nm95hs02 National Semiconductor Corporation, nm95hs02 Datasheet - Page 12

• Manufacturer Part Number: nm95hs02
• Description: Hisec-tm High Security Rolling Code Generator
• Manufacturer: National Semiconductor Corporation

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Security Aspects
NORMAL OPERATION
Once the NM95HS01 02 has been initialized the device will
generate and transmit a new code each time a key is
pressed If a key is held down the same frame (plus any
pauses between frames) is transmitted repeatedly If the
key is held down for longer than 80 seconds the generator
will go into halt mode to conserve battery power and will
stop transmitting data frames (if the TIMEOUTEN option is
enabled)
Another option available during normal generator operation
is the ability to generate a resync after a key has been
pressed for more than 10 seconds (if the AutoResync op-
tion is enabled) This option allows the end user to resyn-
chronize the generator if necessary without having to re-
move and replace the battery
FORWARD CALCULATION AND CODE WINDOWS
Aside from using a sync frame there is another way to en-
sure the NM95HS01 02 remains in sync with its decoder
during normal operation The decoder can perform a for-
ward calculation to predict what the next generator codes
will be This is an important point and should be considered
carefully in designing the decoding system
In a well-designed system the decoder should be able to
calculate forward for some reasonable number of codes
and store the results for future reference This allows the
decoder to remain in sync even if it misses one or more
codes from the generator This could occur if the receiver
did not receive a transmission clearly or if someone activat-
ed the keys outside the range of the receiver
Increasing the depth of this code window would allow the
decoder to miss a greater number of codes from the gener-
ator and still remain in sync One method for implementing
a code window is to include a MICROWIRE
(such as the NM93Cx6) in the decoder design and store the
codes in memory This becomes even more important if the
decoder is designed to accomodate several HiSeC genera-
tor devices In this case the decoder should have a code
window available for each device
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TM
EEPROM
12
Generator Clock Design
Parameters
Tables IV V and VI provide a basis for selecting component
values for both the RC clocked generator (NM95HS01) and
the
(NM95HS02)
The component values shown in the tables have been cho-
sen for low cost general availability and reliable operation
Components are referenced to the circuit schematics
shown in Figure 3 Though there is some flexibility in select-
ing alternate values there are constraints on permissible
component values
All resistors and capacitors should be kept within the follow-
ing ranges 3 kX
R1 (MX)
R (kX)
R (kX)
3 3
5 6
6 8
3 3
5 6
6 8
1
crystal
TABLE VI XTAL Clock Components
TABLE IV RC Clock Components
TABLE V RC Clock Components
C1 (pF)
T
T
C (pF)
C (pF)
A
30
(XTAL)
A
s
100
100
100
100
T
e
82
82
e
A
R
25 C V
e
x s
25 C V
25 C V
C2 (pF)
30 – 36
200 kX and 50 pF
oscillator
CC
CKI (MHz)
CKI (MHz)
2 12 – 2 32
CC
1 1 – 1 17
0 9 – 0 95
1 53 – 1 6
0 8 – 0 83
0 9 – 1
CC
e
e
2 5V – 6 5V
e
5V – 6 5V
CKI (MHz)
2 5V
clocked
4
s
1100 – 1050
1100 – 1000
1250 – 1200
C
470 – 430
870 – 850
650 – 600
CKI (ns)
CKI (ns)
x s
CKI (ns)
generator
250
200 pF