© 2011 Microchip Technology Inc.
DS41098D-page 1
FEATURES
Security
• Programmable 28-bit serial number
• Programmable 64-bit encryption key
• Each transmission is unique
• 66-bit transmission code length
• 32-bit hopping code
• 34-bit fixed code (28-bit serial number,
4-bit button code, 2-bit status)
• Encryption keys are read protected
Operating
• 3.5V-13V operation
(2.0V min. using the Step up feature)
• Three button inputs
• 7 functions available
• Selectable baud rate
• Automatic code word completion
• Battery low signal transmitted to receiver
• Non-volatile synchronization data
Other
• Simple programming interface
• On-chip EEPROM
• On-chip oscillator and timing components
• Button inputs have internal pull-down resistors
• Minimum component count
• Synchronous Transmission mode
• Built-in step up regulator
Typical Applications
• The HCS201 is ideal for Remote Keyless Entry
(RKE) applications. These applications include:
• Automotive RKE systems
• Automotive alarm systems
• Automotive immobilizers
• Gate and garage door openers
• Identity tokens
• Burglar alarm systems
DESCRIPTION
The HCS201 from Microchip Technology Inc. is a code
hopping encoder designed for secure Remote Keyless
Entry (RKE) systems. The HCS201 utilizes the
K
EE
L
OQ®
code hopping technology, incorporating high
security, a small package outline and low cost. The
HCS201 is a perfect solution for unidirectional remote
keyless entry systems and access control systems.
PACKAGE TYPES
HCS201 BLOCK DIAGRAM
The HCS201 combines a 32-bit hopping code,
generated by a nonlinear encryption algorithm, with a
28-bit serial number and 6 information bits to create a
66-bit code word. The code word length eliminates the
threat of code scanning and the code hopping mecha-
nism makes each transmission unique, thus rendering
code capture and resend schemes useless.
1
2
3
4
8
7
6
5
S0
S1
S2
V
DDB
V
DD
STEP
DATA
V
SS
PDIP, SOIC
HCS20
1
V
SS
V
DD
Oscillator
RESET circuit
Controller
Power
latching
and
switching
Button input port
32-bit shift register
Encoder
EEPROM
DATA
S
2
S
1
S
0
Step Up
Controller
V
DDB
V
DD
STEP
HCS201
K
EE
L
OQ®
Code Hopping Encoder
HCS201
DS41098D-page 2
© 2011 Microchip Technology Inc.
The crypt key, serial number and configuration data are
stored in an EEPROM array which is not accessible via
any external connection. The EEPROM data is pro-
grammable but read-protected. The data can be veri-
fied only after an automatic erase and programming
operation. This protects against attempts to gain
access to keys or manipulate synchronization values.
The HCS201 provides an easy-to-use serial interface
for programming the necessary keys, system parame-
ters and configuration data.
1.0
SYSTEM OVERVIEW
Key Terms
The following is a list of key terms used throughout this
data sheet. For additional information on K
EE
L
OQ
and
Code Hopping, refer to Technical Brief 3 (TB003).
• RKE - Remote Keyless Entry
• Button Status - Indicates what button input(s)
activated the transmission. Encompasses the 4
button status bits S3, S2, S1 and S0 (Figure 4-2).
• Code Hopping - A method by which a code,
viewed externally to the system, appears to
change unpredictably each time it is transmitted.
• Code word - A block of data that is repeatedly
transmitted upon button activation (Figure 4-1).
• Transmission - A data stream consisting of
repeating code words (Figure 9-1).
• Crypt key - A unique and secret 64-bit number
used to encrypt and decrypt data. In a symmetri-
cal block cipher such as the K
EE
L
OQ
algorithm,
the encryption and decryption keys are equal and
will therefore be referred to generally as the crypt
key.
• Encoder - A device that generates and encodes
data.
• Encryption Algorithm - A recipe whereby data is
scrambled using a crypt key. The data can only be
interpreted by the respective decryption algorithm
using the same crypt key.
