives synchronously at

the transponder that has just transmitted, but

asynchronously at all the other transponders.

necessary, a WAKE-UP can also be sent if the

code is not received correctly, making sure that it

will not be interpreted as an ACK by the

transmitting transponder. This could speed up the

protocol, but runs the risk of turning transponders

off by accident.

To detect a WAKE-UP, the chip checks for two

GAPs, less than 7 bits apart and each less than

seven bits wide.

As with the MUTE allowance

must made for the 30% spread in

clock

frequencies. To be safely interpreted as a WAKE-

UP, the GAPs should be sent less than 5 bits apart,

and each should be less than 5 bits wide. This

has an implication in the case of the high

frequency ACK, which could theoretically consist

of two very narrow GAPs 6 bits apart. In practice

though, the GAPs will be typically at least one bit

wide, making the separation five bits.

Like the MUTE, the low frequency ACK GAPs

should be at least 1.5 bits wide to serve as a

reliable WAKE-UP.

It should be noted that failure to reliably

recognise WAKE-UPs is not critical. The protocol

might be slowed down marginally, but will still

work, as the chips time-out of the sleep mode

automatically after 128 bits.

Data Encoder

The transmitted code always consists of an 11 bit

preamble followed by the 64 code bits.

The

preamble consists of 8 start bits (ZEROES),

followed by a SYNCH. The SYNCH consists of a

LOW for two bit periods followed by a ONE.

The P4022 can be programmed for one of two

data encoding methods. The first method is a

variation on Manchester II, i.e. a ONE is

represented by a HIGH in the first half of a bit

period, and a ZERO is represented by a LOW in

the first half of a bit period.

The second encoding method is called GLITCH

encoding. A ONE is represented by a HIGH in the

first quarter of the bit period, while a ZERO is

represented by a HIGH in the third quarter of the

bit period.

In GLITCH encoding the longest modulation

period is one quarter of a bit period, compared to

the Manchester encoding, where the longest

modulation period is one full bit period. GLITCH

encoding therefore requires a much smaller power

storage capacitor.

Figure 7

: Data encoding methods

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