Time-interval acquisition on a 6502-based microcomputer

A versatile machine language subroutine and accompanying interface circuitry enable a 6502-based computer to collect time intervals between data events that can be represented by voltage transients. Intervals within the range of 150 microsec to 13.98 min can be collected with a resolution of ±25 microsec. Electrical interface circuitry includes signal conditioning and optical isolation and also allows user-specification of threshold levels on the data line. The machine language software timing routine and sequential interval handler include a 24-bit interval timer, a 16-bit interval counter, user-specified number and length of acquisition periods, and an out-of-memory tester. Although presented for implementation on the Apple II series of microcomputers, the transportability of the routine to other 6502-based machines is addressed.