Flowcharts are useful for providing a graphical representation of the program, for example, for a presentation, but they are time consuming to create. Nevertheless, the flowcharts shown here were drawn just using the drawing tools in Word, so the creation of flowcharts to a reasonable standard is not difficult for the occasional user. Specialist drawing packages are also available, which make the process quicker and easier for the professional software engineer. Pseudocode Pseudocode shows the program as a text outline, using higher-level language constructs to represent the basic processes of sequential processing, selection and repetition. BIN4 is represented in Table 2.6. Interfacing PIC Microcontrollers 46 (a) (b) DELAY Decrement Count = 0? Return No BIN4 Initialise Port B = all outputs Reset ? All LEDs off Run? Increment LED display DELAY using Count Yes No Figure 2.3 BIN4 flowcharts: (a) main routine; (b) delay subroutine Else_IPM-BATES_ch002.qxd 6/29/2006 11:34 AM Page 46 The program outline uses high level key words such as IF and DO…WHILE to control the sequence. It is not an ideal method for a very simple program like this, but is useful for more complex programs. In particular, it translates directly into ‘C’, if the high-level language is preferred. Note that in this case, the program out- line does not make any assumptions about the hardware implementation. Structure Charts Structure charts are also more suited to more complex programs, but the con- cept can be illustrated as in Figure 2.4. Each program component is included under standard headings: inputs, processes and outputs, and can be broken down further in more complex pro- grams, so that components can be created independently and then integrated. ‘C’ Programming The ‘C’ programming language allows applications to be written using syntax whose meaning is a little easier to understand than assembly code. Programs PIC Software 47 Operation Symbol Implementation Start Source code file/project name in start box. End End not needed if program loops endlessly Process BANKSEL TRISB Sequence MOVLW B’00000000’ MOVWF PORTB Input or CLRF PORTB Output Branch BTFSS PORTD,Inres Selection GOTO reset Subroutine MOVLW 0FF Procedure CALL delay or Function Table 2.5 Flowchart implementation DELAY using Count Reset ? All LEDs off Initialise Port B = all outputs BIN4 Else_IPM-BATES_ch002.qxd 6/29/2006 11:34 AM Page 47 Interfacing PIC Microcontrollers 48 Project: BIN4 MPB 22-3-06 Ver 1.0 Hardware: BINX MCU = P16F877 RC clock = 40 KHz Description: LED binary counter with stop and reset buttons Declare Registers Input, Output, Count Bits Reset, Run Initialise Inputs (default) Reset, Run Outputs LEDS Main DO IF Reset pressed Switch off LEDs DO Increment LEDS Load Count DELAY using Count WHILE Run pressed ALWAYS Subroutine DELAY DO Decrement Count WHILE Count not zero RETURN Table 2.6 BIN4 pseudocode BIN4 Inputs OutputsProcesses Reset Run Reset Inc DELAY LEDS Figure 2.4 BIN4 structure chart Else_IPM-BATES_ch002.qxd 6/29/2006 11:34 AM Page 48 in C are converted into assembly language by a compiler, and assembled into machine code, in a two-stage process. ‘C’ is the high-level language of choice for microcontrollers. A range of different development systems and compilers are available, but most use the same basic syntax defined as ANSI (American National Standards Institute) C. Assembly language is syntax which is unique to each type of processor, while C provides a common language for all MCU types. BIN4C Program A version of BIN4, BIN4C, is shown in Figure 2.5 as an example. The func- tion is the same as BIN4. It can be seen that the program is simpler, because each C statement is converted into several assembler instructions. As a result, the program written in C will normally occupy more memory than the equivalent assembler version, so microcontrollers with larger memory are typically needed. Therefore, the more powerful 18XXXX series of PIC chips are usually used for C applications. They also have additional instructions, such as multiply, which makes the conversion more compact. PIC Software 49 // BIN4C.C ************************ MPB 19-11-05 #include <16F877.h> // Include standard MCU labels #byte PortB=6 // Output port data type and address void main() // Start of program { set_tris_b(0); // Initialise output port PortB=0; // Initial output value while(1) // Endless loop between braces { if (!input(PIN_D0)) // Reset button pressed? PortB=0; // if