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8. Input/Output Interfacing
47
avoid overlapping I/O addresses because bus
contention would result. The use of the external flags,
DMA, and the interrupts is discussed in the following
section. For additional information, refer to the
User
Manual for the CDP1802 COSMAC Microprocessor,
MPM-201
Interrupt facility. The discussion concerning SCl-P
and MRD-N in the previous section also applies here.
The Control Module uses DMA-OUT to terminate
IDLE. Isolation of a user-supplied signal by means of a
three-state device, diode, or the like is required on this
line.
System Signals in I/O
Interfacing
External Flags EF1 to EF4
The external flags offer a simple yet powerful input
interface to the CDP1802 COSMAC Microprocessor
in the computer system module of the MCDS. Means
by which a program may test an external flag and
branch conditionally on its value have been discussed.
The use of a flag as a bit-serial data input port was also
described. Note that the serial I/O uses EF4 making it
unavailable for other devices unless link connection
9:8 on link LK36 on the CDP18S601 Microboard
module is opened. Because transmission gate outputs
may be connected as “wire ors”, several devices may
share a specific external flag when necessary. It should
also be noted that all four external flag signals are
pulled up to VDD through 22-kilohm resistors on the
CDP18S601.
In order for an external flag to play a functional role
in a COSMAC-based system, it must be tested by the
program at the time when action is required. Further,
in programs which incorporate a periodic flag test,
there must be some means for the interface logic to
sense that the flag stimulus has caused a response. In
systems where it is necessary to detect failure or error
conditions, one or more flags may be used. If
immediate action is required, the flags may be used as a
means to vector interrupts.
D M A Input
Assuming only one I/O device needs the DMA
input port, the arrangement shown in Fig. 13 is
possible. Systems having more than one DMA channel
(either 2 or more DMA-IN’s or a mixed DMA-IN and
DMA-OUT) must use the Interrupt facility to
establish vectoring. Notice that if interrupts will not be
used in the system of Fig. 13, the SC l-P can be used
directly for the output port and signal flip-flop: it does
not have to be gated with SCO-P. The MRD-N line is
also not strictly necessary unless a mixed DMA-
IN/DM AOUT system is being constructed.
B Y T E
C L O C K
IN
Fig. 13—DMA inputexam ple.
SIGNAL
OUT
D M A Output
Interrupt
Assuming only one device is usingthe DMA-output
channel, the circuit of Fig. 14 can be used. Again, note
that multi-channel DMA systems must use the
A straightforward interrupt implementation is
shown in the
User Manual for the CDP1802
COSMAC Microprocessor,
MPM-201.