PCF7936AS datasheet(23/32 Pages) PHILIPS

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NXP Semiconductors

Product Specification

Security Transponder (HITAG2)

PCF7936AS

2010 May 04

CONFIDENTIAL

7.6.2

Write Direction

Transmission

data

from

the

basestation

the

transponder is accomplished by Amplitude Shift Keying

(ASK) of the LF field with a modulation index as specified.

According to the data designated for transmission, the

basestation coil driver is simply switched ON and OFF (tri-

state) typically. Due to the inductive coupling of the

transponder resonant circuit and the basestation coil, the

voltage of the transponder resonant circuit is modulated

accordingly. Resulting in a two-level amplitude modulation

that is detected by the transponder interface demodulator

circuitry, see Figure 21.

The

PCF7936AS transponder demodulator circuitry has

been optimized for basestations with antenna coil drivers

that perform the LF field modulation by Tri-State switching

of the driver stage.

In write direction Binary Pulse Length Modulation (BPLM) is

applied for data encoding, see Figure 22.

Sending data or commands to the device commences with

an initial write pulse that marks transmission start. A logic

‘0’

‘1’

signaled

the

transponder

the

corresponding repetition time (TLOG_0 respectively TLOG_1) of

the write pulse sequence.

The end of the transmitted bit string is marked by a stop

condition. A stop condition is detected by the transponder,

if no write pulse is detected for the specified time (TSTOP).

In the case the bit string transmitted causes the device to

respond with data, modulation of the LF field by the device

does commence after the specified time out (tWAIT,Tr), see

also section 7.3.1.

Violation of the specified timing causes an error condition,

upon which the device enters the WAIT state, see also

section 7.2.

VLF-LOW

Coil

VLF-HIGH

Coil

Figure 21. ASK Modulation of LF Field by the Basestation

Internal Data

BPLM

Encoding

LF field:

High

Low

Stop

Condition

Start of transmission

'1'

'0'

tWAIT,Tr

End of transmission

Last

Bit

TLOG_0

TLOG_1

TWRP

TSTOP

...

Figure 22. Data Transmission in Write Direction

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