Date : Tue, 23 Jan 1996 00:42:06 GMT0BST
From : "Tom Seddon" <T.W.Seddon@...>
Subject: System VIA help
Thanks to everyone who sent me help with my emulator. I found the
problem later that night as it happened but it was only really by
accident. I really was desparate...
This time my request is rather easier (I hope)... I've started
implementing the System VIA and I've now got read/write support for
all the registers except for port B read and port A read/write. My
questions are:
(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?
(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)?
(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?
(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?
(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?
Thanks for any help. Once I've got the VIAs sorted out I'll be able
to get started on the graphics, and I've played with those on the BBC
a lot so (hopefully) I shan't need any help with it.
--Tom
. ** Tom Seddon ** .
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