IRQ Interrupt Request. The Interrupt Request line is used by the
POKEY chip to initiate communication with the microprocessor. Setting
this line low interrupts the processor and forces the processor to
service the POKEY chip.
Peripheral Interface Adapter (PIA).
The Peripheral Interface Adapter (6520) has two 8-bit programmable
I/O ports and two controller bits for each port, for a total of ten
lines per port. The two control lines for each port are used to
accomplish the handshaking during the exchange of data on the eight
data Iines. The external output lines are described in the following
paragraphs :
D0-D7, Data Bus. The eight-bit bidirectional Data Bus transfers data
to the PIA chip during an input operation, and to the peripheral
ports A and B during an output operation.
PA0-PA7, Port A Lines. The eight-bit peripheral port A lines are
programmed lines which can serve both input and output operations.
The data direction register determines the direction data is
transferred. A logic 1 (high) at pin CA2 specifies an output
operation and a logic 0 (low)specifies an input operation.
PB0-PB7, Port B Lines. The eight-bit peripheral port B lines are
programmed lines which can serve both input and output operations.
The data direction register determines the direction of data
transfer. A logic 1 (high) at pin CB2 specifies an output operation
and a logic 0 (low) specifies an input operation.
VCC, VSS. VCC is the +5V dc supply, and VSS is ground for the
integrated circuit.
Ph 2 or Φ2 - Phase 2 Clock. This signal is the Phase 2 clock signal
generated at the microprocessor. It is used to synchronize operations
using the Data Bus.
RSO, RS1 - Register Select. The two Register Select lines, along with
bit 2 of the Control Register and R/W bit, determine the data path
and function. Selection combinations are shown in Table 4-1.
R/W - Read/Write. This line is high for a Read operation and goes low
for a Write operation. R/W, together with 2, RS, and CS control the
operation of the PIA chip.
CS0, CS1, CS2 - Chip Selects. These inputs are used to select the PIA
chip. CS0 and CS1 must be high and CS2 low in order to select the PIA
chip.
ATARI Personal Computers 4-9
Table 4-1. PIA Register Selection and Function
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