Browse Prior Art Database

Event Latch for Multitasking Environments

IP.com Disclosure Number: IPCOM000100117D
Original Publication Date: 1990-Mar-01
Included in the Prior Art Database: 2005-Mar-15
Document File: 4 page(s) / 101K

Publishing Venue

IBM

Related People

Fasig, JL: AUTHOR

Abstract

Described is a circuit that latches the occurrence of transient events for subsequent software processing in multitasking systems.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 52% of the total text.

Event Latch for Multitasking Environments

       Described is a circuit that latches the occurrence of
transient events for subsequent software processing in multitasking
systems.

      Since the introduction of microprocessors, common practice is
to scan I/O ports via software to detect the occurrence of external
real-time events such as a switch closure.  In multitasking systems
such a practice leads to unreliable operation.  The problem arises
from the fact that in a multitasking system, there is a certain
probability that the scan program is "suspended" or otherwise
inattentive at the instant the external event occurs.  A means is
required to prevent lost start requests.  In addition, another
requirement is that software must recognize and acknowledge an event
independently from all other tasks and events.

      The circuit shown in Fig. 4 solves the problems cited above
through a connection of asynchronous state machines. The circuit
actually includes eight separate event latches, one for each switch
shown, with a common I/O interface to the microprocessor bus.  The
following explanation focuses on the operation of the latch for
switch SW1 with the understanding that the remaining latches function
in a similar manner.

      Under normal conditions, the switch is positioned as shown and
signals REQ$0 (REQuest 0) and SRV$0 (SeRVice 0) are low while -ATN$0
(ATteNtion 0) is high.  When the switch is closed, the switch is
debounced by a S-R latch in U9 so that REQ$0 goes high...