Файл: Programming Microcontrollers in C, 2-nd edit (Ted Van Sickle, 2001).pdf

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compile individually and examine if they create outlandish code. Once these small functions are all debugged, you can integrate them into your program as either inline code or as function calls. Hence, the fundamental rule of writing good high-level code for a microcontroller: the production of good C code for a microcontroller is a joint effort between the programmer and the compiler writer.

Erasure of EEPROM causes component wear, and most EEPROMs will wear out after a large number of erasures. The nature of the deg­ radation is that the component refuses to erase after many erasures. There has been no evidence that data retention is affected by repeated erasures. The number of erasures that can cause problems is tempera­ ture sensitive. Most of these devices are rated for 10,000 write/erase cycles at the maximum rated temperature for the part. At room tem­ perature, the number of write/erase cycles without damage can grow to several hundred thousand. In light of these facts, it is important that programs use care in rewriting the contents of EEPROM.

EXERCISES

1.The EEPROM in microcontrollers erase to the 1 state. Write a function that checks to determine if an erasure cycle erases the contents of a given location in memory.

2.Write a function that compares data to be programmed into EEPROM and determine if it is necessary to erase a byte contain­ ing data before it is reprogrammed. This approach will extend the life of the EEPROM.

Timers

The systems of timers placed on microcontrollers are among the most creative engineering efforts most people will ever see. The func­ tions of these timers cover literally dozens of different operations. This set of peripheral components mainly relieve the computer of much work associated with execution of the peripheral function. We will start with the simplest timer and outline different timer capabili­ ties in increasing complexity. The most basic timer in the M68HC05 family is called the 15-bit timer, and probably the most advanced timer system is the 16-bit timer. These different timer systems are literally unrelated in the features they offer. Another timer feature

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The timer counter register at address 0x09 contains the value found in the 8-bit ripple counter. The contents of this counter can be read at any time, but it cannot be written to. When the part comes out of reset, the timer counter register contains a zero. 4096 clock cycles will follow, during which the TCR will count at the minimum rate.

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At the close of this period, the initialization of the part is complete, and the TCR is again reset to zero prior to execution of the code identified by the reset vector. Whenever the RESET line is asserted, the TOF will be loaded with zeros.

Any program can read the TCR, so it is possible to generate asyn­ chronous time events faster than the TOF or the RTIF would indicate.

When the processor enters the WAIT mode after execution of a WAIT instruction, the CPU clock halts, but the timer clock continues to execute. If the interrupts are not masked, a timer interrupt, an exter­ nal interrupt or a reset will cause the device to exit the WAIT mode.

Table 4-3: RTI And COP Rates for Fxtal = 4.0 MHz

If a STOP instruction is executed, the timer clock is halted along with the CPU clock. The STOP mode is exited when an external interrupt occurs or the RESET line is asserted. In this case, the part performs as described above.

Most microcontrollers are placed in operation with no operator to intervene in the event of a problem. A COP timer will provide one means of recovering if the operation of the microcontroller gets lost. “Gets lost”? The situation that can cause a microcontroller to get lost is usually some type of voltage spike or glitch in the power supply operation. The program counter usually ends up with a value outside of the program, and no one knows what will happen. The COP is sim­ ply a timer that counts for a specified amount of time. If the COP timer has not been reset before the specified time elapse, the COP timer overflow causes an internal reset of the microcontroller. If the cause of the problem is a drop in power or other error, in most instances forcing a reset will bring the microcontroller back into normal operation.

The COP control register is located at address 0x7f0 in the M68HC05J1. To service the COP from the program, the program must merely write a zero to bit 0 of this address to reset the COP portion of the timer system.

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Good programming practice dictates that microcontroller-specific information be placed in a header file like that shown here:

#pragma portrw PORTA @ 0x00; #pragma portrw PORTB @ 0x01; #pragma portrw DDRA @ 0x04; #pragma portrw DDRB @ 0x05; #pragma portrw TCST @ 0x08; #pragma portrw TCR @ 0x09;

#pragma portrw __COPSVS @ 0x7f0; #pragma vector __TIMER @ 0x07f8; #pragma vector __IRQ @ 0x07fa; #pragma vector __SWI @ 0x07fc ; #pragma vector __RESET @ 0x07fe; #pragma has STOP ;

