Date   : Wed, 24 Jan 1996 19:20:29 +0000 (GMT)
From   : James Fidell <james@...>
Subject: Re: System VIA help

Tom Seddon wrote:

> (1) Why is there a 'write enable' for the keyboard interface as part
> of the addressable latch? What does writing to the keyboard do? And
> can I safely ignore completely any writes to the sound or speech
> chips, and any reads from the speech chip?

It's more a way of enabling keyboard polling rather than writing to the
keyboard. I worked it all out from the circuit diagram of the keyboard
in the back of the AUG, though the key values are wrong in places, I
believe.

> (2) The addressable latch has bits enabling the various devices
> connected to port A, so when accessing port A, how does the system
> decide which device is to be accessed (since you could enable all the
> devices at once)?

Not so.  The output part of port A is connected to a de-multiplexer
whose outputs form the enable inputs of each of the relevant bits of
hardware.

> (3) When reading from Port B the bottom 4 bits are unused (or is it 
> the bottom 3?) and not linked to anything. I assume that if you set 
> DDRB to all inputs and read from Port B these unused bits would 
> return 0... is this correct?

From memory, if latching is enabled, they would return whatever was on
the input lines last time the latch was set.  If it isn't, it returns
whatever is on the input lines at the time the read is done.  If there
is no input connected to the pin, I'd guess that they float high, but it's
also possible that you would get a random input value.

> (4) Timer 2 can be set to decrement on pulses from PB6, but what
> exactly is a 'pulse'? Does PB6 have to be 1, then 0, or is a 'pulse'
> any time it changes?

The documentation appears to indicate that it decrements with negative
transitions of PB6, which means that it counts each time PB6 changes
from high to low.

> (5) Does anything use the shift register? XBeeb and BeebEm don't seem 
> to support it, so I assume it can't be that important... does anybody 
> know otherwise?

I don't support the shift register because there's nothing in my emulation
that needs it.  I assume that it is used for the RS423 port and the
cassette system.

James.

-- 
 "Yield to temptation --             | Work: james@...          
  it may not pass your way again"    | Play: james@...                 
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