HCS412 Datasheet, PDF (18/44 Pages) Microchip Technology – KEELOQ Code Hopping Encoder and Transponder

HCS412
4.0 TRANSPONDER OPERATION
4.1 IFF Mode
The HCS412’s IFF Mode allows it to function as a bi-
directional token or transponder. IFF mode capabilities
include the following.
• A bi-directional challenge and response sequence
for IFF validation. HCS412 IFF responses may be
directed to use one of two available encryption
algorithms and one of two available crypt keys.
• Read selected EEPROM areas.
• Write selected EEPROM areas.
• Request a code hopping transmission.
• Proximity Activation of a code hopping transmis-
sion.
4.2 IFF Communication
The transponder reader initiates each communication
by turning on the low frequency field, then waits for a
HCS412 to Acknowledge the field.
The HCS412 enters IFF mode upon detecting a signal
on the LC0 LF antenna input pin. Once the incoming
signal has remained high for at least the power-up time
TPU, the device responds with a field Acknowledge
sequence indicating that the it has detected the LF
field, is in IFF Mode and is ready to receive commands
(Figure 4-1). The HCS412 will repeat the field Acknowl-
edge sequence every 255 LFTE‘s if the field remains
but no command is received (Figure 4-1).
The transponder reader follows the HCS412’s field
Acknowledge by sending the desired 5-bit command
and associated data. LF commands are always pre-
ceded by a 2 LFTE low START pulse and are Pulse
Position Modulated (PPM) as shown in Figure 4-2. The
last command or data bit should be followed by leaving
the field on for a minimum of 6 LFTE.
HCS412 PPM data responses are preceded by a 1
LFTE low pulse, followed by a 01b preamble before the
data begins (Figure 4-4). The responses are sent either
on the LC antenna output alone or on both the LC out-
put and the DATA pin, depending on the device config-
uration (Section 4.4.2). This allows for short-range LF
responses as well as long-range RF responses.
Data to and from the HCS412 is always sent Least Sig-
nificant bit first. The data length and modulation format
vary according to the command and the transmission
path.
Data Length and Commands:
• Read and Write transfers 16 bits of data.
• Challenge and Response transfers 32 bits of data.
Modulation Format and Transmission Path:
• LF responses on the LC output are Pulse Position
Modulated (PPM) according to Figure 4-2.
• RF responses on the DATA pin modulate accord-
ing to standard encoder transmissions
(Figure 3-5, Figure 3-6).
Communication with the HCS412 over the low fre-
quency path (LC pins) uses a basic Timing Element,
LFTE. The Low Frequency Baud Rate Select option,
LFBSL, sets LFTE to either 100 µs or 200 µs
(Table 4-1).
The response on the DATA pin uses the Encoder
mode’s RF Timing Element (RFTE) and the modulation
format set by the MOD configuration option (Table 3-6).
The RF responses use the standard Encoder mode for-
mat with the 32-bit hopping portion replaced by the
response data (Figure 4-19). If the response is only 16
bits, the 32 bits will contain 2 copies of the response
(Figure 4-16).
TABLE 4-1: LOW FREQUENCY BAUD
RATE SELECT BITS
LFBSL
LFTE
0
200 µs
1
100 µs
4.2.1 CALCULATING COMMUNICATION TE
The HCS412’s internal oscillator will vary ±10% over
the device’s rated voltage and temperature range.
When the oscillator varies, both its transmitted TE and
expected TE when receiving will vary.
Communication reliability with the token may be
improved by calculating the HCS412’s TE from the field
Acknowledge sequence and using this measured time
element in communication to and in reception routines
from the token.
Always begin and end the time measurement on rising
edges. Whether LF or RF, the falling edge decay rates
may vary but the rising edge relationships should
remain consistent. A common TE calculation method
would be to time an 8 TE sequence, then divide the
value down to determine the single TE value. An 8 TE
measurement will give good resolution and may be
easily right-shifted (divide by 2) three times for the math
portion of the calculation (Figure 4-1).
Accurately measuring TE is important for communicat-
ing to an HCS412 as well as for inductive programming
a device. The configuration word sent during program-
ming contains the 4-bit oscillator tuning value. Accu-
rately determining TE allows the programmer to
calculate the correct oscillator tuning bits to place in the
configuration word, whether the device oscillator needs
to be sped up or slowed down to meet its desired TE.
DS41099C-page 18
Preliminary
© 2002 Microchip Technology Inc.