Technical Note HW550

VIA IC Q&As

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This Technical Note contains a collection of archived Q&As relating to a specific topic--questions sent the Developer Support Center (DSC) along with answers from the DSC engineers. Current Q&A's can be found on the Macintosh Technical Q&A's web site. [Oct 01 1990]

Use Time Manager instead of VIA2 timer T1

Date Written: 11/27/91

Last reviewed: 11/27/91

The Apple Macintosh Family Hardware Reference, first edition, (page 17-28) says VIA2 timer T1 can generate interrupts up to every 1.2766 microsecond. Is there any documentation for programming the timer to generate the interrupt at high speeds and installing the interrupt service routine? By the way, the Time Manager documented in Inside Macintosh Volume IV (page 299) is capable of generating interrupts at the accuracy of only one millisecond.

Apple discourages direct use of the VIA2 because it's not present on all Macintosh CPUs. It's absent on the popular Macintosh IIci and IIfx models, for example.

Contrary to the outdated Inside Macintosh Volume IV Time Manager documentation, the Time Manager in System 6.0.3 and later has accuracy on the order of 1 microsecond. See Inside Macintosh Volume VI for details on new Time Manager features.

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