Intel MCS-80/85 manual Application Example

APPLICATION EXAMPLE 1

MINIMUM SYSTEM APPLICATION AS A TEMPERATURE SENSOR

Overview

Following is an application example that illustrates the use of the interrupt and SOD pins on the 8085A, software for a block search routine, and the procedure for using and reading the 8155 counter. It is a simple application showing the use of the small but powerful 3-chip MCS-85 system as a temperature sensor (SDK-85 board used). This example can be modified to be an accurate industrial temperature controller, for several locations if desired.

The basic operation behind this application is a monostable multivibrator having its timing pulse duration controlled by a thermistor. The counter in the 8155 converts this timing pulse to a decimal count that is software mapped into a temperature and displayed in degrees C in the address field of the display in the .SDK-85 Kit. For the purpose of keeping the software relatively simple, many approximations were incorporated into the code.

Detailed Hardware

The basic SDK kit was used for the initial hardware. This Kit provides for everything necessary to develop and debug a program through the use of the SDK-85 monitor, keyboard and display board. The kit provides for 256 bytes of RAM resident in the 8155 and 2K bytes of ROM or EPROM where the SDK-85 monitor is placed. (See the Intel SDK-85 User's Manual for copy of monitor software code.)

Figure 20 is a schematic of the SDK-85 Kit with only one 8155 and 8355. There is no buffering in this system as all compo-

nents are on the same board and far below the maximum component loading. A monostable multivibrator (74121) is also shown with a thermistor connected to RE/CE.

The SOD output pin from the B085A is used for the purpose of starting the monostable multivibrator in generating its tem- perature controlled timing pulse. This pulse is created by the RC time constant provided for by the thermistor acting as a variable resistor and a .1 J.lF capacitor to put the timing pulse in the desired timing range.

The inverted output of the monostable multivibrator (one shot) has been directly connected to the RST 6.5 pin on the 8085A. Since this pin is high level sensitive, it is necessary to disable interrupts in the program until after the pulse from the one shot goes low.

The hardware addressing in the configuration shown allows for several code spaces that could be used. The RST and TRAP interrupt lines on the 8085A also have hardware start addresses but many of these are altered by the SDK monitor. Table 7 should be useful in understanding the addresses used in the software that follows. Each memory/ I/O compo- nent in the basic SDK-85 system is enabled by a signal com- ing from the 8205 address decoder. Since no expansion chips are used, output enables 00 (8355 monitor ROM), 03 (8279 Keyboard) and 04 (8155 RAM) were the only ones needed. Additional memory and/or I/O could have been incorporated using other output enables from the 8205.

Memory/ I/O Device Function				Output from 8205		code space
8155		RAM space			04	2000 - 20FF
						(20 - 20FF are reserved
						for monitor RAM locations)
8355		ROM space			00	0000 - 07FF
8279		Keyboard/display			03	1800 - 1FFF
		controller
stack pointer
Since the monitor uses locations 10C8 through 20FF,
the stack pointer must be initialized to 20C8 or less.
8085A jump address				Usage	monitor mapped address
trap	24H		TO of 8155			0157
RST 5.5	2CH			8279 interrupt		028E
RST 6.5	34H			oneshot interrupt		20CE
RST 7.5	3CH			vector interrupt
I/O ports address		Function

00Monitor ROM Port A (8355)

01Monitor ROM Port B (8355)

02Monitor ROM Port A (8355) Data direction register

03Monitor ROM Port B (8355) Data direction register

20Basic command/status register

21Basic RAM Port A

22Basic RAM Port B

23Basic RAM Port C

24Basic RAM LOW order byte of timer count

25Basic RAM HIGH order byte of timer count

Table 7. Addressing

A1-26