HCS301 - KeeLoQ Code Hopping Encoder

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© 2011 Microchip Technology Inc.

DS21143C-page 1

HCS301

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 - 13.0V operation
• Four button inputs
• No additional circuitry required
• 15 functions available 
• Selectable baud rate
• Automatic code word completion
• Battery low signal transmitted to receiver
• Battery low indication on LED
• Non-volatile synchronization data

Other

• Functionally identical to HCS300
• Easy-to-use programming interface
• On-chip EEPROM
• On-chip oscillator and timing components
• Button inputs have internal pull-down resistors
• Current limiting on LED output
• Low external component cost

Typical Applications

The HCS301 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 HCS301 from Microchip Technology Inc. is a code
hopping encoder designed for secure Remote Keyless
Entry (RKE) systems. The HCS301 utilizes the
K

EE

L

OQ®

 code hopping technology, which incorpo-

rates high security, a small package outline and low
cost, to make this device a perfect solution for unidirec-
tional remote keyless entry systems and access control
systems.

PACKAGE TYPES

HCS301 BLOCK DIAGRAM

The HCS301 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

S3

V

DD

LED

PWM

V

SS

PDIP, SOIC

HCS30

1

V

SS

V

DD

Oscillator

RESET circuit

LED driver

Controller

Power

latching

and

switching

Button input port

32-bit shift register

Encoder

EEPROM

PWM 

  

LED 

  

S3 S2

S1 S0

K

EE

L

OQ

®

 Code Hopping Encoder

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HCS301

DS21143C-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 HCS301 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 9-2).

• 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 HCS301 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 HCS301 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 HCS301, 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 HCS301 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

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DS21143C-page 3

HCS301

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
HCS301 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 1-1). 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 HCS301 based transmitter. Section 7.0
provides detail on integrating the HCS301 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

.

.

.

HCS301

Production

Programmer

EEPROM Array

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HCS301

DS21143C-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

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© 2011 Microchip Technology Inc.

DS21143C-page 5

HCS301

2.0

DEVICE OPERATION

As shown in the typical application circuits (Figure 2-1),
the HCS301 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 HCS301 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.

Note:

When V

DD

 > 9.0V and driving low capaci-

tive loads, a resistor with a minimum value
of 50

Ω should be used in line with V

DD

.

This prevents clamping of PWM at 9.0V in
the event of PWM overshoot.

B0

Tx out

S0

S1
S2
S3

LED

V

DD

PWM

V

SS

2 button remote control

B1

Tx out

S0

S1
S2
S3

LED

V

DD

PWM

V

SS

5 button remote control 

(1)

B4  B3  B2 B1  B0

Note 1:

Up to 15 functions can be implemented by pressing

more than one button simultaneously or by using a
suitable diode array.

2:

Resistor R is recommended for current limiting.

+12V

R

+12V

R

(2)

(2)

Name

Pin

Number

Description

S0

1

Switch input 0

S1

2

Switch input 1

S2

3

Switch input 2 / Clock pin when in
Programming mode

S3

4

Switch input 3

V

SS

5

Ground reference

PWM

6

Pulse Width Modulation (PWM)
output pin / Data pin for
Programming mode

LED

7

Cathode connection for LED

V

DD

8

Positive supply voltage

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HCS301

DS21143C-page 6

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FIGURE 2-2:

ENCODER OPERATION 

3.0

EEPROM MEMORY 
ORGANIZATION

The HCS301 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,
synchronization 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.

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) LSb’s

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) MSb’s

4

SYNC

16-bit synchronization
value 

5

RESERVED Set to 0000H

6

SER_0

Device Serial Number
(word 0) LSb’s

7

SER_1(Note) Device Serial Number

(word 1) MSb’s

8

SEED_0

Seed Value (word 0)

9

SEED_1

Seed Value (word 1)

10

RESERVED Set to 0000H

11

CONFIG

Config Word

Note:

The MSB of the serial number contains a bit 
used to select the Auto-shutoff timer.

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DS21143C-page 7

HCS301

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.

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. 

3.4.1

AUTO-SHUTOFF TIMER ENABLE

The Most Significant bit of the serial number (Bit 31) is
used to turn the Auto-shutoff timer on or off. This timer
prevents the transmitter from draining the battery
should a button get stuck in the on position for a long
period of time. The time period is approximately
25 seconds, after which the device will go to the Time-
out mode. When in the Time-out mode, the device will
stop transmitting, although since some circuits within
the device are still active, the current draw within the
Shutoff mode will be higher than Standby mode. If the
Most Significant bit in the serial number is a one, then
the Auto-shutoff timer is enabled, and a zero in the
Most Significant bit will disable the timer. The length of
the timer is not selectable.

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.

3.6

CONFIG (Configuration Word)

The Configuration Word is a 16-bit word stored in
EEPROM array that is used by the device to store
information used during the encryption process, as well
as the status of option configurations. The following
sections further explain these bits.

TABLE 3-2:

CONFIGURATION WORD 

3.6.1

DISCRIMINATION VALUE 
(DISC0 TO DISC9) 

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 10 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 discrimina-
tion value was programmed as the 10 LSb’s of the
serial number then it may merely be compared to the
respective bits of the received serial number; saving
EEPROM space.

3.6.2

OVERFLOW BITS 
(OVR0, OVR1) 

The overflow bits are used to extend the number of
possible synchronization values. The synchronization
counter is 16 bits in length, yielding 65,536 values
before the cycle repeats. Under typical use of
10 operations a day, this will provide nearly 18 years of
use before a repeated value will be used. Should the
system designer conclude that is not adequate, then
the overflow bits can be utilized to extend the number

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

Discrimination Bit 8

9

Discrimination Bit 9

10

Overflow Bit 0 (OVR0)

11

Overflow Bit 1 (OVR1)

12

Low Voltage Trip Point Select 
(V

LOW

 

SEL

)

13

Baud rate Select Bit 0 (BSL0)

14

Baud rate Select Bit 1 (BSL1)

15

Reserved, set to 0

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HCS301

DS21143C-page 8

© 2011 Microchip Technology Inc.

of unique values. This can be done by programming
OVR0 and OVR1 to 1s at the time of production. The
encoder will automatically clear OVR0 the first time that
the synchronization value wraps from 0xFFFF to
0x0000 and clear OVR1 the second time the counter
wraps. Once cleared, OVR0 and OVR1 cannot be set
again, thereby creating a permanent record of the
counter overflow. This prevents fast cycling of 64K
counter. If the decoder system is programmed to track
the overflow bits, then the effective number of unique
synchronization values can be extended to 196,608. 

3.6.3

 BAUD RATE SELECT BITS 
(BSL0, BSL1)

BSL0 and BSL1 select the speed of transmission and
the code word blanking. Table 3-3 shows how the bits
are used to select the different baud rates and
Section 5.7 provides detailed explanation in code word
blanking.

TABLE 3-3:

BAUD RATE SELECT

3.6.4

LOW VOLTAGE TRIP POINT 
SELECT

The low voltage trip point select bit is used to tell the
HCS301 what 

V

DD

 level is being used. This information

will be used by the device to determine when to send the
voltage low signal to the receiver. When this bit is set to
a one, the 

V

DD

 level is assumed to be operating from a

9V or 12V 

V

DD

 level. If the bit is set low, then the 

V

DD

 level

is assumed to be 6.0 volts. Refer to Figure 3-1 for volt-
age trip point. 

FIGURE 3-1:

VOLTAGE TRIP POINTS 
BY CHARACTERIZATION  

BSL1

BSL0

Basic Pulse 

Element

Code Words 

Transmitted

0

0

400 

μs

All

0

1

200 

μs

1 out of 2

1

0

100 

μs

1 out of 2

1

1

100 

μs

1 out of 4

-40

20 40

100

8.5

7.5

8.0

7.0

9.0

2.5

3.0

3.5

4.0

V

LOW

Temp (C)

Volts (V)

V

LOW

 sel = 1

V

LOW

 sel = 0

4.5

-20

0

60 80

5.0

5.5

Max

Min

Max

Min

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DS21143C-page 9

HCS301

4.0

TRANSMITTED WORD

4.1

Code Word Format

The HCS301 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 HCS301 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

    

Repeat

(1 bit)

V

LOW

(1 bit)

Button 

Status

S2 S1 S0 S3 

Serial Number

(28 bits)

Button 

Status

S2 S1 S0 S3 

OVR

(2 bits)

DISC

(10 bits)

Sync Counter

(16 bits)

Repeat

(1 bit)

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

Note: SEED replaces Encrypted Portion when all button inputs are activated at the same time.

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HCS301

DS21143C-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

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+ OVR

(12 bits)

Sync Counter

(16 bits)

82 Data bits

Transmitted

LSb first.

LSb

MSb

 Fixed Portion

Encrypted Portion

Maker
Microchip Technology Inc.