• Decoder - A device that decodes data received
from an encoder.
• Decryption algorithm - A recipe whereby data
scrambled by an encryption algorithm can be
unscrambled using the same crypt key.
• Learn – Learning involves the receiver calculating
the transmitter’s appropriate crypt key, decrypting
the received hopping code and storing the serial
number, synchronization counter value and crypt
key in EEPROM. The K
EE
L
OQ
product family facil-
itates several learning strategies to be imple-
mented on the decoder. The following are
examples of what can be done.
- Simple Learning
The receiver uses a fixed crypt key, common
to all components of all systems by the same
manufacturer, to decrypt the received code
word’s encrypted portion.
- Normal Learning
The receiver uses information transmitted
during normal operation to derive the crypt
key and decrypt the received code word’s
encrypted portion.
- Secure Learn
The transmitter is activated through a special
button combination to transmit a stored 60-bit
seed value used to generate the transmitter’s
crypt key. The receiver uses this seed value
to derive the same crypt key and decrypt the
received code word’s encrypted portion.
• Manufacturer’s code – A unique and secret 64-
bit number used to generate unique encoder crypt
keys. Each encoder is programmed with a crypt
key that is a function of the manufacturer’s code.
Each decoder is programmed with the manufac-
turer code itself.
The HCS201 code hopping encoder is designed specif-
ically for keyless entry systems; primarily vehicles and
home garage door openers. The encoder portion of a
keyless entry system is integrated into a transmitter,
carried by the user and operated to gain access to a
vehicle or restricted area. The HCS201 is meant to be
a cost-effective yet secure solution to such systems,
requiring very few external components (Figure 2-1).
Most low-end keyless entry transmitters are given a
fixed identification code that is transmitted every time a
button is pushed. The number of unique identification
codes in a low-end system is usually a relatively small
number. These shortcomings provide an opportunity
for a sophisticated thief to create a device that ‘grabs’
a transmission and retransmits it later, or a device that
quickly ‘scans’ all possible identification codes until the
correct one is found.
The HCS201, on the other hand, employs the K
EE
L
OQ
code hopping technology coupled with a transmission
length of 66 bits to virtually eliminate the use of code
‘grabbing’ or code ‘scanning’. The high security level of
the HCS201 is based on the patented K
EE
L
OQ
technol-
ogy. A block cipher based on a block length of 32 bits
and a key length of 64 bits is used. The algorithm
obscures the information in such a way that even if the
transmission information (before coding) differs by only
one bit from that of the previous transmission, the next
© 2011 Microchip Technology Inc.
DS41098D-page 3
HCS201
coded transmission will be completely different. Statis-
tically, if only one bit in the 32-bit string of information
changes, greater than 50 percent of the coded trans-
mission bits will change.
As indicated in the block diagram on page one, the
HCS201 has a small EEPROM array which must be
loaded with several parameters before use; most often
programmed by the manufacturer at the time of produc-
tion. The most important of these are:
• A 28-bit serial number, typically unique for every
encoder
• A crypt key
• An initial 16-bit synchronization value
• A 16-bit configuration value
The crypt key generation typically inputs the transmitter
serial number and 64-bit manufacturer’s code into the
key generation algorithm (Figure 1-2). The manufac-
turer’s code is chosen by the system manufacturer and
must be carefully controlled as it is a pivotal part of the
overall system security.
FIGURE 1-1:
CREATION AND STORAGE OF CRYPT KEY DURING PRODUCTION
The 16-bit synchronization counter is the basis behind
the transmitted code word changing for each transmis-
sion; it increments each time a button is pressed. Due
to the code hopping algorithm’s complexity, each incre-
ment of the synchronization value results in greater
than 50% of the bits changing in the transmitted code
word.
