The CPU core module
talks to the display using an
8bit wide data bus
and two low active strobe signals
which I call AS (address
strobe) and DS (data strobe).
The CPU transmits
data always in pairs, one address
byte (latched into
the display with AS) and one data
byte (latched in
with DS). The timing varies a little,
but it seems that
the address byte is set up about 32us
before AS, the AS
pulse is about 16us, the byte remains
stable for about
48us. Then the data byte is set up,
after about 32us
comes the DS pulse which lasts for
about 32us. The byte
remains stable for about 16us.
This could be called
a write cycle. All display data
and control commands
are transmitted this way.
The addresses 0..39
(0x00..0x27) match the 40 display
positions, beginning
in the upper left corner:
0.....9
10....19
20....29
30....39
The display has a
256 character ROM, which is more or less
ASCII compatible,
full table see here.
A character is written
on the display by writing the
character code to
the address of the desired display position.
There is a second
address area from 64 to 103 (0x40..0x67)
that seems to be
the character attribute memory.
The addresses also
match the display positions (constant
offset 64). Writing
a 1 here makes the character blink,
writing a 0 turns
blinking off.
Control commands are
write cycles with address 0x70..0x7F
and a data byte (usual
0x00). I saw the following commands:
0x71 second
init command (data byte was 0x02 here)
0x75 'begin'
0x79 contrast
+
0x7A contrast
-
0x7B first
init command
0x7C 'end'
0x7D first
clear command
0x7E second
clear command
The 0x7B 0x71 sequence
happens only after RESET.
All character write
sequences begin with the 0x75 command
and end with the
0x7C command. If the display has to
be cleared (e.g.
mode switching), 0x7D 0x7E follows the 0x75.
After the 0x75 comes
always 15ms pause.
After the 0x7D 0x7E
pair comes 1.5ms pause.
In normal time display
mode, the two seconds digits are
refreshed every second,
and every minute, the entire
display gets refreshed.
In time setting mode,
the entire display gets refreshed
every second.
