Jonathan A. Titus

Microcomputer Pioneer

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Lesson 11: Advanced Topics

by David G. Larsen, Peter R. Rony, Jonathan A. Titus and Christopher A. Titus

Direct Memory Access

DIRECT MEMORY ACCESS is a technique that allows a computer user direct access to individual memory locations without first having to go through the computer's central processing unit. The direct memory access, or DMA, interfacing technique has a number of applications but few microcomputer users fully understand its use, and even fewer have fully implemented it for scientific data acquisition or control purposes.

Calculation shows that a very simple software routine, one that will input an eight-bit data point and transfer it to a memory location, will take many microseconds. Further, if additional points are to be taken and stored in some sort of a sequential file, additional software steps are needed for housekeeping tasks that count the number of data points, increment the address pointer, etc. When these additional software steps are introduced into the test program, an eight-bit data point may be acquired only every 30-40 usee. In many cases, however, higher data acquisition or data transfer rates are required. The idea of providing the computer user with a direct access to the memory section of the computer is not new. A direct memory access scheme was provided for in the Digital Equipment Corporation's PDP-8 series of minicomputers.
Read More: Adobe Acrobat PDF FileDirect Memory Access.pdf [44.5KB]

Real Time Clocks: An Introduction
IN MANY microcomputer applications it is necessary to have the computer perform actions at accurately timed intervals. This allows the computer to make accurate measurements of an analog signal at 100-msec intervals, for example. The period of 100 msec may be timed through the use of a time-delay loop in which software commands are used, or through the use of an external clock.

At this point, it must be realized that while a time-delay software routine may generate a delay of the required accuracy, the computer can do nothing else while it is performing the timing software steps. This is a serious limitation. Although probably less obvious, the time-delay software steps may be interrupted by an external device that requires immediate servicing by the computer. The overall effect is to lengthen the time required for the time-delay software steps. The actual time delay would be the sum of the time spent in the software steps and the time spent servicing the external interrupt.

In most instances in which accurate periods are required, an external circuit is used to time the necessary periods with as little interaction between the computer and the external clock as possible. Such clocks are immune to external interrupts and to changes in the normal flow of a program. Once started, they continue to time a period until it is completed and the time is up. In this way, the clock runs parallel with the computer, allowing the computer to perform other tasks and service interrupts while the clock is running. This type of an external clock is often called a real-time clock, since its time is real, in that it cannot be altered or delayed by events that would normally affect a program.
Read More: Adobe Acrobat PDF FileReal Time Clocks An Introduction.pdf [127KB]

Real Time Clocks: Hardware and Software
IN OUR last column, we described real-time clocks and their characteristics. In this column, we will describe some of the hardware devices and software instructions that can be used to build and use a realtime clock.

Perhaps the simplest clock that can be built is a free-running realtime clock. The schematic for one of these devices is shown in Figure 1. The "heart" of this circuit is the Mostek Corporation MK5009, which is driven by a I-MHz quartz crystal. This device contains a number of divide- by-IO counters, and you can select which divide-by-IO counter will drive the MK5009's output. There are four digital inputs to the device that are used to select the required decade counter. The truth table for these inputs and the resulting output frequencies are listed in Table 1. For input values greater than 1000" the MK5009 generates frequencies that are not multiples of ten.
Read More: Adobe Acrobat PDF FileReal Time Clocks Hardware And Software.pdf [127KB]

QUITE OFTEN, a program will execute much. faster if the values it accesses are sorted. Sorting also is useful when data values or alphanumeric strings have to be' output to a printer or a cathode-ray tube (CRT) in a specific order. Sorting is defined as the rearrangement of data values in a list, table, record, or file in an ascending or descending order. If the data values consist of alphanumeric strings, then the strings will be sorted in alphabetic or alphanumeric order.

A number of different methods can be used to sort data, including: straight insertion sort, binary insertion sort, Shell's sort, bubble sort, quick sort, heap sort, merge-exchange sort, and two-way merge sort. In general, all of these sorting methods can be classified as one of the following sorting methods: insertion method, exchange method, selection method, merge method, distribution method.

This column discusses only the exchange method of sorting numeric values. Specifically, this column is concerned with the bubble sort method. This method is perhaps one of the slowest methods that can be used. However, it is probably easier to write a bubble sort subroutine than any other type of sort subroutine.
Read More: Adobe Acrobat PDF FileSorting.pdf [127KB]