Figure 1-2 shows how the key values in EEPROM are
used in the encoder. Once the encoder detects a button
press, it reads the button inputs and updates the syn-
chronization counter. The synchronization counter and
crypt key are input to the encryption algorithm and the
output is 32 bits of encrypted information. This data will
change with every button press, its value appearing
externally to ‘randomly hop around’, hence it is referred
to as the hopping portion of the code word. The 32-bit
hopping code is combined with the button information
and serial number to form the code word transmitted to
the receiver. The code word format is explained in
greater detail in Section 4.0.
A receiver may use any type of controller as a decoder,
but it is typically a microcontroller with compatible firm-
ware that allows the decoder to operate in conjunction
with an HCS201 based transmitter. Section 7.0
provides detail on integrating the HCS201 into a sys-
tem.
A transmitter must first be ‘learned’ by the receiver
before its use is allowed in the system. Learning
includes calculating the transmitter’s appropriate crypt
key, decrypting the received hopping code and storing
the serial number, synchronization counter value and
crypt key in EEPROM.
In normal operation, each received message of valid
format is evaluated. The serial number is used to deter-
mine if it is from a learned transmitter. If from a learned
transmitter, the message is decrypted and the synchro-
nization counter is verified. Finally, the button status is
checked to see what operation is requested. Figure 1-3
shows the relationship between some of the values
stored by the receiver and the values received from
the transmitter.
Transmitter
Manufacturer’s
Serial Number
Code
Crypt
Key
Key
Generation
Algorithm
Serial Number
Crypt Key
Sync Counter
.
.
.
HCS201
Production
Programmer
EEPROM Array
HCS201
DS41098D-page 4
© 2011 Microchip Technology Inc.
FIGURE 1-2:
BUILDING THE TRANSMITTED CODE WORD (ENCODER)
FIGURE 1-3:
BASIC OPERATION OF RECEIVER (DECODER)
NOTE: Circled numbers indicate the order of execution.
Button Press
Information
EEPROM Array
32 Bits
Encrypted Data
Serial Number
Transmitted Information
Crypt Key
Sync Counter
Serial Number
K
EE
L
OQ®
Encryption
Algorithm
Button Press
Information
EEPROM Array
Manufacturer Code
32 Bits of
Encrypted Data
Serial Number
Received Information
Decrypted
Synchronization
Counter
Check for
Match
Sync Counter
Serial Number
K
EE
L
OQ®
Decryption
Algorithm
1
3
4
Check for
Match
2
Perform Function
Indicated by
button press
5
Crypt Key
© 2011 Microchip Technology Inc.
DS41098D-page 5
HCS201
2.0
ENCODER OPERATION
As shown in the typical application circuits (Figure 2-1),
the HCS201 is a simple device to use. It requires only
the addition of buttons and RF circuitry for use as the
transmitter in your security application. A description of
each pin is given in Table 2-1.
FIGURE 2-1:
TYPICAL CIRCUITS
TABLE 2-1:
PIN DESCRIPTIONS
The HCS201 will wake-up upon detecting a button
press and delay approximately 10 ms for button
debounce (Figure 2-2). The synchronization counter,
discrimination value and button information will be
encrypted to form the hopping code. The hopping code
portion will change every transmission, even if the
same button is pushed again. A code word that has
been transmitted will not repeat for more than 64K
transmissions. This provides more than 18 years of use
before a code is repeated; based on 10 operations per
day. Overflow information sent from the encoder can be
used to extend the number of unique transmissions to
more than 192K.
If in the transmit process it is detected that a new but-
ton(s) has been pressed, a RESET will immediately
occur and the current code word will not be completed.
Please note that buttons removed will not have any
effect on the code word unless no buttons remain
pressed; in which case the code word will be completed
and the power-down will occur.
V
DD
B0
Tx out
S0
S1
S2
V
DDB
STEP
V
DD
DATA
V
SS
Two button remote control
B1
Tx out
S0
S1
S2
STEP
V
DD
DATA
V
SS
Four button remote control
B3 B2 B1 B0
Note:
Up to 7 functions can be implemented by pressing
more than one button simultaneously or by using a
suitable diode array.
