Apple II
Technical Notes
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                                                  Developer Technical Support


Mouse
#2:    Varying VBL Interrupt Rate

Revised by:    Matt Deatherage                                  November 1988
Revised by:    Rilla Reynolds                                   November 1985

This Technical Note describes a method to make the AppleMouse peripheral card 
interrupt at a rate other than the default 60 Hz.  This method does not work 
on the Apple IIc or IIGS.
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This Technical Note describes a previously undocumented call to the AppleMouse 
II firmware which allows the user to set the interrupt rate to 50 or 60 Hz.  
(The default is 60 Hz, which keeps the card-generated VBL interrupts 
synchronized with the actual VBL rate on standard North American Apples; 
European Apples use 50 Hz as a standard.)


    Call:               TimeData
    Offset Location:    $Cn1C
    Input:              Accumulator bit 0:    0 for 60 Hz
                                              1 for 50 Hz

    Note:    All other accumulator bits are reserved, and must be set to 0.

    Output:             carry bit clear
                        screen holes unchanged

You must make this call just prior to calling InitMouse to be effective.  If 
you want to change the interrupt rate in the middle of an application, you 
must call TimeData with the appropriate value in the accumulator, then call 
InitMouse (which generates an interrupt).  InitMouse resets the mouse 
position, mode, clamps, etc. to their default values.  If you fail to call 
TimeData, InitMouse will use a default interrupt rate of 60 Hz.

Note:    This call exists only on the AppleMouse card for the IIe or 
][+ and should only be used when you know you are working with a 
IIe or ][+.  A user may configure a IIGS to 50 Hz by holding down 
the Option key while rebooting.  The standard North American Apple 
IIc will not generate 50 Hz VBL interrupts.