so, switch off LEDs if (!input(PIN_D1)) // Run button pressed? PortB++; // if so, increment binary display } } // End of program Figure 2.5 BIN4C source code Else_IPM-BATES_ch002.qxd 6/29/2006 11:34 AM Page 49 In the BIN4C source code, the header file with the standard register labels for the 16F877 is included in the same way as in the assembler version. The output port is declared as an 8-bit variable (PortB), and its address assigned (6). The main program block starts with the statement ‘void main()’ and is en- closed in braces (curly brackets). The output port is then initialised using a li- brary function provided with the compiler ‘set_tris_b(0)’, where 0 is the data direction code in decimal form. An initial value of 0 is output to switch off the LEDs. The control loop starts with the loop condition statement ‘while(1)’, which means repeat the statements between the braces endlessly. The buttons are tested using ‘if (condition)’ statements, and the actions following carried out if the condition is true. The condition is that the input is low ( ! = not ), and pin labels as defined in the header file are used. BIN4C Assembler Code The C source code is compiled into assembler code, and then into machine code. The list file in Figure 2.6 shows the assembler version of BI4C. It can be seen that some statements are converted into a single instruction, for example, PortB++; >>> INCF 06,F Others need several instructions, if (!input(PIN_D0)) >>> BSF 03.5 BSF 08.0 BCF 03.5 BTFSS 08.0 The total number of instructions for the C version is 28. The original as- sembler version used 20, giving an increase of 40% for the C version, in this case. We are not going to look at the C language in any further detail here, but this example is given so that the advantages of C programming for microcontrollers can be appreciated. When assembly language has been mastered, the developer can then decide if C would be a better choice for given applications. For those needing complex mathematical calculations, for example, C is a better choice. For simpler programs comprising more bit Interfacing PIC Microcontrollers 50 Else_IPM-BATES_ch002.qxd 6/29/2006 11:34 AM Page 50 PIC Software 51 CCS PCM C Compiler, Version 3.207 19-Nov-05 15:01 Filename: C:\PIC\bin4c\bin4c.LST ROM used: 28 words (0%) Largest free fragment is 2048 RAM used: 5 (3%) at main() level 5 (3%) worst case Stack: 0 locations * 0000: MOVLW 00 0001: MOVWF 0A 0002: GOTO 004 0003: NOP //BIN4C.C #include <16F877.h> #device PIC16F877 #list #byte PortB=6 void main() { 0004: CLRF 04 0005: MOVLW 1F 0006: ANDWF 03,F 0007: BSF 03.5 0008: BSF 1F.0 0009: BSF 1F.1 000A: BSF 1F.2 000B: BCF 1F.3 set_tris_b(0); 000C: MOVLW 00 000D: MOVWF 06 PortB=0; 000E: BCF 03.5 000F: CLRF 06 while(1) { if (!input(PIN_D0)) 0010: BSF 03.5 0011: BSF 08.0 0012: BCF 03.5 0013: BTFSS 08.0 PortB=0; 0014: CLRF 06 if (!input(PIN_D1)) 0015: BSF 03.5 0016: BSF 08.1 0017: BCF 03.5 0018: BTFSS 08.1 PortB++; 0019: INCF 06,F } 001A: GOTO 010 } 001B: SLEEP Figure 2.6 BIN4C list file Else_IPM-BATES_ch002.qxd 6/29/2006 11:34 AM Page 51 I/O operations and fewer calculations, assembler is generally faster and more compact. C and assembler can be mixed in the same program to retain the advantages of both. SUMMARY 2 • The standard development system consists of a source code editor, assem- bler, simulator and programmer • Machine code instructions can be broken down into operation and operand • Programs should be well commented and structured for ease of analysis and debugging • Assembler directives can be used to improve the efficiency and flexibility of code production ASSESSMENT 2 1 Describe the advantages of in-circuit programming and debugging over the corresponding conventional development process. (3) 2 Refer to the instruction set in the PIC16F877 data sheet. State the binary codes for the operation and operands in the instruction DECFSZ 0C. (3) 3 State three commonly used assembler directives. (3) 4 Identify two instructions, one of which must be placed last in the PIC source code. What happens if one of these is not used? (3) 5 Identify two types of label used assembly language programming. (3) 6 State three PIC chip options, which are determined by the configuration code. (3) 7 State the function of the EQU directive. (3) 8 State the difference between the subroutine and macro, and one advantage of each. (3) 9 Describe the function of the standard header file “P16F877A.INC”. (3) 10 State