PCF7936AS datasheet(24/32 Pages) PHILIPS

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NXP Semiconductors

Product Specification

Security Transponder (HITAG2)

PCF7936AS

2010 May 04

CONFIDENTIAL

7.7 LF Field Power On Reset

When the transponder enters a LF field a rectifier circuitry

becomes operational and the internal transponder supply

voltage (VDD) develops. As soon as the supply voltage

exceeds the LF Field Power-On Reset threshold voltage

(VTHR) the device performs a chip reset and starts its

initialization sequence, see Figure 23.

Subsequently, the transponder is muted and does not

respond to any command prior to termination of the

initialization sequence, tINIT. The startup time, tSTART,

depends on the basestation configuration, the resonance

circuit properties and the system coupling factor, however,

is small compared with the initialization time typically.

For proper device operation, after a LF Filed Power-On

Reset condition, command execution must commence

within

the

specified

Idle

time,

tIDLE, see Figure 23.

Otherwise the device may stop command decoding,

disabling any communication with the device. In this case a

LF Field Power-On Reset has to be applied, in order to

reset and initialize the circuitry. Consequently, the device

would resume WAIT state. As indicated, the Idle time is

specified as the time interval following the initialization

sequence until the last bit of the Command Sequence that

is send to the transponder.

In case one of the Read Only modes is enabled, the device

enters READ ONLY state, if the first two bits of the

START_AUTH command are not being recognized within

the time-out period tWAIT,SA. In this case, Read Only

operation commences tWAIT,RO after termination of the

initialization sequence, tINIT, see Figure 23. For details refer

to section 7.5.

In order to force a LF Field Power-On Reset and proper

device initialization at any time, the LF field OFF condition

must be applied for at least tRESET,SETUP, in order to ensure

that the internal device supply voltage, VDD, drops below

the threshold voltage (VTHR), see Figure 24.

LF field applied

VTHR

tINIT

VDD

tSTART

LF field power on reset (POR)

threshold voltage

tIDLE

Command Sequence

READ ONLY Mode

tWAIT,RO

tWAIT,SA

Figure 23. LF field power on reset timing

LF field OFF

VTHR

VDD

tRESET_SETUP

LF field power on reset (POR)

threshold voltage

Figure 24. LF field power on reset setup timing

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