PCF7936AS datasheet(22/32 Pages) PHILIPS

22 / 32 page

NXP Semiconductors

Product Specification

Security Transponder (HITAG2)

PCF7936AS

2010 May 04

CONFIDENTIAL

7.6 Transponder Data Transmission Format

Reading from and writing to the device is accomplished by

modulating the LF field in amplitude. Since the LF field also

provides

the

device

power

supply,

the

modulation

characteristics have to be verified carefully, in order to

avoid a device reset due to a power low condition.

7.6.1

Read Direction

Transmission

data

from

the

transponder

the

basestation is accomplished by absorption modulation

applied to the LF field. According to the data designated for

transmission,

the

transponder

interface

activates

additional load that modulates the current drawn from the

transponder resonant circuit. Due to the inductive coupling

of the transponder resonant circuit and the basestation coil,

the current in the basestation coil is modulated accordingly,

resulting

corresponding

two-level

amplitude

modulation, see Figure 19.

In read direction the device employs either Manchester or

CDP encoding of data, see Figure 20, according to the

setting of the Immobilizer Configuration bit DCS, which is

part of the Transponder and Memory Configuration bits,

TMCF, see also section 7.1.4.

In case of Manchester encoding, a logic ‘1’ is modulated by

loading the LF field during the first half of the bit frame,

while no load is applied during the second half. A logic ‘0’ is

modulated in the opposite manner.

In case of CDP encoding, a logic ‘1’ corresponds to a state

change at the end of the bit frame. A logic ‘0’ corresponds

to a state change after the first half and at the end of the bit

frame.

In any case, the device starts with a „Load ON“ condition,

when data transmission commences.

The bit duration is a fixed multiple of the system clock

recovered from the LF field carrier.

After reception of the last bit, the basestation and control

software have to consider the indicated delay, tWAIT,Bs,

before any command or data is transmitted to the device,

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