Z85C3010VSG Datasheet, PDF (27/83 Pages) Zilog, Inc. – CMOS SCC Serial Communications Controller

CMOS SCC Serial Communications Controller
Product Specification
19
Interrupt Pending (IP), Interrupt Under Service (IUS), and Interrupt Enable (IE). Opera-
tion of the IE bit is straight forward. If the IE bit is set for a given interrupt source, then
that source can request interrupts. The exception is when the MIE (Master Interrupt
Enable) bit in WR9 is reset and no interrupts can be requested. The IE bits are write-only.
The other two bits are related to the interrupt priority chain (see Figure 8). As a micropro-
cessor peripheral, the SCC can request an interrupt only when no higher priority device is
requesting one, that is, when IEI is High. If the device in question requests an interrupt, it
pulls down INT. The CPU responds with INTACK, and the interrupting device places the
vector on the data bus.
+5 V
Peripheral
IEI D7–D0 INT INTACK IEO
Peripheral
IEI D7–D0 INT INTACK IEO
Peripheral
IEI D7–D0 INT INTACK
+5 V
D7–D0
INT
INTACK
Figure 8. SCC Interrupt Priority Schedule
The SCC can also execute an interrupt acknowledge cycle through software. In some CPU
environments, it is difficult to create the INTACK signal with the necessary timing to
acknowledge interrupts and allow the nesting of interrupts. In these cases, the INTACK
signal can be created with a software command to the SCC.
In the SCC, the Interrupt Pending (IP) bit signals a need for interrupt servicing. When an
IP bit is 1 and the IEI input is High, the INT output is pulled Low, requesting an interrupt.
In the SCC, if the IE bit is not set by enabling interrupts, then the IP for that source is
never set. The IP bits are readable in RR3A.
The IUS bits signal that an interrupt request is being serviced. If an IUS is set, all interrupt
sources of lower priority in the SCC and external to the SCC are prevented from request-
ing interrupts.
The internal interrupt sources are inhibited by the state of the internal daisy chain, while
lower priority devices are inhibited by the IEO output of the SCC being pulled Low and
propagated to subsequent peripherals. An IUS bit is set during an Interrupt Acknowledge
cycle, if there are no higher priority devices requesting interrupts.
There are three types of interrupts:
• Transmit
PS011707-1013
Functional Descriptions