S0
S1
S2
STEP
V
DD
DATA
V
SS
V
DDB
V
DDB
V
DD
Tx out
Three button remote control with Step up regulator
V
DD
2.0-6.0V
External components sample values:
R = 5.1 K
Ω L = 390 uH Q = 2N3904
C = 1.0 uF D = ZHCS400CT (40V 0.4A Zetex)
(see Section 5.6 for a description of the Step Up circuit)
R
L
D
C
Q
Pin
Name
Pin
Number
Pin Description
S0
1
Switch input 0
S1
2
Switch input 1
S2
3
Switch input 2 / Clock pin for
Programming mode
V
DDB
4
Battery input pin, supplies power
to the step up control circuitry
V
SS
5
Ground reference connection
DATA
6
Pulse Width Modulation (PWM)
output pin / Data pin for
Programming mode
STEP
7
Step up regulator switch control
V
DD
8
Positive supply voltage
HCS201
DS41098D-page 6
© 2011 Microchip Technology Inc.
FIGURE 2-2:
ENCODER OPERATION
3.0
EEPROM MEMORY
ORGANIZATION
The HCS201 contains 192 bits (12 x 16-bit words) of
EEPROM memory (Table 3-1). This EEPROM array is
used to store the encryption key information, synchro-
nization value, etc. Further descriptions of the memory
array is given in the following sections.
TABLE 3-1:
EEPROM MEMORY MAP
3.1
KEY_0 - KEY_3 (64-Bit Crypt Key)
The 64-bit crypt key is used to create the encrypted
message transmitted to the receiver. This key is calcu-
lated and programmed during production using a key
generation algorithm. The key generation algorithm
may be different from the K
EE
L
OQ
algorithm. Inputs to
the key generation algorithm are typically the transmit-
ter’s serial number and the 64-bit manufacturer’s code.
While the key generation algorithm supplied from
Microchip is the typical method used, a user may elect
to create their own method of key generation. This may
be done providing that the decoder is programmed with
the same means of creating the key for
decryption purposes.
3.2
SYNC (Synchronization Counter)
This is the 16-bit synchronization value that is used to
create the hopping code for transmission. This value
will increment after every transmission.
Power-Up
RESET and Debounce Delay
(10 ms)
Sample Inputs
Update Sync Info
Encrypt With
Load Transmit Register
Buttons
Added
?
All
Buttons
Released
?
(A button has been pressed)
Transmit
Stop
No
Yes
No
Yes
Crypt Key
Complete Code
Word Transmission
WORD
ADDRESS
MNEMONIC
DESCRIPTION
0
KEY_0 64-bit
encryption
key
(word 0)
1
KEY_1
64-bit encryption key
(word 1)
2
KEY_2
64-bit encryption key
(word 2)
3
KEY_3 64-bit
encryption
key
(word 3)
4
SYNC
16-bit synchronization
value
5
RESERVED Set to 0000H
6
SER_0
Device Serial Number
(word 0)
7
SER_1
Device Serial Number
(word 1)
8
SEED_0
Seed Value (word 0)
9
SEED_1
Seed Value (word 1)
10
DISC
Discrimination Word
11
CONFIG
Config Word
© 2011 Microchip Technology Inc.
DS41098D-page 7
HCS201
3.3
Reserved
Must be initialized to 0000H.
3.4
SER_0, SER_1
(Encoder Serial Number)
SER_0 and SER_1 are the lower and upper words of
the device serial number, respectively. Although there
are 32 bits allocated for the serial number, only the
lower order 28 bits are transmitted. The serial number
is meant to be unique for every transmitter.
3.5
SEED_0, SEED_1 (Seed Word)
The 2-word (32-bit) seed code will be transmitted when
all three buttons are pressed at the same time (see
Figure 4-2). This allows the system designer to imple-
ment the secure learn feature or use this fixed code
word as part of a different key generation/tracking pro-
cess.
