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Lesson 3 Programming the Motorola 68HC705C8 microcontroller In Lesson 2, we covered the following topics: (1) Mne­monics; (2) Addressing Modes; and (3) the 6805 Instruction Set. This month, we look further at addressing modes. By BARRY ROZEMA Table 1 Opcode Mnemonic A. Mode No. Bytes No. Cycles A6 LDA IMM 2 2 C6 LDA EXT 3 4 86 SWI INH 1 10 C7 STA EXT 3 5 CC JMP EXT 3 3 43 COMA INH 1 3 A3 CPX IMN 2 2 42 MUL INH 1 11 Before moving into new territory, let’s first take a look at the answers to the exercises in Lesson 2. Exercise 1: fill in the blank spaces in Table 2 (Lesson 2). A look at the instruction set in Lesson 2 and/or the op code map on page 30 of the Motorola MC68HC705C8 technical summary (BR594/D) should have given you the information to get the re­sults shown in Table 1. You should also have modified your programs in last months lesson to get the following results. Each exercise is a modifica­tion of the previous program. You add the same instructions on to the program with a change to the data. The flow charts have the new or modified processes shown with an asterisk (*). Exercise 2: rewrite the flow chart and the program to make the LEDs give a smooth inward pattern; ie, to make the LEDs close in one bit at a time instead of two. To do this, you will need to examine the bit pattern to get the hex values. Table 2 shows the bit map. Notice that the bits come “on” one bit at a time in an inward direction. The asterisks show the data that’s added to give a smoother display pattern. Table 3 shows the modified program. To do this, you need to change the immediate byte that sets the time delay. Remember, the time delay uses a subroutine that is in the MAL-4 monitor ROM. There are seven time delay subrou­tines and each one is set by loading the accumulator before jumping to the subroutine. You may have found that experimenting with various delay values by changing the immediate byte in the LDA #$?? instruction quickly became rather tedious. A better way is to make the delay time adjustable while the program is running. To do this, you load the accumulator with the value of the input port instead of a fixed value; ie, you use extended addressing instead of imme­diate. The PORT A input port address is $0000. The instruction looks like this: LDA $$0000. The modified process is shown with an asterisk in the flow chart of Fig.2 and Table 4 shows the modified program. Note: remember to enable PORT A by turning switch 7 of DIP SW2 (Bit 6) on. Kitt scanner Exercise 4: rewrite the flow chart and the program to make an 8-bit “Kitt” scanner; ie, one LED on at a time switching from left to right and back again. Better visual effect This program will need to produce a display with a pattern like the one in Exercise 3: experiment with the time the bit map of Table 5. If you make the delay to give a better visual effect. time delay adjustable as in exercise 3, Table 2 the program will repeat the following instructions. B7 B6 B5 B4 B3 B2 B1 B0 HEX A6 xx LDA #$xx 0 0 0 0 0 0 0 0 $00 C7 00 01 STA $$0001 1 0 0 0 0 0 0 1 $81 * C6 00 00 LDA $$0000 CD 14 D9 JSR $$14D9 1 1 0 0 0 0 1 1 $C3 The xx will contain the pattern data. 1 1 1 0 0 1 1 1 $E7 * There are 14 patterns to be displayed 1 1 1 1 1 1 1 1 $FF before the program starts again. The December 1993  53 Table 3 ADDRESS CODE LABEL MNEMONIC OPERAND COMMENT 0030 A600 START LDA #$00 0032 C70001 STA $$0001 Clear accumulator & store at output 0035 A632 LDA #$32 0037 CD14D9 JSR $$14D9 003A A681 LDA #$81 003C C70001 STA $$0001 003F A632 LDA #$32 0041 CD14D9 JSR $$14D9 0044 A6C3 LDA #$C3 0046 C70001 STA $$0001 0049 A632 LDA #$32 004B CD14D9 JSR $$14D9 004E A6E7 LDA #$E7 0050 C70001 STA $$0001 0053 A632 LDA #$32 0055 CD14D9 JSR $$14D9 0058 A6FF LDA #$FF 005A C70001 STA $$0001 005D A632 LDA #$32 005F CD14D9 JSR $$14D9 Set time delay 50 ($32) x ACC = 0.5 seconds 0062 CC0030 JMP $$0030 Jump to start ADDRESS CODE LABEL MNEMONIC OPERAND COMMENT 0030 A600 START LDA #$00 0032 C70001 STA $$0001 Clear accumulator & store at output 0035 C60000 LDA $$0000 0038 CD14D9 JSR $$14D9 003B A681 LDA #$81 003D C70001 STA $$0001 Set time delay 50 ($32) x ACC = 0.5 seconds Load accumulator & store at output Set time delay 50 ($32) x ACC = 0.5 seconds Load accumulator & store at output Set time delay 50 ($32) x ACC = 0.5 seconds Load accumulator & store at output Set time delay 50 ($32) x ACC = 0.5 seconds Load accumulator & store at output Table 4 Fig.1: the flow chart for Exercise 2. The program first turns on the two outside LEDs at the output port, then turns the remaining LEDs on to give a smooth inward pattern; ie, the LEDs