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1992-03-01
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VIDCClock
---------
This module provides integrated software control of AtomWide and compatible
Turbo VIDC hardware when the system concerned is fitted with RISC OS 2.01 or
later. It does this by sitting on Service_ModeChange, reading the
VIDCClockSpeed VDU variable, and switching the VIDC clock to 24 or 36 MHz as
appropriate. This requires that the modes you use are in A540 VIDC list
format so that the kernel knows what the VIDC clock rate should be. Sound
will then play at the correct pitch and speed in 36 MHz modes.
For this to work, the Turbo VIDC hardware must be installed for software
control on the middle pin (pin 3) of the AUXIO connector, and the Turbo VIDC
must be designed such that logic 0 (power-on value) is 24 MHz and logic 1 is
36 MHz. If your Turbo VIDC uses the logic the other way round, then you will
need to modify the VIDCClock module to cope with this. Also your machine
will fail the Power-On Self Test built into RISC OS 2.01 or later as your
VIDC will be running at 36 MHz when it shouldn't be. Wait long enough (a
minute or two while the floppy disk light flashes) and your machine will
eventually start up.
The source code is supplied in AAsm format, and furthermore it uses Acorn
internal header files which are not supplied. This is because the software
was originally written for my own personal use. The binary of the module is
supplied so if you need to change the source you can work out what all the
constants are by dissasembling the binary. Apologies for any inconvenience
this causes.
Owen Smith (osmith@acorn.co.uk)
1st March 1992