Commodore Amiga - Page 304

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Once sync is restored, the keyboard will have clocked a garbage
character into the CODputer. That is why the key-up/key-down
flag is always transmitted last. Since the keyboard clocks out
l'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 CODputer failed to receive the keycode that
it had been trying to transmit. Since the CODputer is unable
to detect lost sync, it is the keyboard's responsibility to
inform the conputer of the disaster. It does this by transmit-
ting a "lost sync" code (value F9H = 11111001) to the CODputer.
Then it retransmits the code that had been garbled.
Note: the only reason to transmit the "lost sync" code to the
conputer is to alert the software that something may be screwed
up. 'llle "lost sync" code does not help the recovery process,
because the garbage keycode can't be deleted, and the correct
key code could sinply be retransmitted without telling the
conputer 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: the conputer can be turned on
with the keyboard plugged in, or the keyboard can be plugged
into an already "on" conputer. 'llle keyboard and conputer
III\lSt handle either case without causing any upset.
'llle first thing the keyboard does on power-up is to perform
a self-test. This involves a ROM checksum test, sinple 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 CODputer.
'llle 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 CODputer
struggles to boot up and get running. 'llle keyboard IlJJ.St
continue clocking out Is 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 CODputer 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 III\lSt
anticipate this happening and handle it, as should any
application that uses raw keycodes.
Note: the keyboard I1lIlSt not transmit a "lost sync" code after
resyncing due to a power-up or restart; only after resyncing
due to a handshake time-out.
Once the keyboard and CODputer are in sync, the keyboard III\lSt
inform the conputer of the results of the self-test. If the
self-test failed for any reason, a "self test failed" code
(value FCH = 11111100) is transmitted (the keyboard does not
wait for a handshake pulse after sending the "self test 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. 'llle 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 (this last test isn't inplemented yet) .
If the self-test succeeds, then the keyboard will proceed to
transmit any keys that are currently down. First, it sends
an "initiate powerup key stream" code (value FDH = 11111101),
followed by the key codes of all depressed keys (with
keyup/down set to "down" for each key). After all keys are
sent (usually there won't be any at all), a "terminate key
stream" code (value FEH = 11111110) is sent. Finally, the
CAPS LOCK LED is shut off. This marks the end of the
start-up sequence, and normal processing c:omnences.
Note: These special codes, (that is, FCH et al) are a-bit
numbers; there is no up/down flag associated with them.
However, the transmission bit order is the same as previously
described.
'llle usual sequence of events will therefore be: power up;
synchronize; transmit "initiate powerup key stream" (FDH);
transmit "terminate key stream" (FEH).
Hard Reset
'llle keyboard has the additional task of resetting the ~nputer
on the conmand of the user. The user initiates hard reset by
simultaneously pressing the CTRL key and the two "JlMlGA." keys.
'llle keyboard responds to this input by pulling KCLK low and
starting a SOO-ms timer. At the end of the 500 ms, the
processor checks the three keys to see if they are still down,
and if so, restarts the SOO-ms timer. This continues until one
or more of the three keys is released .
When one or more keys is released, then the processor will wait
until the end of the 500 ms. Then it junps to its start-up
code, which releases KCLK and restarts the keyboard.
Special Codes
'llle special codes that the keyboard uses to conmunicate with
the main unit are summarized here.