THE
COMPLETE AMIGA HARDWARE
REFERENCE MANUAL
APPENDIX H
KEYBOARD
This appendix contains the keyboard interface specification for A1000,
A500 and A2000.
The keyboard plugs into the Amiga computer via a cable with four primary
connections. The four wires provide 5-volt power, ground, and signals
called KCLK (keyboard clock) and KDAT keyboard data). KCLK is
unidirectional and always driven by the keyboard; KDAT is driven by both
the keyboard and the computer. Both signals are open-collector, there are
pullup resistors in both the keyboard (inside the keyboard
microprocessor) and the computer.
- Appendix H 343 -
KEYBOARD COMMUNICATIONS
The keyboard transmits 8-bit data words serially to the main unit. Before
the transmission starts, both KCLK and KDAT are high. The keyboard starts
the transmission by putting out the first data bit (on KDAT), followed by
a pulse on KCLK (low then high); then it puts out the second data bit and
pulses KCLK until all eight data bits have been sent. After the end of
the last KCLK pulse, the keyboard pulls KDAT high again.
When the computer has received the eighth bit, it must pulse KDAT low for
at least 1 (one) microsecond, as a handshake signal to the keyboard. The
handshake detection on the keyboard end will typically use a hardware
latch The keyboard must be able to detect pulses greater than or equal to
1 microsecond. Sofware MUST pulse the line low for 85 microseconds to
ensure compatibility with all keyboard models.
All codes transmitted to the computer are rotated one bit before
transmission. The transmitted order is therefore 6-5-4-3-2-1-0-7. The reason
for this is to transmit the up/down flag last, in order to cause a key-up
code to be transmitted in case the keyboard is forced to restore lost
sync (explained in more detail below).
The KDAT line is active low; that is, a high level (+5V) is interpreted
as 0, and a low level (0V) is interpreted as 1.
___ ___ ___ ___ ___ ___ ___ ___ _______
KCLK \_/ \_/ \_/ \_/ \_/ \_/ \_/ \_/
_______________________________________________________
KDAT \_____x_____x_____x_____x_____x_____x_____x_____/
(6) (5) (4) (3) (2) (1) (0) (7)
First Last
sent sent
The keyboard processor sets the KDAT line about 20 microseconds before it
pulls KCLK low. KCLK stays low for about 20 microseconds, then goes high
again. The processor waits another 20 microseconds before changing KDAT.
Therefore, the bit rate during transmission is about 60 microseconds per
bit, or 17 kbits/sec.
- 344 Appendix H -
KEYCODES
Each key has a keycode associated with it (see accompanying table).
Keycodes are always 7 bits long. The eighth bit is a "key-up"/"key-down"
flag; a 0 (high level) means that the key was pushed down, and a 1 (low
level) means the key was released (the CAPS LOCK key is different - see
below).
For example, here is a diagram of the "B" key being pushed down. The
keycode for "B" is $35=00110101; due to the rotation of the byte, the
bits transmitted are 01101010.
___ ___ ___ ___ ___ ___ ___ ___ _______
KCLK \_/ \_/ \_/ \_/ \_/ \_/ \_/ \_/
_______ ______ ______ ____________
KDAT \__________/ \____/ \____/
0 1 1 0 1 0 1 0
In the next example, the "B" key is released. The keycode is still $35,
except that bit 7 is set to indicate "key-up," resulting in a code of $B5
= 10110101. After rotating, the transmission will be 01101011:
___ ___ ___ ___ ___ ___ ___ ___ _______
KCLK \_/ \_/ \_/ \_/ \_/ \_/ \_/ \_/
_______ ______ ______ ______
KDAT \__________/ \____/ \__________/
0 1 1 0 1 0 1 0
CAPS LOCK KEY
This key is different from all the others in that it generates a keycode
only when it is pushed down, never when it is released. However, the
up/down bit is still used. When pushing the CAPS LOCK key turns on the
CAPS LOCK LED, the up/down bit will be 0; when pushing CAPS LOCK shuts
off the LED, the up/down bit will be 1.
- Appendix H 345 -
"OUT-OF-SYNC" CONDITION
Noise or other glitches may cause the keyboard to get out of sync with
the computer. This means that the keyboard is finished transmitting a
code, but the computer is somewhere in the middle of receiving it.
If this happens, the keyboard will not receive its handshake pulse at the
end of its transmission. If the handshake pulse does not arrive within
143 ms of the last clock of the transmission, the keyboard will assume
that the computer is still waiting for the rest of the transmission and
is therefore out of sync. The keyboard will then attempt to restore sync
by going into "resync mode."
In this mode, the keyboard clocks out a 1 and waits for a handshake
pulse. If none arrives within 143 ms, it clocks out another 1 and waits
again. This process will continue until a handshake pulse arrives.
Once sync is restored, the keyboard will have clocked a garbage character
into the computer. That is why the key-up/key-down flag is always
transmitted last. Since the keyboard clocks out 1's to restore sync, the
garbage character thus transmitted will appear as a key release, which is
less dangerous than a key hit.
Whenever the keyboard detects that it has lost sync, it will assume that
the computer failed to receive the keycode that it had been trying to
transmit. Since the computer is unable to detect lost sync, it is the
keyboard's responsibility to inform the computer of the disaster. It does
this by transmitting a "lost sync" code (value $F9 = 11111001) to the
computer. Then it retransmits the code that had been garbled.
NOTE
The only reason to transmit the "lost sync" code to the computer is to
alert the software that something may be screwed up. The "lost sync" code
does not help the recovery process, because the garbage keycode can't be
deleted, and the correct key code could simply be retransmitted without
telling the computer that there was an error in the previous one.
