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Lesson 1 Programming the Motorola 68HC705C8 microcontroller Following our series on the MAL-4 Microcon­ troller Aid for Learning, we now present a series of lessons on programming its central device: the MC68HC705C8 micro­controller. If you’ve always wanted to learn about microcon­trollers, this series will give you a good grounding. By BARRY ROZEMA Welcome to the fascinating world of microprocessors and micro­con­trollers. Over the next few months, we will be showing you how to program the Motorola MC68HC705C8 single chip microcontroller. This micro­controller unit (MCU) is a member of the large 6805 family and the lessons will be based on its internal architecture and instructions set. We will be using the MAL-4 Microcontroller Aid for Learning for the practical parts of the lessons (see SILICON CHIP – Nov-Dec. 1992 & Feb. 1993). The lessons are aimed at everyone from keen beginners to those with some programming experience but who need to refresh their knowledge of microcontrollers. However, to get the most from the lessons, you should at least have a reasonable grounding in digital electronics. Each lesson will consist of: • Theory. • Practical program examples. • Things to do between lessons. • A new program to try. Note, however, that the lessons Fig.1: this diagram is called a “programming model” & shows the various internal registers of the 68HC705C8 microcontroller. 7 6 5 4 3 2 ACCUMULATOR 7 6 5 4 3 INDEX REGISTER 7 1 6 1 5 4 3 STACK POINTER 1 0 2 1 0 2 1 0 8 BITS 8 BITS 15 0 14 0 13 0 12 0 11 0 10 0 9 0 8 0 Hardware 6 BITS 15 0 14 0 13 0 12 11 10 9 8 7 6 5 PROGRAM COUNTER 4 3 2 1 0 4 H 3 I 2 N 1 Z 0 C 13 BITS CONDITION CODE REGISTER 7 1 6 1 5 1 HALF CARRY (FROM BIT 3) INTERRUPT BIT NEGATIVE To carry out the practical aspects of the course, you will require a MAL-4 Microcontroller Aid for Learning with a power supply and loudspeaker. You should be well read with regard to the MAL-4 operations manual and you should be able to load pro­grams and run them. Programming concepts ZERO CARRY (FROM BIT 7) 5 BITS 22  Silicon Chip will be specific to the MC68HC705C8 micro­ controller and its associated hardware. It will also be necessary for you to do some further reading after completing each lesson. As you are probably aware, when it comes to electronics, you can never have too many text and reference books. The micro­controller area is no exception. There have been many good text books written about micro­processors, most of which have been written for specific devices. Unfortunately, I have not seen any based on the 6805 family to date. However, any microprocessor text written around an 8-bit Motorola processor or 6502 processor may suffice; eg, Micropro­cessors & Microcomputers, The 6800 Family or The 6502 Family, by Ronald J. Tocci & Lester P. Laskowski (Prentice-Hall). A good digital text book will also be of value; eg, Digital Systems, Principles & Applications, by Ronald J. Tocci (Prentice-Hall). You will also need the MAL-4 Operation and Construction Manual, plus two publications from Motorola: (1) Microprocessor, Microcontroller & Peripheral Data, Volumes 1 & 2. (2) MC68HC705C8 8-Bit Micro­ con­ troller Unit. Technical Summary. BR­ 594/D Motorola. All microprocessors and microcon­ trollers, from those used in main­ frames to the simplest MCUs, must be capable of running a “program”. A program is simply a list of instructions that are to be carried out (executed) in a given order. All micro­controllers run in “machine code”, although this is usually hidden from us by what are called “higher level languages”. Some typical higher level languages include Pascal, Forth, Basic, DOS and Windows. Even the humble word processor can be regarded as a higher level language. The level of instructions in a given application depends on the power of the processor. For example, an adult human can easily understand and carry out an instruction “go to the shop and get some milk please”. No other information would be neces­sary. But if you wanted a child to perform the same task, you would have to give the correct change for the milk, instruct him on how to cross the road safely, and provide other information that would not be necessary with an adult. Now try to give the same task to a robot. You would have to give every detail of the job. For example, you would have to tell the robot how and when to lift its legs, how to maintain its balance and how to get out of the door – and that’s before you even tell it how to successfully find its way to the shop. In fact, the task is probably impossible given today’s level of robot development. Machine code All microcontrollers are similar in the way they are pro­ grammed. You cannot say “go and get the milk please”. Instead, you have to lay out each step in machine code form. Machine code – also called opcode (for operation code) – is issued in binary form and is simply an instruction code for a given machine or micro­processor. In the case of the 8-bit MC68HC705C8 microcontroller, it takes the form of an 8-bit binary code. To make it easy for us, this binary code is hidden in the form of other numbering formats, such as octal, hexadecimal or decimal. In most cases, the hexadecimal format is used for program­ming and this also applies to the MAL-4. But what ever the pro­gramming format used, the processor still works in binary. Hexa­decimal numbers in these lessons will have a “$” sign in front of them; eg, $1234, $EA, $1FFE. If we wanted a program larger than say 10 bytes, it would be very difficult to write in either binary or hexadecimal for­mat. That’s because we are not very good at remembering numbers and because we find it hard to relate hexadecimal numbers to events. Although the microprocessor understands binary instruc­tions, we need to program in “plain” language. For example, to load an accumulator we could use the in­ struction “Load Accumulator”, or other in1 START structions which are then converted into hexadecimal op-code. This code is stored in the memory and the MCU then “runs” the program. Programming model In order to program the MC68HC705C8, we need to find out what internal registers are available. Fig.1 shows the details for this device. This diagram is called the “programming model” (all microprocessors have one) and it enables the programmer to best utilise the CPU. A brief explanation of the registers in the MC68HC705C8 follows. Accumulator 2 SET REGISTER 3 DECREMENT REGISTER 4 ZERO? NO YES 5 END Fig.2: this flowchart shows an MCU counting down to zero. Symbols 1 & 5 are terminators, symbols 2 & 3 are process blocks, & symbol 4 is a decision block. structions like “Store Accumulator” and “Or Accumulator”. This is how the instructions are written for the human side of the program, though in a short­ened form called “mnemonics” (pronounced nu-mon-ics). The last three instructions in mnemonic form would look like this:    LoaD Accumulator - LDA    STore Accumulator - STA    OR Accumulator - ORA To write a program using mnemonics, the programmer first thinks of the instruction in plain language. He then finds the correct mnemonic and looks up the applicable binary machine code (usually given in hexadecimal). The MC68HC705C8 has 62 of these mnemonic instructions – see p.28 of the MC68HC705C8 Technical Summary. A typical microprocessor program consists of a series of mnemonic in- The accumulator is the workhorse of the CPU. Some micropro­ cessors have more than one accumulator but the 6805 only has one. A better name for the accumulator would be “general purpose, do-almost-everything register”. Like the MCU, the accumulator is eight bits wide. Its cont­ ents can be loaded from memory or stored in memory. We can add, subtract and multiply with it; perform logical functions (AND, OR, NOT & XOR) with it; shift it right or left; rotate it right or left; increment or decrement it; and negate it (2s complement). To understand how the accumulator is used, picture yourself at the kitchen sink washing dishes. With one hand you grab a dirty plate and place it in the sink. You wash it. With your other hand, you then take the clean plate out of the sink and place it on the dish rack to dry. This process is analogous to the way in which we use the accumulator – for example, to take data from the input port and place it on the output port. To do this, we must first LOAD the accumulator from the input port, then STORE the accumulator at the output port. Going back to our analogy, when washing up, we can not send the dirty plate directly to the dish rack – we must handle it and wash it up. A similar situation applies to data – it must be “handled” by the accumulator. The 6805 family has no instruction to direct data around the memory; eg, from output to input. Index register The index register operates in a similar manner to the