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

132 10 AVR Development Board

Fig. 10.1. Schematic of the AVR development board

The idle status of the input lines is HIGH. An input line is active when pulled to GND (for example by the contact of a relay). In this case, the associated LED will light, indicating the active input.

The analog input lines use PORTA, and are provided with RC filters for noise rejection. Note that these lines can be programmed to work as digital I/O lines. When using the lines of PORTA for digital output, reduce the values of the filter capacitors C11–C19, or remove them completely.

For digital output, the system uses the eight lines of PORTB of the microcontroller. They are buffered with IC4 – ULN2803, an array of eight open collector transistors able to sink up to 300 mA, at 30 V. This allows driving higher current loads, like relays, lamps, coils, etc. Each output line is also provided with a LED-resistor group to indicate its active status.

The output lines OC1A, OC1B of the MCU are buffered with two 2N2222 transistors, Q2 and Q3. They can be programmed to work either with timer 1 in output compare mode, or can be used as PWM outputs. LED18 and LED19 indicate the status of these lines.

Fig. 10.2. PCB layout of the AVR development board

The input lines INT0, INT1, and INPUT CAPTURE of the MCU are not available on the external connectors. Instead, they are connected to test points and can be used for extensions of the board in the custom area.

The ISP connector SV2 is configured to make the board compatible with AVRISP, for programming the internal flash memory of the microcontroller. The ISP uses the MOSI, MISO, SCK and RESET lines of the microcontroller. The SS (Slave Select) signal has been added on the ISP connector, to allow the use of the ISP connector for a standard SPI interface.

The RS232 interface is a typical MAX232 implementation, and uses the connector SV1. Note that all the projects and examples in this book use the same circuits and connector layouts for RS232 and ISP.

The power supply circuit comprises the diode D1, the voltage regulator IC1, and filter capacitors C5, C6, C20. LED1 indicates when the circuit is powered.

Figure 10.2 shows the component layout of the printed circuit board for this module. Detailed execution drawings of the printed circuit board, as well as the complete component list, are available on the accompanying CD.

134 10 AVR Development Board

10.3 Testing the Circuit

Once you have the board assembled and visually checked for short-circuits, execute the following steps to test the circuit. It is recommended to use appropriate sockets for all the ICs, except IC1.

1.Power up the circuit without having the ICs inserted in their sockets.

2.Check for a value of +5 V of Vcc .

3.LED1 must be ON.

4.The RESET line (pin 9 of IC3) must be around +5 V.

5.Pins 10, 30, and 32 of IC3 must be at +5 V.

6.Pins 31 and 11 of IC3 must be at GND.

If everything is OK, remove the power and insert the ICs in their sockets.

Using the ISP interface, load one of the test programs described in the following paragraphs into the MCU internal flash memory and check the behavior of the module on the LEDs associated with the I/O lines.

10.4 The Software

Atmel Corporation offers an excellent, free programming and debugging tool to support AVRs, called AVRStudio. The latest version of AVRStudio is available for download at the Atmel Corporation web site.

Step 1. Installing the Avrstudio

To install AVRSTUDIO, unzip the downloaded archive in a new folder on your hard disk, then launch SETUP.EXE and follow the instructions on screen.

Once the program has installed, read the detailed help file included, for an overview of the capabilities of the program. This help file also contains a good description of the instruction set of AVR microcontrollers.

Step 2. Creating a New Project

When launched, AVRStudio4 prompts the user whether to create a new project, or open an existing one. Figure 10.3 shows a snapshot of the first dialog window of AVRStudio.

If you choose the option Create new project, the following information is required:

•the name for the new project,

•the name of the initial file,

•the working directory where the project will be placed.

Fig. 10.3. Snapshot of the first dialog window in AVRSTUDIO4

Fig. 10.4. Snapshot of the dialog window for Create new project in AVRSTUDIO4

See Fig. 10.4 for a snapshot of the dialog window for Create new project. Fill in the information required and press the Next button.

One final dialog window is presented, as shown in Fig. 10.5, where the user is prompted to choose the debugging platform and the device type. For this particular development board, choose AVR Simulator as the debugging platform, and AT90S8535 for the device, then press Finish to start writing the new program.

Step 3. Writing and Testing Simple Programs in AVRStudio4

Before starting, note a few syntax differences between the assembler included in AVRStudio and ASHC11, the assembler used for HC11.

The definitions file 8535def.inc is provided by AVRStudio and contains a number of equation directives (.equ) that associate symbolic names to the addresses of all the ressources. Here is a fragment of the contents of this file:

.device AT90S8535

;***** I/O Register Definitions

.equ PINB = $16

.....

The following Include file, MAP.ASM, contains application-specific variables and definitions. Below is the listing of MAP.ASM for this particular application:

.def tmp3=r18

;define variables in data segment

;constants used by timer

.equ prescaler_constant=5

.equ ft_div=177

.equ time_s_timer0=50