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PCCchip2
3
3-8
to the CD2401. Note that the PCCchip2 drives the CD2401 A7-A0 pins
with $01 for modem interrupt acknowledges, $02 for transmit interrupt
acknowledges and $03 for receive interrupt acknowledges.) The use of the
auto vector mode is not recommended because the CD2401 can supply the
vector and the CD2401 requires an interrupt acknowledge cycle.
In order to support polling with the CD2401, the PCCchip2 supports
pseudo interrupt acknowledge (PIACK) cycles to the CD2401. (This is
required since the CD2401 has no other way of clearing its interrupt
requests.) PIACK cycles happen as follows:
1. The MPU waits for an IRQ bit to be set in one of the three SCC
interrupt control registers.
2. The Local Bus master starts a normal read cycle to one of the three
PIACK registers in the PCCchip2. (The three PIACK registers
correspond to modem, transmit, and receive interrupts
respectively.)
3. The PCCchip2 upon detecting the start of the read, performs an
interrupt acknowledge cycle to the CD2401. (The PCCchip2 drives
the CD2401 A7 through A0 pins with a value that corresponds to the
PIACK register that is being read. If the Modem PIACK Register is
being read, then A7 through A0 = $01. If the Transmit PIACK
Register is being read, then A7 through A0 = $02. If the Receive
PIACK Register is being read, then A7 through A0 = $03.)
4. As the interrupt acknowledge cycle completes, the PCCchip2 places
the vector being driven by the CD2401 onto the Local Bus D0
through D8 and D16 through D23 signals. (From the MPU point of
view, the status read from the selected PCCchip2 PIACK register is
the vector from the CD2401.)
5. The PCCchip2 signals to the local MPU (via TA*) that the read
cycle is complete.
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