nm95hs02 National Semiconductor Corporation, nm95hs02 Datasheet - Page 11

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

Operational Timing Issues
As an example consider the following situation A designer
wishes to design an RF data transmitter using RF bit coding
format 5 with a bit time of 1 ms The designer also wishes to
use a 3 MHz crystal oscillator as the system clock
The required bit time of 1 ms encompasses three RF clock
periods for RF bit coding format 5 Therefore the RF clock
time needs to be
a target value of 2 5 ms (2500 ms) as the output of Prescal-
er3 Since the RF clock signal is divided by Prescaler3 Pre-
scaler3 divides the signal by 2500 333
rounded off to become 8
One point of possible confusion should be clarified here
Whenever a division value is calculated for any of the 3
prescalers the prescaler should be configured with one unit
less than that division value For example in this case we
calculated a division value of 8 (after rounding) for Prescal-
er3 Therefore Prescaler3 should be programmed with 8
1
Next we calculate values for Prescaler1 and Prescaler2
Although the crystal oscillator uses both the CKI and CKO
pins only the CKI input is relevant here The CKI input fre-
quency is 3 MHz and
input frequency to the HiSeC timer block and the corre-
sponding timing signal is 1 33 ms
Since the RF clock must be 333 ms Prescalers1 and 2 to-
gether must divide by 333 1 33
would be to make Prescaler1 divide by 10 and Prescaler2
divide by 25
Therefore load Prescaler1 with 10
er2 with 25
DEBOUNCE LOGIC
The key switch input signals are connected to the debounce
logic block which continuously polls the inputs to determine
if a key switch has been asserted If a key switch has been
asserted its normally high input will be seen as a low lf the
input is seen low for four continuous debounce strobe sig-
nals it is considered to be a stable signal and its associat-
ed output from the debounce logic block is set high This
enables the generator control logic and a code is generat-
ed and transmitted
This debounced output signal is deasserted as soon as the
key is released and its signal goes high again This assumes
normal operation However if a key remained pressed for a
long time the generator might time-out before seeing the
signal go high again (if TIMEOUTEN
would then enter halt mode even if the key remained
pressed The generator would come out of halt mode when
it saw the falling edge of another key input which would
occur when another key is pressed
LOW BATTERY DETECT OPTION
The NM95HS01 02 contains an internal comparator circuit
that detects low battery voltage and indicates this condition
to the data frame generator The CompareEnable parame-
ter in EEPROM enables this function (CompareEnable
1) During halt mode the comparator is switched off com-
pletely to minimize power consumption The BatteryType
parameter in EEPROM selects the threshold voltage range
for the comparator If BatteryType
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7
b
1
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of 1 ms (
24
of that is 0 75 MHz This is the
e
333 ms) The timer block has
e
250 A convenient choice
b
e
e
1
e
e
1) The generator
1 the compara-
7 5 This figure is
9 and Prescal-
(Continued)
e
b
11
tor assumes a 6V battery and sets the low battery detect
region to approximately 4 4V to 4 8V If BatteryType
the comparator assumes a 3V battery and sets the low
battery detect region to approximately 2 2V to 2 4V
Data output signals are sampled for low voltage at the start
of the data field during frame transmission If a low battery
voltage level is detected and the detect option is enabled
the LED will signal the condition by flashing at the first
pause in the data frame transmission and alternating nor-
mal data field data with a data field containing all ones This
procedure is explained more fully in the Data Field section
Security Aspects
The basis of the HiSeC Generator is to provide a means
of communicating information between the device and its
decoder across some distance Since data is transmitted
at a distance there is a possibility of signal interception
and unauthorized use of the data by a third party The
NM95HS01 02 has been designed to provide such a high
level of complexity and correlation immunity that intercept-
ing the signal is immaterial
INITIALIZATION SYNCHRONIZATION
Initialization is the process of synchronizing the gen-
erator with its decoder for the first time The NM95HS01 02
uses the following procedure to initialize the device
The user inserts a new battery into the HiSeC-based device
which causes the LED to light The LED also has a second-
ary function for synchronization and initialization proce-
dures It will light to prompt the end user that it expects
some action and therefore serves as a guide
When the LED lights the user presses a key The LED will
go off as the generator begins randomizing its registers and
configuring its internal logic When the user releases the
key the LED will light a second time This is a signal for the
user to press a key again This second action shifts the
generator into sync mode This causes the NM95HS01 02
to transmit at least four sync frames allowing the decoder
to synchronize to the generator The generator then exits
sync mode and is ready tor normal operation
RESYNCHRONIZATION
If synchronization is lost between the generator and its de-
coder resynchronization is accomplished using a sync
frame A sync frame is generated in two cases when the
battery is removed and replaced or the user initiates an
initialization procedure by holding Key Switch 1 and Key
Switch 2 simultaneously for 5 seconds
A sync frame provides the decoder with enough information
to ‘‘learn’’ the key and synchronize to it
For the highest possible security protection resynchroniza-
tion can be completely excluded by configuring the decoder
to recognize and refuse to act upon the transmission of a
sync frame The sync frame format is discussed more fully
elsewhere but briefly it can be recognized by the presence
of all zeroes in the data field In this case if synchronization
is lost between the generator and decoder they could not
be made to function together
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