POWER-UP SEQUENCE
There are two possible ways for the keyboard to be powered up under
normal circumstances: <1> the computer can be turned on with the keyboard
plugged in, or <2> the keyboard can be plugged into an already "on"
computer. The keyboard and computer must handle either case without
causing any upset.
- 346 Appendix H -
The first thing the keyboard does on power-up is to perform a self-test.
This involves a ROM checksum test, simple RAM test, and watchdog timer
test. Whenever the keyboard is powered up (or restarted - see below), it
must not transmit anything until it has achieved synchronization with the
computer. The way it does this is by slowly clocking out 1 bits, as
described above, until it receives a handshake pulse.
If the keyboard is plugged in before power-up, the keyboard may continue
this process for several minutes as the computer struggles to boot up and
get running. The keyboard must continue clocking out ls for however long
is necessary, until it receives its handshake.
If the keyboard is plugged in after power-up, no more than eight clocks
will be needed to achieve sync. In this case, however, the computer may
be in any state imaginable but must not be adversely affected by the
garbage character it will receive. Again, because it receives a key
release, the damage should be minimal. The keyboard driver must
anticipate this happening and handle it, as should any application that
uses raw keycodes.
NOTE
The keyboard must not transmit a "lost sync" code after re-synchronizing
due to a power-up or restart; only after re-synchronizing due to a
handshake time-out.
Once the keyboard and computer are in sync, the keyboard must inform the
computer of the results of the self-test. If the self-test failed for any
reason, a "selftest failed" code (value $FC = 11111100) is transmitted
(the keyboard does not wait for a handshake pulse after sending the"
selftest failed" code). After this, the keyboard processor goes into a
loop in which it blinks the CAPS LOCK LED to inform the user of the
failure. The blinks are coded as bursts of one, two, three, or four
blinks, approximately one burst per second:
One blink ROM checksum failure.
Two blinks RAM test failed.
Three blinks Watchdog timer test failed.
Four blinks A short exists between two row lines
or one of the seven special keys (not implemented).
If the self-test succeeds, then the keyboard will proceed to transmit any
keys that are currently down. First, it sends an "initiate power-up key
stream" code (value $FD = 11111101), followed by the key codes of all
depressed keys (with keyup/down set to "down" for each key). After any
keys are sent (usually there won't be any at all), a "terminate key
stream" code (value $FE = 11111110) is sent. Finally, the CAPS LOCK LED
is shut off. This marks the end of the start-up sequence, and normal
processing commences.
The usual sequence of events will therefore be: power-up; synchronize;
transmit "initiate power-up key stream" ($FD); transmit "terminate key
stream2 ($FE).
- Appendix H 347 -
RESET WARNING
NOTE
Available on some A1000 and A2000 keyboards. You cannot rely on this
feature for all Amigas.
The keyboard has the additional task of resetting the computer on the
command of the user. The user initiates Reset Warning by simultaneously
pressing the CTRL key and the two "AMIGA" keys.
The keyboard responds to this input by syncing up any pending transmit
operations. The keyboard then sends a "reset warning" to the Amiga. This
action alerts the Amiga software to finish up any pending operations
(such as disk DMA) and prepare for reset.
A specific sequence of operations ensure that the Amiga is in a state
where it can respond to the reset warning. The keyboard sends two actual
"reset warning" keycodes. The Amiga must handshake to the first code like
any normal keystroke, else the keyboard goes directly to Hard Reset. On
the second "reset warning" code the Amiga must drive KDAT low within 250
milliseconds, else the keyboard goes directly to Hard Reset. If the all
the tests are passed, the Amiga has 10 full seconds to do emergency
processing. When the Amiga pulls KDAT high again, the keyboard finally
asserts hard reset.
If the Amiga fails to pull KDAT high within 10 seconds, Hard Reset is
asserted anyway.
HARD RESET
NOTE
This happens after Reset Warning. Valid for all keyboards except the
Amiga 500.
The keyboard Hard Resets the Amiga by pulling KCLK low and starting a 500
millisecond timer. When one or more of the keys is released AND 500
milliseconds have passed, the keyboard will release KCLK. 500
milliseconds is the minimum time KCLK must be held low. The maximum KCLK
time depends on how long the user holds the three keys down.
NOTE
Circuitry on the Amiga motherboard detects the 500 millisecond KCLK
pulse.
- 348 Appendix H -
After releasing KCLK, the keyboard jumps to its start-up code (internal
RESET). This will initialize the keyboard in the same way as cold power-
on.
NOTE
The keyboard must resend the "powerup key stream"!
SPECIAL CODES
The special codes that the keyboard uses to communicate with the main
unit are summarized here.
NOTE
The special codes are 8-bit numbers; there is no up/down flag associated
with them.
However, the transmission bit order is the same as previously described.
CODE NAME MEANING
-----------------------------------------------------------------------
78 Reset warning. CTRL-AMIGA, AMIGA has been hit -
computer will be reset in 10 seconds. (see text)
F9 Last key code bad, next code i9 the same code
retransmitted (used when keyboard and main unit
get out of sync).
FA Keyboard output buffer overflow
FB Unused (was controller failure)
FC Keyboard selftest failed
FD Initiate power-up key stream (keys pressed at powerup)
FE Terminate power-up key stream
FF Unused (was interrupt)
- Appendix H 349 -
MATRIX TABLE
Row 5 Row 4 Row 3 Row 2 Row 1 Row 0
Column (Bit 7) (Bit 6) (Bit 5) (Bit 4) (Bit 3) (Bit 2)
+-------+-------+-------+-------+-------+-------+
15 |(spare)|(spare)|(spare)|(spare)|(spare)|(spare)|
(PD.7)| | | | | | |
| (0E) | (1C) | (2C) | (47) | (48) | (49) |
+-------+-------+-------+-------+-------+-------+
14 | * |
|