TABLE 3-2:
DISCRIMINATION WORD
3.6
DISC
(Discrimination Word)
The discrimination value aids the post-decryption
check on the decoder end. It may be any value, but in
a typical system it will be programmed as the 12 Least
Significant bits of the serial number. Values other than
this must be separately stored by the receiver when a
transmitter is learned. The discrimination bits are part
of the information that form the encrypted portion of
the transmission (Figure 4-2). After the receiver has
decrypted a transmission, the discrimination bits are
checked against the receiver’s stored value to verify
that the decryption process was valid. If the discrimi-
nation value was programmed as the 12 LSb’s of the
serial number then it may merely be compared to the
respective bits of the received serial number; saving
EEPROM space.
3.7
CONFIG
(Configuration Word)
The Configuration Word is a 16-bit word stored in
EEPROM array that is used by the device to store infor-
mation used during the encryption process, as well as
the status of option configurations. Further explana-
tions of each of the bits are described in the following
sections.
TABLE 3-3:
CONFIGURATION WORD
3.7.1
OSCILLATOR TUNING BITS
(OSC0 AND OSC3)
These bits are used to tune the frequency of the
HCS201 internal clock oscillator to within ±10% of its
nominal value over temperature and voltage.
3.7.2
LOW VOLTAGE TRIP POINT
SELECT (V
LOWS
)
The low voltage trip point select bit (V
LOWS
) and the S3
setting bit (S3SET) are used to determine when to send
the V
LOW
signal to the receiver.
* See also Section 3.7.6
Bit Number
Bit Description
0
Discrimination Bit 0
1
Discrimination Bit 1
2
Discrimination Bit 2
3
Discrimination Bit 3
4
Discrimination Bit 4
5
Discrimination Bit 5
6
Discrimination Bit 6
7
Discrimination Bit 7
8 Discrimination
Bit
8
9
Discrimination Bit 9
10
Discrimination Bit 10
11
Discrimination Bit 11
12
Not Used
13
Not Used
14
Not Used
15
Not Used
Bit Number
Bit Name
0
OSC0
1
OSC1
2
OSC2
3
OSC3
4
V
LOWS
5
BRS
6
MTX4
7
TXEN
8 S3SET
9
XSER
10
Not Used
11
Not Used
12
Not Used
13
Not Used
14
Not Used
15
Not Used
TABLE 3-4:
TRIP POINT SELECT
V
LOWS
S3SET*
Trip Point
0
0
4.4
0
1
4.4
1
0
9
1
1
6.75
HCS201
DS41098D-page 8
© 2011 Microchip Technology Inc.
3.7.3
BAUD RATE SELECT BITS (BRS)
BRS selects the speed of transmission and the code
word blanking. Table 3-5 shows how the bit is used to
select the different baud rates and Section 5.5 provides
detailed explanation in code word blanking.
TABLE 3-5:
BAUDRATE SELECT
3.7.4
MINIMUM FOUR TRANSMISSIONS
(MTX4)
If this bit is cleared, only one code is completed if the
HCS201 is activated. If this bit is set, at least four com-
plete code words are transmitted, even if code word
blanking is enabled.
3.7.5
TRANSMIT PULSE ENABLE (TXEN)
If this bit is cleared, no transmission pulse is transmit-
ted before a transmission. If the bit is set, a START
pulse (1 T
E
long) is transmitted after button de-bounc-
ing, before the preamble of the first code word.
3.7.6
S3 SETTING (S3SET)
This bit determines the value of S3 in the function code
during a transmission and the high trip point selected
by V
LOWS
in section 3.6.2. If this bit is cleared, S3 mir-
rors S2 during a transmission. If the S3SET bit is set,
S3 in the function code (Button Status) is always set,
independent of the value of S2.
3.7.7
EXTENDED SERIAL NUMBER
(XSER)
If this bit is set, a long 32-bit Serial Number is transmit-
ted. If this bit is cleared, a standard 28-bit Serial Number
is transmitted followed by 4 bits of the function code
(Button Status).
