Z85C3008PEG Datasheet, PDF (28/81 Pages) Zilog, Inc. – CMOS SCC Serial Communications Controller

CMOS SCC Serial Communications Controller
Product Specification
20
• External/Status
Each interrupt type is enabled under program control with Channel A having higher prior-
ity than Channel B, and with Receiver, Transmit, and External/Status interrupts prioritized
in that order within each channel.
When enabled, the receiver interrupts the CPU in one of three ways:
• Interrupt on First Receive Character or Special Receive Condition
• Interrupt on All Receive Characters or Special Receive Conditions
• Interrupt on Special Receive Conditions Only
Interrupt on First Character or Special Condition and Interrupt on Special Condition Only
are typically used with the Block Transfer mode. A special Receive Condition is one of the
following. receiver overrun, framing error in Asynchronous mode, end-of-frame in SDLC
mode and, optionally, a parity error. The Special Receive Condition interrupt is different
from an ordinary receive character available interrupt only by the status placed in the vec-
tor during the Interrupt Acknowledge cycle. In Interrupt on First Receive Character, an
interrupt occurs from Special Receive Conditions anytime after the first receive character
interrupt.
The main function of the External/Status interrupt is to monitor the signal transitions of
the CTS, DCD, and SYNC pins, however, an External/Status interrupt is also caused by a
Transmit Underrun condition; a zero count in the Baud Rate Generator; by the detection of
a Break (Asynchronous mode), Abort (SDLC mode) or EOP (SDLC Loop mode)
sequence in the data stream. The interrupt caused by the Abort or EOP has a special fea-
ture allowing the SCC to interrupt when the Abort or EOP sequence is detected or termi-
nated. This feature facilitates the proper termination of the current message, correct
initialization of the next message, and the accurate timing of the Abort condition in exter-
nal logic in SDLC mode. In SDLC Loop mode, this feature allows secondary stations to
recognize the primary station regaining control of the loop during a poll sequence.
Software Interrupt Acknowledge
On the CMOS version of the SCC, the SCC interrupt acknowledge cycle can be initiated
through software. If Write Register 9 (WR9) bit D5 is set, Read Register 2 (RR2) results in
an interrupt acknowledge cycle to be executed internally. Like a hardware INTACK cycle,
a software acknowledge causes the INT pin to return High, the IEO pin to go low and set
the IUS latch for the highest priority interrupt pending.
Similar to using the hardware INTACK signal, a software acknowledge cycle requires that
a Reset Highest IUS command be issued in the interrupt service routine. Whenever an
interrupt acknowledge cycle is used, hardware or software, a reset highest IUS command
is required. If RR2 is read from channel A, the unmodified vector is returned. If RR2 is
read from channel B, then the vector is modified to indicate the source of the interrupt.
PS011706-0511
Functional Descriptions