HCS200 Datasheet, PDF (8/16 Pages) Microchip Technology – KEELOQ CODE HOPPING ENCODER

HCS200
5.0 SPECIAL FEATURES
5.1 Code Word Completion
Code word completion is an automatic feature that
makes sure that the entire code word is transmitted,
even if the button is released before the transmission is
complete. The HCS200 encoder powers itself up when
a button is pushed and powers itself down after the
command is finished, if the user has already released
the button. If the button is held down beyond the time
for one transmission, then multiple transmissions will
result. If another button is activated during a
transmission, the active transmission will be aborted
and the new code will be generated using the new
button information.
5.2 Blank Alternate Code Word
Federal Communications Commission (FCC) part 15
rules specify the limits on fundamental power and
harmonics that can be transmitted. Power is calculated
on the worst case average power transmitted in a
100 ms window. It is therefore advantageous to
minimize the duty cycle of the transmitted word. This
can be achieved by minimizing the duty cycle of the
individual bits and by blanking out consecutive words.
Blank Alternate Code Word (BACW) is used for
reducing the average power of a transmission
(Figure 5-1). This is a selectable feature that is
determined in conjunction with the baud rate selection
bit BSL0. Using the BACW allows the user to transmit a
higher amplitude transmission if the transmission
length is shorter. The FCC puts constraints on the aver-
age power that can be transmitted by a device, and
BACW effectively prevents continuous transmission by
only allowing the transmission of every second word.
This reduces the average power transmitted and
hence, assists in FCC approval of a transmitter device.
5.3 Seed Transmission
In order to increase the level of security in a system, it is
possible for the receiver to implement what is known as
a secure learn function. This can be done by utilizing the
seed value on the HCS200 which is stored in EEPROM
and can only be transmitted when all three button inputs
are pressed at the same time (Table 5-1). Instead of the
normal key generation method being used to create the
encryption key, this seed value is used.
5.4 VLOW: Voltage LOW indicator
The VLOW bit is transmitted with every transmission
(Figure 4-2 and Figure 8-5) and will be transmitted as a
zero if the operating voltage is above the voltage trip
point. The VLOW signal is transmitted so the receiver
can give an indication to the user that the transmitter
battery is low.
FIGURE 5-1: BLANK ALTERNATE CODE WORD (BACW)
Amplitude
One Code Word
BACW Disabled
A
(All words transmitted)
100ms
100ms
100ms
100ms
BACW Enabled
(1 out of 2 transmitted)
2A
TABLE 5-1: PIN ACTIVATION TABLE
External
Internal
S2 S1 S0 S3 S2 S1 S0
Standby after CC
0
0
0
0
0
0
0
0
0
1
0
0
0
1
0
1
0
0
0
1
0
Hop
0
1
1
0
0
1
1
1
0
0
1
1
0
0
1
0
1
1
1
1
0
1
1
0
1
1
1
0
Fixed
1
1
1
1
1
1
1
DS40138A-page 8
Preliminary
© 1996 Microchip Technology Inc.