BRS
Basic Pulse
Element
Code Words
Transmitted
0
400
μs
All
1
200
μs
1 out of 2
© 2011 Microchip Technology Inc.
DS41098D-page 9
HCS201
4.0
TRANSMITTED WORD
4.1
Code Word Format
The HCS201 code word is made up of several parts
(Figure 4-1). Each code word contains a 50% duty
cycle preamble, a header, 32 bits of encrypted data and
34 bits of fixed data followed by a guard period before
another code word can begin. Refer to Table 9-4 for
code word timing.
4.2
Code Word Organization
The HCS201 transmits a 66-bit code word when a
button is pressed. The 66-bit word is constructed from
a Fixed Code portion and an Encrypted Code portion
(Figure 4-2).
The 32 bits of Encrypted Data are generated from 4
button bits, 12 discrimination bits and the 16-bit sync
value. The encrypted portion alone provides up to four
billion changing code combinations.
The 34 bits of Fixed Code Data are made up of 2 sta-
tus bits, 4 button bits and the 28-bit serial number. The
fixed and encrypted sections combined increase the
number of code combinations to 7.38 x 10
19
.
FIGURE 4-1:
CODE WORD FORMAT
FIGURE 4-2:
CODE WORD ORGANIZATION
LOGIC ‘0’
LOGIC ‘1’
Bit
Period
Preamble
Header
Encrypted Portion
of Transmission
Fixed Portion of
Transmission
Guard
Time
T
P
T
H
T
HOP
T
FIX
T
G
T
E
T
E
T
E
50% Duty Cycle
1
V
LOW
(1 bit)
Button
Status
S2 S1 S0 S3
Serial Number
(28 bits)
Button
Status
S2 S1 S0 S3
DISC
(12 bits)
Sync Counter
(16 bits)
1
V
LOW
(1 bit)
Button
Status
1 1 1 1
Serial Number
(28 bits)
SEED
(32 bits)
34 bits of Fixed Portion
32 bits of Encrypted Portion
66 Data bits
Transmitted
LSb first.
LSb
MSb
MSb
LSb
SEED replaces Encrypted Portion when all button inputs are activated at the same time.
HCS201
DS41098D-page 10
© 2011 Microchip Technology Inc.
4.3
Synchronous Transmission Mode
Synchronous Transmission mode can be used to clock
the code word out using an external clock.
To enter Synchronous Transmission mode, the Pro-
gramming mode start-up sequence must be executed
as shown in Figure 4-3. If either S1 or S0 is set on the
falling edge of S2 (or S3), the device enters Synchro-
nous Transmission mode. In this mode, it functions as
a normal transmitter, with the exception that the timing
of the PWM data string is controlled externally and 16
extra bits are transmitted at the end with the code word.
The button code will be the S0, S1 value at the falling
edge of S2 or S3. The timing of the PWM data string is
controlled by supplying a clock on S2 or S3 and should
not exceed 20 kHz. The code word is the same as in
PWM mode with 16 reserved bits at the end of the
word. The reserved bits can be ignored. When in Syn-
chronous Transmission mode S2 or S3 should not be
toggled until all internal processing has been com-
pleted as shown in Figure 4-4.
FIGURE 4-3:
SYNCHRONOUS TRANSMISSION MODE (TXEN=0)
FIGURE 4-4:
CODE WORD ORGANIZATION (SYNCHRONOUS TRANSMISSION MODE)
“01,10,11”
PWM
S2
S[1:0]
T
PS
T
PH
1 T
PH
2
t = 50ms
Preamble
Header
Data
Reserved
(16 bits)
Padding
(2 bits)
Button
Status
S2 S1 S0 S3
Serial Number
(28 bits)
Button
Status
S2 S1 S0 S3
DISC
(12 bits)
Sync Counter
(16 bits)
82 Data bits
Transmitted
LSb first.
LSb
MSb
Fixed Portion
Encrypted Portion