HCS473 Datasheet, PDF (23/68 Pages) Microchip Technology – Code Hopping Encoder and Transponder

TABLE 3-4: HCS473 RESPONSE OPTIONS
RFRSP LCRSP
Description
0
0 No response
0
1 Response over the LC pins
1
0 Response through the DATA pin
1
1 Response through the DATA pin
first and then the LC pins
3.2.1.8 Skip Field Acknowledge (SKIPACK)
The initial Field Acknowledge sequence, occurring dur-
ing the wake-up pulse, may be disabled by enabling the
Skip Field Acknowledge configuration option (SKI-
PACK=1). Omitting the ACK slightly minimizes a
HCS473’s average communication current draw, but
conversely will increase average authentication time as
the wake-up pulse must then be the maximum start-up
filter charge time, TSF_MAX.
3.2.2 TRANSPONDER COMMUNICATION
Data to and from the HCS473 is always sent Least Sig-
nificant bit first. The data length and modulation format
vary with the particular command sequence and the
transmission path.
3.2.2.1 LC Communication Format
Commands from the transponder reader to the
HCS473 as well as the responses from the HCS473
over the low frequency path (LC pins) are Pulse Posi-
tion Modulated (PPM) according to Figure 3-10.
Communication from the transponder reader to a
HCS473 is via the reader amplitude shifting a 125kHz
low frequency (LF) field.
LF responses back to the transponder reader are
achieved by the HCS473 applying a low-resistance
short from the LC pins to LCCOM (configuration option
LCRSP enables LF talkback). This short across the
antenna inputs is detected by the reader as a load on
its 125kHz transmitting antenna.
See Section 5.4 for further details on inductive commu-
nication principles.
HCS473
FIGURE 3-10:
‘0’
125kHz
LC PIN PULSE POSITION
MODULATION (PPM)
Digital
Representation
1LFTE 1LFTE
‘1’
125kHz
START or
previous bit
Digital
Representation
2LFTE 1LFTE
3.2.2.2 RF DATA Communication Format
The RF responses on the DATA pin vary with the infor-
mation being returned.
• Acknowledge responses are based on the LFTE.
• Data code words responses are based on the
RFTE, using the format determined by the
Encoder mode options, Section 3.1.4.
3.2.2.3 Wake-up Sequence
The transponder reader initiates each communication
sequence by turning on the low frequency field, then
waits for a HCS473 to Acknowledge the field.
The HCS473 enters Transponder Mode after detecting
a signal on any LC low frequency antenna input pin that
has remained high for at least the start-up filter time
TSF, Table 7-5. The device then responds with a Field
Acknowledge sequence indicating that it has detected
the LF field, is in Transponder Mode and is ready to
receive commands (Figure 3-11). The wake-up pulse’s
falling edge must then occur within TCMD of the end of
the Field Acknowledge sequence.
The Field Acknowledge sequence may optionally be
disabled by enabling the Skip Field ACK configuration
option, Section 3.2.1.8.
In both cases, the first command bit must begin within
TCMD of the wake-up pulse’s falling edge or the
HCS473 will return to Low Power mode.
 2002 Microchip Technology Inc.
Preliminary
DS40035C-page 21