#pragma has WAIT ; #pragma has MUL ;

#pragma memory RAMPAGE0 [64] @ 0xc0; #pragma memory ROMPROG [1024] @ 0x300;

#define RT0 0 /* TSCR Bits */ #define RT1 1

#define RTIE 4 #define TOFE 5 #define RTIF 6 #define TOF 7

Listing 4-3: Header File For The M68HC05J1

The #pragma and several important #define commands are microcontroller specific. Therefore, to change the program from one microcontroller to another, the programmer need only change the microcontroller header file. Listing 4-3 is a header file for the M68HC05JJ1 controller. The first six entries identify the locations of the I/O ports, the data direction registers, timer status/control reg­ ister, and the timer counter Register. The next five entries specify the COP service address and the vector locations for this part. Note that the names associated with the vector locations all start with a double

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underscore. These names are also listed in all upper-case letters. These entries are the names of the various interrupt service routines. Since C is case sensitive, the names of the functions to be used as interrupt service routines must have the same form.

The three #pragma entries identified as has notifies the com­ piler that the microcontroller has the STOP, WAIT, and MUL instructions. The next pair of entries defines the memory map for this microcontroller. Finally, the next six entries are #defines that identify the bits in the TCSR. Therefore, mnemonic representations of all registers and bits can be used in the C program.

Several header files for the M68HC05 family are found on the CD-ROM. The conventions in these files are to use bit names and register names that are identical to those used in the technical data books that describe the devices. Therefore, the programmer can safely use register names and bit names found in the books without having to look up the values in the header files. These files include commands to prevent listing of these files in the compiler listing output files.

Listed below is a simple program that shows the use of the 15-bit timer in the M68HC05J1. This program is not aimed at doing more than showing the use of the timer operation. The system will create an inaccurate clock in which the time in hours, minutes, and seconds will be recorded in memory, but no provision to display these values or even set the values will be considered at this time.

Most clocking operations should be interrupt driven. If a periodic interrupt can be generated, the operation of the clock will be transpar­ ent to any other operations being conducted in the microcontroller.

#include “hc05j1.h” enum {FALSE,TRUE); enum {OFF,ON};

#define FOREVER while(TRUE) #define MAX_SECONDS 59

#define MAX_MINUTES MAX_SECONDS #define MAX_HOURS 12

#define MAX_COUNT 121

/* define the global variables */ int hrs,mts,sec;

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0xc0 and the program memory starts at 0x300. Note that the com­ piler places the global variables in 0xc0 through 0xc3, and the executable program begins at 0x300 as one would expect.

The first several instructions clear the count location and set or reset proper bits in the TSCR. The instruction CLI clears the inter­ rupt bit in the status register of the microcontroller. When this bit is cleared, interrupts will detected and processed.

The beginning of the loop defined by the macro command FOR­ EVER is at address 0x30d. This macro causes no code at the beginning of the loop. At the end of the loop, address 0x32b, there is an instruc­ tion BRA 0x30d that causes control of the program to start at the beginning of the loop. That is the total code created by the macro FOREVER. Within this loop, the code created by the compiler is straightforward and not very different from code that would be cre­ ated by a competent assembly language programmer.

#include “hc05j1.h”

0000 #pragma portrw PORTA @ 0x00;

0001 #pragma portrw PORTB @ 0x01;

0003 #pragma portr PORTD @ 0x03;

0004 #pragma portrw DDRA @ 0x04;

0005 #pragma portrw DDRB @ 0x05;

0008 #pragma portrw TCST @ 0x08;

0009 #pragma portrw TCNT @ 0x09;

07F0 #pragma portrw __COPSVS @ 0x7f0;

07F8 #pragma vector __TIMER @ 0x07f8; 07FA #pragma vector __IRQ @ 0x07fa; 07FC #pragma vector __SWI @ 0x07fc ; 07FE #pragma vector __RESET @ 0x07fe;

#pragma has STOP ; #pragma has WAIT ; #pragma has MUL ;

00C0 0040 #pragma memory RAMPAGE0 [64] @ 0xc0;

0300 0400 #pragma memory ROMPROG [1024] @ 0x300;

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0000 #define RT0 0

0001 #define RT1 1

0004 #define RTIE 4

0005 #define TOFE 5

0006 #define RTIF 6

0007 #define TOF 7