Файл: Microcontrollers in Practice (Mitescu M., 2005).pdf

CODE

ORG ROMBASE

END

The TOC2 initialization routine, the interrupt service routine, and the code that scans and decrements the software timers are placed in TIMER.ASM.

To improve the readability of the program, a MACRO, called DJEQ (Decrement or Jump if EQual to zero), has been defined. It has the following structure:

DJEQ receives a parameter, which is the symbolic address of the destination. It checks the content of the destination, and if greater than zero, it decrements it by one. If the destination is zero, DJEQ returns leaving the destination unchanged.

Here is the full listing of TIMER.ASM:

TITLE MAIN TIMER

CODE

JMP TMRMAIN

9.5 Exercises

SX 9.1

Using the software timers described in this paragraph, write a program that toggles PORTA bit 7 every 1 second, and PORTA bit 6 every 2.5 seconds.

Solution

Out of RESET, all I/O lines are configured as inputs. Therefore, we must configure the selected bits of PORTA as outputs, by adding the following lines to INIT.ASM.

130 9 HC11 Development Board

The main program looks like this:

The program uses two timers with the quantum 100 milliseconds, T100MS0 and T100MS1, which are tested one by one in an endless loop. When a timer reaches zero, the associated I/O line is toggled, and the timer is reloaded with the desired value.

X 9.2

Write a program that reads all the analog inputs every 20 milliseconds, and updates a set of variables AN0–AN7.

X 9.3

Write a program that uses the SCI reception interrupt. Upon reception of an ASCII code for ‘A’ ($41) the program answers with the last value read from the analog input AN0. The byte is transmitted as two ASCII characters, corresponding to its hexadecimal representation.

X 9.4

Modify the schematics of the module described in this chapter, by adding an external 32 K RAM circuit, selected by CSGEN. Describe the initialization sequence in this case.

10

AVR Development Board

10.1 In this Chapter

This chapter describes a simple, yet flexible development board, based on AT90S8535, for the study of the AVR microcontrollers. This board can be used to test most of the AVR projects presented in this book.

10.2 The Hardware

The schematic of the development board is shown in Fig. 10.1. The circuit comprises the following functional blocks:

1.Microcontroller

2.Clock circuit

3.RESET circuit

4.Output buffers

5.ISP interface

6.RS232 interface

7.Power supply circuit.

The microcontroller is an AT90S8535-P in a DIP40 package. Note that this microcontroller is pin by pin, hardware compatible with other members of the analog series of AVR microcontrollers, like ATMega8535 and ATMega16. However, there are many differences between these microcontrollers. Consult the data sheets before making the replacement.

The external clock circuit uses an 8 MHz crystal, Q1, and the capacitors C10, C11 (12–47 pF). The RESET circuit consists of the resistor R1 (10 K), and the capacitor C1 (10 F/10 V). These values are not critical, because AT90S8535 contains internal signal conditioning circuits for the RESET signal.

The digital input lines are connected to PORTC, and are pulled up to Vcc with resistors. A group of LEDs has been included, to show the status of each input.