User's Manual

Chapter 4 Register-Level Programming

3. Install an interrupt service routine.

You must install an interrupt service routine for the slot containing the Lab-NB. Consult the

Inside Macintosh manual for information regarding the installation of interrupt service routines.

The interrupt service routine can use the TIMERUP bit in the Interrupt Status Register to

determine whether the interrupt was a counter A2 (or EXTUPDATE*) generated interrupt,

which is useful when interrupts from other sources such as data acquisition and digital I/O have

been enabled on the Lab-NB. The interrupt service routine must write to either the DAC0,

DAC1, or DAC0 and DAC1 Data Registers or to TMRINTCLR to reset the TIMERUP bit and

acknowledge the current interrupt. Another interrupt is generated when a rising edge (low-to-

high) is detected on OUTA2 or EXTUPDATE*.

4. Enable timer interrupts.

Timer interrupts refer to the interrupts generated by rising edges on OUTA2 or EXTUPDATE*.

A rising edge on OUTA2 or EXTUPDATE* sets the TIMERUP bit high in the Interrupt Status

TMRINTEN bit in the Interrupt Control Register high to enable timer interrupts.

Programming the Digital I/O Circuitry

The digital I/O circuitry is designed around an 82C55A integrated circuit. The 82C55A is a

general-purpose PPI containing 24 programmable I/O pins. These pins represent the three 8-bit

I/O ports (A, B, and C) of the 82C55A. These ports can be programmed as two groups of 12

signals or as three individual 8-bit ports. The following paragraphs include programming

information for the Lab-NB along with program examples written in C.

The three 8-bit ports are divided into two groups: group A and group B (two groups of 12

signals). One 8-bit configuration (or control) word specifies the mode of operation for each

group. Group A’s control bits configure port A (A0 through A7) and the upper 4 bits (nibble) of

port C (C4 through C7). Group B’s control bits configure port B (B0 through B7) and the lower

nibble of port C (C0 through C3). These configuration bits are defined later in this chapter.

82C55A Modes of Operation

The three basic modes of operation for the 82C55A are as follows:

• Mode 0 – Basic I/O

• Mode 1 – Strobed I/O

• Mode 2 – Bidirectional bus

The 82C55A also has a single bit set/reset feature for port C. The 8-bit control word also

programs this function. For additional information, refer to Appendix D, OKI 82C55A Data

Sheet.