close in one bit at a time. Set time delay input port A times ACC Load accumulator & store at output 0040 C60000 LDA $$0000 0043 CD14D9 JSR $$14D9 0046 A6C3 LDA #$C3 0048 C70001 STA $$0001 004B C60000 LDA $$0000 004E CD14D9 JSR $$14D9 0051 A6E7 LDA #$E7 0053 C70001 STA $$0001 0056 C60000 LDA $$0000 0059 CD14D9 JSR $$14D9 Addressing modes 005C A6FF LDA #$FF In Lesson 2, we covered the following three addressing modes: Inherent, Immediate and Extended. This lesson, we will introduce another 005E C70001 STA $$0001 0061 C60000 LDA $$0000 0064 CD14D9 JSR $$14D9 Set time delay input port A times ACC 0067 CC0030 JMP $$0030 Jump to start program will take 14 x 11 bytes = 154 bytes plus two jumps (one jump to go back to the start and one to jump to the RAM at location $0100). The last jump is needed because the MAL-4 only has 144 bytes of user RAM at page zero ($0030 - $00BF) – see the memory map in the November 1993 issue of SILICON CHIP. 54  Silicon Chip Set time delay input port A times ACC Load accumulator & store at output Set time delay input port A times ACC Load accumulator & store at output Set time delay input port A times ACC Load accumulator & store at output Table 2 B7 B6 B5 B4 B3 B2 B1 B0 HEX 0 0 0 0 0 0 0 1 $01 0 0 0 0 0 0 1 0 $02 0 0 0 0 0 1 0 0 $04 0 0 0 0 1 0 0 0 $08 0 0 0 1 0 0 0 0 $10 0 0 1 0 0 0 0 0 $20 0 1 0 0 0 0 0 0 $40 1 0 0 0 0 0 0 0 $80 0 1 0 0 0 0 0 0 $40 0 0 1 0 0 0 0 0 $20 0 0 0 1 0 0 0 0 $10 0 0 0 0 1 0 0 0 $08 0 0 0 0 0 1 0 0 $04 0 0 0 0 0 0 1 0 $02 addressing mode called “Direct” or “Zero Page”. Direct Addressing Mode Symbol for Direct add­r ess­ ing: $ or Z or none. The CPU requires two bytes of data to process this instruc­tion. The first byte is the op-code, while the second byte is the Low address of the operand in memory. This is like the ex­tended addressing mode except that you can only address zero page. The CPU sets the High address to $00 and so you can only address data from $0000 to $00FF. The CPU requires the operand (byte) which is “directly” at zero page in memory. The memory location for this byte is known at the time the program is writ­ten. The opcode for direct addressing is found in Lesson 2, Table 1, column 5. In these lessons, we will use the “Z” symbol to indicate Direct addressing (Z for Zero page). The mnemonics will have a “Z” tacked on the end of them, eg: ADDZ, ANDZ, ORAZ, EORZ, LDAZ, STAZ and LDXZ. The MAL-4 can make good use of zero page (Direct) address­ing. Most of the user RAM is located at zero page, as are all of the input/output, timer, watchdog and other control registers. Thus, it is possible to write programs that use more direct (zero page) addressing than extended. This saves one byte of program each time you use direct instead of extended addressing. The program in Table 6 is the same as in Table 3 from Lesson 2 except that direct addressing was used instead of ex­tended addressing, were possible. The four asterisks (*) in Table 6 indicate instructions with direct (zero page) addressing. The first three (STAZ $01) are used because the output port (PORT B) is at address $0001 which is in zero page of memory. The last in­ struction (JMPZ $30) is used because our program is written in zero page RAM, from $0030. Running the program Load the program into the MAL-4 and run it from location $0030. The output port LEDs should flash in the pattern described in Lesson 2. If not, go back and check that the program Table 6 ADDRESS CODE LABEL MNEMONIC OPERAND COMMENT 0030 A600 0032 B701 START LDA #$00 * STAZ $01 Clear accumulator & store at output 0034 A632 LDA #$32 0036 CD14D9 JSR $$14D9 0039 A6C3 LDA #$C3 003B B700 STAZ $00 003D A632 LDA #$32 003F CD14D9 JSR $$14D9 0042 A6FF LDA #$FF 0044 B700 STAZ $01 0046 A632 LDA #$32 0048 CD14D9 JSR $$14D9 Set time delay 50 ($32) x ACC = 0.5 seconds 004B BC30 JMPZ $30 Jump to start * * * Set time delay 50 ($32) x ACC = 0.5 seconds Load accumulator & store at output Set time delay 50 ($32) x ACC = 0.5 seconds Load accumulator & store at output Fig.2: this modified flow chart will provide a better visual effect than the flow chart shown in Fig.1. In this case, the time delays have been set by loading the accumulator with the value of the input port instead of a fixed value. has been entered correctly. If there are problems, Mode 2 (the disassemble mode) may help you find your mistake. Things to do In Lesson 4, we will continue with addressing modes and we will explain relative addressing. Make sure that you read up on 8-bit 2’s complement negative numbers. Finally, rewrite all your programs in Exercises 2-4 to use direct addressing were possible. Note SC the saving on memory. December 1993  55