accumulator. July 1993  23 Table 1: Noise 1 Laser/Spaceship Sound Generator Program Address Date Label Mnemonic 0030 A6 00 Start LDA #$00 ;Set highest frequency 0032 B7 BF STAZ TEMP ;Store at temporary memory 0034 B6 BF LDAZ TEMP ;Get delay time 0036 CD 14 A1 JSR D100US ;100us X ACC delay 0039 B6 02 LDAZ PORTC ;Get speaker 003B A8 80 EOR #$80 ;Complement speaker 003D B7 02 STAZ PORTC ;Store speaker 003F 3C BF INCZ TEMP ;Inc delay time 0041 B6 BF LDAZ TEMP ;Get delay time 0043 B1 00 CMPZ PORTA ;Compare with Port A 0045 27 E9 BEQ START ;Start again if equal 0047 20 EB BRA LOOP1 ;Back to Loop 1 Loop 1 Comments Comment: this program causes the MAL-4 to generate a laser/spaceship sound from its loudspeaker. The program is loaded from RAM location $0030. The DIP switch on PORT A changes the sound. Be sure to enable PORT A by turning DIP SW2 7 (E) on. Try changing location $0031 from $00 to other values. Like the accumulator, it is an 8-bit register but it can not do all the arithmetic and logic operations that the accumulator can do. However, the index register can perform a very powerful form of programming called indexed addressing, whereby it is used to point to individual memory locations. START SET DELAY TIME Stack pointer This 6-bit register is used by the CPU itself. The stack pointer points to a RAM location where the CPU can store or load data. The stack pointer moves down to store new data or moves up to load old data. In this respect, it functions like a FILO (First In Last Out) register. The stack pointer memory range is $00FF-$00C0. Program counter The program counter is a 13-bit register that keeps track of a program’s location in memory. When we start a program from a given memory location, the program counter must be loaded with the start location. As the program progresses through memory, the program counter keeps track of the location. If the program has to jump or branch to another memory location, this is achieved by changing the program counter. The 13 bits of the register give an addressing range of 8Kb ($0000-$1FFF). Condition code register The condition code register (some24  Silicon Chip DELAY 100us X ACC Flowcharts The old saying “a picture is worth a thousand words” is very true, especially when we are programming MCUs. The “picture” that programmers use is called a flow chart. A flow chart con­ sists of symbols “strung” together, usually stacked vertically from top to bottom. Each symbol indicates a particular function, either carrying out an instruction or making a decision. For example, the flowchart in Fig.2 shows an MCU counting down to zero. This could be used to give a time delay. Symbols 1 and 5 are terminators – they indicate the start and finish of a program (or part of a program). Symbols 2 and 3 are process blocks and indicate a process or operation. Symbol 4 is a decision symbol. It has one input and two outputs. One output is for a result that is true, while the other is for a result that is untrue. The result from a decision is bi­ nary; ie, true/false, yes/no or zero/one. The best way to study a flow chart is to follow it with your finger in the direction of the arrows until you come out at the end. Although there are many flow chart symbols, we will only use five basic types during this course. Subjects to study COMPLEMENT SPEAKER INCREMENT DELAY EQUAL TO PORT A? decide if it should branch. A detailed look at each instruction will show how and which flag is affect­ed. YES NO Fig.3: this is the flowchart for the laser/spaceship sound program listed in Table 1. times called the flag register) is a 5-bit register. This register tells us what hap­pened in the ALU (arithmetic logic unit) on the previous instruc­tion(s). If we use conditional branching, the CPU looks at the flags in this register to Following is a list of things to study before next month’s lesson, listed in order of importance: (1.) Microprocessor fundamentals: basic operation; block dia­grams; central processing unit (CPU) (2.) Numbering systems: binary to hexadecimal to decimal (3.) Motorola MC68HC705C8 8-Bit MCU Technical Summary; pages 1-11, 28-31. Sample program Finally, Table 1 list a simple program that generates laser/spaceship sounds using the MAL-4. Try entering the program as set out in the instruction manual and be sure to enable PORT A by turning DIP SW2 7 (E) on. The program is loaded from RAM loca­tion $0030 and the DIP switch on SC PORT A changes the sound.