the only assembler directive, which is essential in any program, and its function. (3) Interfacing PIC Microcontrollers 52 Else_IPM-BATES_ch002.qxd 6/29/2006 11:34 AM Page 52 11 Identify the five main symbols, which are used in a flowchart. (5) 12 Rewrite the BIN4 program pseudocode outline with the delay in-line (eliminate the subroutine). (5) ASSIGNMENTS 2 2.1 MPLAB Test Download and install the current version of the MPLAB development system (if necessary). Enter or download program BIN4. If entered manually, leave out the comments. Assemble (V7 Quickbuild) and run the program. Set up the input simulator buttons to represent the push buttons at Port D (toggle mode). Set the MCU clock to 40 kHz. Display Port B and the Timer register in a suit- able window. Demonstrate that the program runs correctly. 2.2 MPLAB Debugging Use the MPLAB debugging tools to single step the program BIN4 and observe the changes in the MCU registers. Operate the simulated inputs to enable the output count to Port B. Set a break point at the output instruction and run one loop at a time, checking that Port B is incremented. Use the stopwatch to meas- ure the loop time. Comment out the delay routine call in the source code, re- assemble and check that the delay does not execute, and note the effect on the loop time. Re-instate the delay, change the delay count to 03 and note the ef- fect on the loop time. 2.3 C Program Write a minimal ‘C’ program, which will perform the same function as BIN1, and save as a plain text file BIN1.C. Discuss the advantages and disadvantages of programming in ‘C’ and assembler. Obtain access to a suitable ‘C’ develop- ment system and test your program. Predict the assembler code, which will be produced by the same compiler that was used to produce the list file BIN4C.LST. Add comments to explain the meaning of each assembler state- ment produced by the compiler. PIC Software 53 Else_IPM-BATES_ch002.qxd 6/29/2006 11:34 AM Page 53 This page intentionally left blank 54 3 Circuit Simulation In the past, the electronics engineers needed to have a fairly comprehensive knowledge of both electronic component operation and circuit analysis, before setting out to design new applications. The circuit would be designed on paper and a prototype built to test the design, using a hardware prototyping technique such as stripboard; further refinement of the design would often then be required. When the circuit was fully functional, a production version could be developed, with the printed circuit board (PCB) being laid out by hand. Further testing would then be needed on the production prototype to make sure that the layout was correct, and that the variation in component values due to tolerances would not prevent the circuit from functioning correctly. Learning how electronics systems worked also required a good imagination! Unlike mechanical systems, it is not obvious how a circuit works from simple observation. Instruments (voltmeters, oscilloscopes, etc.) must be used to see what is happening, and these also need complex skills to use them effectively. We now have computer-based tools that make the job easier, and perhaps more enjoyable. An early ECAD (Electronic Computer-Aided Design) tool was a system of mathematical modelling used to predict circuit behaviour. SPICE was developed at University of Berkeley, California, to provide a consistent and commonly understood set of models for components, circuits and signals. This system uses nodal analysis to predict the signal flow between each point in a circuit, based on the connections between the components. The results would be displayed or printed numerically. The simplest component is the resistor, and the simplest mathematical model Ohm’s law, V = IR, which relates the current and voltage in the resistor. For two Else_IPM-BATES_ch003.qxd 6/27/2006 12:58 PM Page 55 55 . outputs BIN4 Else_IPM-BATES_ch002.qxd 6/29/2006 11:34 AM Page 47 Interfacing PIC Microcontrollers 48 Project: BIN4 MPB 22-3-06 Ver 1.0 Hardware: BINX MCU = P16F 877 RC clock = 40 KHz Description: LED binary counter. comprising more bit Interfacing PIC Microcontrollers 50 Else_IPM-BATES_ch002.qxd 6/29/2006 11:34 AM Page 50 PIC Software 51 CCS PCM C Compiler, Version 3.2 07 19-Nov-05 15:01 Filename: C: PIC bin4cin4c.LST. //BIN4C.C #include <16F 877 .h> #device PIC1 6F 877 #list #byte PortB=6 void main() { 0004: CLRF 04 0005: MOVLW 1F 0006: ANDWF 03,F 00 07: BSF 03.5 0008: BSF 1F.0 0009: