Microcontrollers in Practice (Mitescu M., 2005)

142 10 AVR Development Board

Here is the listing of the main module UARTMAIN.ASM:

.include ‘‘8535def.inc’’

.include ‘‘map.asm’’

.cseg

.org 0

reset:

rjmp init

.org 9 vector_t0_ov:

rjmp isr_t0_ov

init:

.include ‘‘init.asm’’ sei

,include ‘‘timer.asm’’

.include ‘‘uart.asm’’

.include ‘‘io.asm’’

.include ‘‘lib.asm’’

Note the two new subroutine calls included in the main loop: get_uart, and send. Get_uart checks if a character is available in the receiver’s data register, reads the character in tmp1, and sets the carry bit to inform the main program. When no

character is available, get_uart returns carry = 0.

get_uart:

Send prepares and sends over the communication line a string comprising two ASCII digits corresponding to the hexadecimal value of each nibble of port B, plus CR and LF.

send:

Put_uartw waits for the status bit UDRE to be set, then writes the content of tmp1 to UDR. It does not check for valid ASCII characters before sending.

put_uartw:

144 10 AVR Development Board

10.5 Exercises

SX 10.1

Use two software timers to make the upper and lower nibble of port B toggle at different time intervals.

Solution

The two software timers s_timer0 and s_timer1 described in the previous paragraphs must be loaded with different constants to obtain different time intervals for toggling the lines of port B. Here is how LEDMAIN.ASM should be modified for this:

Each of the subroutines toggle1 and toggle2 must affect only one nibble of port B, leaving the other nibble unchanged:

toggle1:

11

8051 Development Board

11.1 In this Chapter

This chapter contains the description of a simple development board for the study of the 8051 family of microcontrollers.. Unlike the development boards dedicated to the HC11 and AVR families, presented in the previous chapters, this project allows the user to load and execute programs in an external RAM area, addressed to be visible both in the program memory and data memory address space.

11.2 Hardware

The schematic of the board is presented in Fig. 11.1, 11.2, and 11.3. Figure 11.1 shows the microcontroller IC10, the bus demultiplexer IC7, the RS232 interface IC12, the RESET and clock circuits, and the ISP connector SV1.

Note the presence of the two NAND gates IC6C and IC6D, which implement the logic function AND between the signals RD\ (Read) and PSEN\ (Program Store Enable), both active LOW, and generate the signal RDPSEN. RDPSEN is active LOW when either RD\ or PSEN\ is LOW. Connecting this signal to the RD\ input of an external RAM circuit allows the use of this RAM to store program as well as data.

The microcontroller can be any of 8032, 8051, 8052, AT89C51, AT89C52, etc. in a DIP40 package. The input EA\ (External Access) of the MCU is connected to the jumper JP1 to allow the use of the internal ROM, if this is available. Connect the jumper so that EA\ = 1 to use the internal ROM. When the external ROM is used, EA\ must be connected to GND.

The clock circuit comprises the crystal Q1 (14.74 MHz) and the capacitors C6, C7 (22 pF). C5 (10 µF) and R4 (10 K) implement the RESET circuit. The pushbutton S1 is provided to allow manual reset of the circuit.

The latch SN74HC573 (IC7), controlled by the signal ALE (Address Latch Enable), is used to demultiplex the external data bus, by storing the lower half (A0–A&) of the address bus.

Fig. 11.4. Component layout for the PCB of the 8051 development board

The output port is buffered with IC4 (ULN2804). LED2–LED9 and the resistor network R2 have been included to indicate the status of the output lines at any moment. Similarly, LED10–LED17 and R5 are used to indicate the status of the input lines.

Figure 11.3 also shows the voltage regulator IC1 (7805) which generates the power supply voltage Vcc for the board. LED1 indicates the power present. The external power is applied on X1-1, X1-2, with the polarity indicated, and must be in the range +12 V to +16 V/250 mA.

The component layout of the PCB for this schematic is shown in Fig. 11.4.

11.3 The Software

11.3.1 Installing the Cross-Assembler

From the many freeware 8051 cross-assemblers available, we chose ASEM-51, created by W.W. