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FlexiCoderTM K25C81
Versatile PC/XC/AT/PS/2 Compatible
Keyboard Encoder
HID & SYSTEM MANAGEMENT PRODUCTS, KEYCODERTM FAMILY
DESCRIPTION
The FlexiCoderTM is a versatile, low-
power keyboard encoder. It offers
two bi-directional channels for
communicating with a
PC/XT/AT/PS2 system, and/or any
optional keyboard-compatible
devices, such as an 83 or 101/102
standard desktop keyboard, OCR,
barcode reader, etc. The K25C81
directly interfaces a PC keyboard
port, making custom keyboard to
system communications fully
transparent.
On an 8 x 18 matrix, the
FlexiCoderTM will scan, debounce
and encode up to 144 keys. Scan
codes corresponding to single keys
of the IBM 101 keyboard, or a
combination of these keys when
depressed with Shift, Ctrl, and Alt
keys, are generated with each key
press. Custom keypad input is not
effected by the Shift States of an
external keyboard.
Users can conveniently define key
assignments on the matrix as single
or repeating actions. All keys must
be released between key strokes or
scanning is suspended. The
encoder can buffer up to 122
keycodes, and control inputs and
outputs are provided for interfacing
with contact keyboards.
The FlexiCoderTM is ideal for use
with either BIOS-compatible chip
sets or single-board computers for
PC systems requiring a custom
keyboard / keypad as an alternate
or additional input device. Boards
and evaluation kits are immediately
obtainable and ready to connect.
FEATURES
• Directly interfaces a PC
keyboard port
• Encodes up to 144 custom keys
on an 8 X 18 matrix
• Buffers up to 122 keycodes
• Interfaces to 83/101/102
standard keyboard or other
8042-compatible device
APPLICATIONS
• Custom keyboards/keypads
• Control panels
• Automatic teller machines
• Embedded control systems
• Public Information Kiosks
PIN ASSIGNMENTS
• Enables custom keypad scan
codes to remain unaffected by
the Shift States of an external
keyboard
• Includes jumper-selectable auto-
repeat for designated keys
• Offers two-key inhibit scanning
mode
• Instrumentation
• Test and Measurement equipment
• Point of Sales terminals
• Industrial Controllers
• Medical Instruments
_RESET 1
_IRQ 2
VX 3
C17 4
R6 5
RP 6
KD 7
KC 8
EKC 9
EKD 10
C16 11
C0 12
C1 13
C2 14
C3 15
C4 16
C5 17
C6 18
C7 19
Vss 20
DIP
40 Vcc
39 OSCI
38 OSCO
37 EKC1
36 R7
35 NC
34 R5
33 R4
32 R3
31 R2
30 R1
29 R0
28 C8
27 C9
RP
KD
KC
EKC
EKD
C16
C0
C1
C2
C3
C4
6
7
12
17
18
26 C10
25 C11
24 C12
23 C13
22 C14
21 C15
1
PLCC
23
40
39 R7
NC
R5
R4
R3
34 R2
R1
R0
C8
C9
29 C10
28
R6
RP
KD
KC
EKC
EKD
C16
C0
C1
C2
C3
1
QFP
NC
R5
R4
R3
R2
R1
R0
C8
C9
C10
C11
FlexiCoderTM is a trademark of Semtech Corp. All
other trademarks belong to their respective
companies.
Copyright Semtech, 1997-2001
DOC5-C81-DS-105
1
www.semtech.com

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FUNCTIONAL DIAGRAM
ORDERING CODE
Package options
40-pin, Plastic DIP
44-pin, Plastic PLCC
44-pin, Plastic QFP
Pitch In mm’s
2.54 mm
1.27 mm
0.8 mm
TA = -40°C to +85°C
K25C81-XX-P
K25C81-XX-FN
K25C81-XX-FB
Data Buffer
Keyboard Encoder
EKC1
Interrupt Control
Row Data Inputs
Column Select Ouputs
R0-R7
8
C0-C17
18
KC PC
Communication
KD Channel
EKC
EKD
8042 Emulation
(Standard Keyboard)
Communication Channel
Mode Control
RP
Copyright Semtech, 1997-2001
DOC5-C81-DS-105
2
www.semtech.com

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FUNCTIONAL DESCRIPTION
The FlexiCoderTM consists of six
major sections (see functional
diagram, previous page). These
are the Keyboard Encoder, the
Mode Control Unit, the PC
Communication Channel, the 8042
Emulation Channel, the Interrupt
Control Unit and the Data Buffer.
These sections communicate with
each other and operate
concurrently.
The controller continuously scans a
keyboard organized as an 8 row by
18 column matrix, for a maximum of
144 keys. Smaller-size keyboards
can be connected provided that all
unused row pins are connected to
Vcc. The microcontroller selects
one of the 18 column lines (C0-C17)
every 512 µS and then reads the
row data lines (R0-R7).
A key closure is detected as a zero
in the corresponding position of the
matrix. A complete scan cycle for
the entire keyboard takes approxi-
mately 9.2 mS. Each key press is
debounced for a period of 20 mS.
Once the key is verified, the
corresponding key code(s) are
loaded into the transmit buffer of
the PC Keyboard Communication
Channel.
PIN DEFINITIONS
Mnemonic
Vcc
Vss
OSCI
OSCO
_RESET
DIP
40
20
39
38
1
_IRQ
VX
RP
2
3
6
KC
KD
EKD
EKC
EKC1
8
7
10
9
37
C0-C7
C8-C15
C16
C17
12-19
28-21
11
4
R0-R5
R6
R7
NC
29-34
5
36
35
PLCC
44
22
43
42
1
2
4
7
9
8
11
10
41
3-17
18
19-21
31-24
12
5
32-37
6
39
38, 3
18, 23
40
QFP
38
17
37
36
41
42
43
2
4
3
6
5
35
8-15
26-18
7
44
27-32
1
34
16, 33
22, 39
40
TYPE
I
I
I
O
I
I
I
I
I/O
I/O
I/O
I/O
I
O
O
I/O
I
I
I
-
-
Name and Function
Power Supply: +5V
Ground
Oscilator Input
Oscilator Output
Reset: apply 0 V to provide orderly
start-up.
Must be tied to Vcc
Tie to Vcc.
Rollover Mode Selection: tie to
Vcc to select N-Key Rollover. Tie to
Ground to select Two-Key Inhibit
Mode.
Keyboard Clock: connects to PC’s
keyboard port Clock Line.
Keyboard Data: connects to PC’s
keyboard port Data Line.
External Keyboard Data:
connects to external keyboard Data
Line.
External Keyboard Clock:
connects to external keyboard Clock
Line.
External Keyboard Clock 2:
connects to external keyboard Clock
Line and is used to generate an
interrupt for every Clock Line transition.
This signal must be inverted for PC/XT
keyboards.
Column Select Outputs: select
one of columns.
Keyboard type selection: this pin
is used both as outputs for column
select and as input for standard
keyboard-type selection.
Row data inputs
No Connects: these pins are
unused.
Note: An underscore before a pin mnemonic denotes an active low signal.
Copyright Semtech, 1997-2001
DOC5-C81-DS-105
3
www.semtech.com

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REPEAT KEYS
In custom masks, each key can be
defined individually to be either
Typematic or Single-Touch Action.
In the standard mask, all keys are
defined to be Single-Touch Action,
except the Arrow Keys. Pin RP
controls the repeat action of keys
that are defined to be Typematic.
Repeat Mode
RP
No repeat action
L
Repeat action
H
Table 1: Typematic Action Control
The Typematic rate can be
controlled by issuing the
appropriate commands from the
system.
SPECIAL HANDLING
External Keyboard Connection
If an external keyboard was not
connected during power-on and
then connected at a later time, the
controller will proceed with the
normal reset routine in order to
initialize the external keyboard
properly. Thereafter, the
FlexiCoderTM checks for the
presence of an external keyboard
every 2 seconds. After
communication has been
established, the controller continues
checking for the external
keyboard’s presence. Should the
external keyboard be removed at a
later time, the K25C81 detects the
disconnection and will reinitiate the
reset sequence upon reconnection.
This feature allows the user to
connect and disconnect an external
keyboard at any time without
resetting the system.
FUNCTIONAL DESCRIPTION, (CON’T)
Switch Matrix Encoding
Each matrix location is programmed to represent either a single key or a
combination of keys of the IBM-standard 101/102 keyboard.
Scan Code Table Sets
The FlexiCoderTM supports two scan code table sets. Scan Code Sets 1 and
2 are the default sets for PC/XT and AT/PS/2 systems respectively. For
more information, refer to the IBM Technical Reference Manual. Custom
scan code tables, including macros, are also available. Operating modes
are defined by the logic level of the relevant mode pins in the Mode Control
Unit.
SPECIAL HANDLING, (CON’T)
Scan Matrix Data Integrity
The FlexiCoderTM maintains the integrity of the codes corresponding to the
scanned matrix locations, independently of the Shift Status of the external
keyboard or the system. For example, if a key has been defined on the
matrix to send code corresponding to lower case letter “a”, it will still send
the scan code for “a” even if the Shift Key has been pressed or the Caps
Lock has been set by the external keyboard. This is accomplished by
internally maintaining a set of flags to monitor the Shift Status of the external
keyboard and the system. If, for instance, the lower case key “a” is found to
be pressed and Caps Lock is set, the codes sent will be as follows:
1. Caps Lock Make Code
2. Caps Lock Break (this will reset the Caps Lock status of the system)
3. “a” Make Code
4. “a” Break Code
5. Lock Make Code
6. Caps Lock Break Code (this will restore the Caps Lock status of the
system)
Copyright Semtech, 1997-2001
DOC5-C81-DS-105
4
www.semtech.com

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EXT. KBD COMMUNICATION
The external keyboard
communication port of the
FlexiCoderTM fully emulates a
standard 8042, available to an
83/101/102 external keyboard or
other compatible device.
Communication with an external
keyboard is accomplished via EKC
and EKD, Clock and Data lines
respectively. A third pin, EKC1
which connects to the Clock Line,
interrupts the controller whenever
an external keyboard initiates a
communication.
When power is first applied, the
controller proceeds with the
standard reset sequence with the
external keyboard. Data and
commands coming from the
external keyboard are buffered in
the controller’s FIFO, along with
data from the scanned matrix, and
then presented to the system as if
originating from a single source.
Commands and data from the
system, after being acknowledged,
are then transmitted to the external
keyboard.
MODE CONTROL
The FlexiCoderTM implements all the
standard functions of
communication with a BIOS-
compatible PC/XT or AT/PS/2 host
system. Two lines, KC and KD,
provide bi-directional clock and
data signals according to the
protocol (PC or AT) selected. In
addition, the K25C81 supports
commands from and to the system,
as described in the IBM Technical
Reference Manuals. The following
table shows the commands that the
system may send and their value in
hex.
Command
Hex Value
Set/Reset Status
Indicators
ED
Echo
EE
Invalid Command
EF
Select Alternate
Scan Codes
F0
Invalid Command
F1
Read ID
F2
Set Typematic
Rate/Delay
F3
Enable
F4
Default Disable
F5
Set Default
F6
Set All Keys
Typematic
Make/Break
Make
Typematic/Make/Break
F7
F8
F9
FA
Set Key Type
Typematic
Make/Break
Make
FB
FC
FD
Resend
FE
Reset
FF
Table 2: Keyboard Commands from the
System (AT/PS/2 protocol)
These commands are supported in
the AT/PS/2 protocol and can be
sent to the keyboard at any time.
Mode 1 accepts only the ‘reset’
command. Commands shown in
italics do not affect the operation of
the K25C81. Nevertheless, they
are acknowledged and relayed to
the external keyboard, if an
external standard keyboard is
present.
The following table shows the
commands that the keyboard may
send to the system.
Command
Hex Value
Key Detection
Error/Overrun
00*
Keyboard ID
83AB
BAT Completion Code
AA
BAT Failure Code
FC
Echo
EE
Acknowledge (Ack)
FA
Resend
FE
Key Detection
Error/Overrun
FF*
*Scan Code Set 2
Table 3: Keyboard Commands to the
System (AT/PS/2 protocol)
When an external keyboard is
connected, commands from the
system will also be directed to the
external keyboard. Presence or
absence of an external keyboard
will not affect the normal operation
of the FlexiCoderTM.
Copyright Semtech, 1997-2001
DOC5-C81-DS-105
5
www.semtech.com