Microcontrollers in Practice (Mitescu M., 2005)

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

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The CODE segment starts with the interrupt vector area. In this example, only two vectors are used: the reset vector, and the vector for the interrupt for timer0 overflow. The reset vector directs the program to the initialization sequence, and the timer0 overflow vector contains a jump instruction to the associated interrupt service routine, isr_t0_ov.

INIT.ASM contains the initialization sequence, which starts by loading the stack pointer SP with the last address of the internal RAM (Ramend), then calls the initialization routines for timer0, and I/O port B.

After the initialization sequence, the program enables the interrupts, loads the software timer s_timer0 with a constant (time_s_timer0), which defines the time between the moments when port B changes its status, then enters the main loop.

This software timer is the key element of the program. Basically, s_timer0 is an 8-bit RAM variable defined in MAP.ASM, and decremented every 10 milliseconds by the interrupt service routine for timer0 overflow. If the timer is loaded with the value N, it will reach zero after a time T = N × 10 ms.

In this example, N is the constant time_s_timer, defined in MAP.ASM with the value 50, which gives a total time of 500 ms between the moments when port B is toggled. When the software timer expires, port B changes its status, then the timer is reloaded, and the program continues in an endless loop.

The resource-specific routines are placed outside the main loop in the modules TIMER.ASM and IO.ASM. TIMER.ASM contains the initialization sequence for timer0, organized as a subroutine init_timer0, and the interrupt service routine that implements the software timer.

Here is the listing of TIMER.ASM:

init_timer0:

ldi tmp1,prescaler_constant out tccr0,tmp1

ldi tmp1,ft_div mov rint,tmp1 out tcnt0,tmp1 ldi tmp1,$01 out timsk,tmp1 ret

This routine chooses the division factor for the prescaler, and the number of clocks before overflow, then enables the interrupt for timer0 overflow.

isr_t0_ov:

Note how this routine saves and restores the CPU status. The register rint (an alias for r1) is preloaded with the number of clocks before tcnt0 overflows, so that tcnt0 can be reloaded as fast as possible.

soft_timer:

lds tmp1,s_timer0 tst tmp1

breq sft1 dec tmp1

sts s_timer0,tmp1

sft1:

lds tmp1,s_timer1 tst tmp1

breq ex_sft dec tmp1

sts s_timer1,tmp1

ex_sft:

ret

The routine soft_timer implements two independent software timers, s_timer0 and s_timer1. In principle, the number of distinct timers that can be implemented using this technique is limited only by the time required to execute the interrupt service routine. If a larger number of software timers is required, use the technique described in Chap. 9 for HC11 to reduce the execution time of the interrupt service routine.

Step 6. Adding a New Task to an Existing Application

The structure of the software described in the previous paragraph can be used for almost any application. Adding a new task to an existing application involves the following steps:

1.Modify MAP.ASM to define more variables if needed.

2.Write a new software module if a new resource is required. It is recommended to keep all the routines associated with specific resources in separate files (e.g. TIMER.ASM, IO.ASM, UART.ASM, ADC.ASM, etc.)

3.Modify the main program so that it includes calls to subroutines that actually execute the new task.

In the following example, the program reports the status of port B as a string of two ASCII digits, followed by CR+LF, over the asynchronous serial communication line, when the ASCII code for ‘?’ is received on the serial line. Note that port B is toggled every 500 ms, as described in the previous example.

The UART initialization routine and the communication routines are placed in a separate file UART.ASM, invoked in the main module using the “.include” directive. A separate file, LIB.ASM, is used for miscellaneous library routines – in this case hex-to-ASCII conversion routines used to prepare data for serial communication.