National Instruments PC-TIO-10, User Manual

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Chapter 4 Programming
© National Instruments Corporation 4-9 PC-TIO-10 User Manual
MC6821 Control Registers
The MC6821 Control Registers control the overall operation of the MC6821 and the selection of
the two internal registers that are accessed through each of the MC6821 Data Registers. Some of
the bits in the Control Registers are not used because of the design of the PC-TIO-10. These bits
should be set as follows.
Address: Base a 05 (hex) for Port A
Base a 07 (hex) for Port B
Type: Read-and-write
Word Size: 8-bit register, 8-bit port
Bit Map:
7 6 5 4 3 2 1 0
IRQ 0 0 0 0 DRS EDGE INTEN
Bit Name Description
7 IRQ This is a read-only bit that reflects the current status of the
interrupt input for the selected Control Register. If this bit is one
in the
Port A Control Register, an interrupt request is pending on the
external interrupt line EXTIRQ1. If this bit is one in the Port B
Control Register, an interrupt request is pending on the external
interrupt line EXTIRQ2. Always write a zero to this bit.
6-3 Reserved These bits are not used on the PC-TIO-10. Always write a zero to
each of these bits.
2 DRS This is the Data Register Select bit. Writing a one to this bit
selects the Output Register, while writing a zero to this bit selects
the Data Direction Register. Reading this bit shows the bit's
current state. Refer to the description of the Data Register for
more information.
1 EDGE This is the control bit for selecting the edge that will cause an
interrupt. Writing a one to this bit selects rising-edge interrupts,
while writing a zero to this bit selects falling-edge interrupts. The
Port A Control Register controls external interrupt line EXTIRQ1,
while the Port B Control Register controls external interrupt line
EXTIRQ2. Reading this bit shows the bit's current state.
0 INTEN This bit enables and disables the interrupt generation capability of
EXTIRQ1 or EXTIRQ2. Writing a one to this bit enables
interrupts, while writing a zero to this bit disables interrupts. The
Port A Control Register controls EXTIRQ1, while the Port B
Control Register controls EXTIRQ2. Reading this bit shows the
bit's current state.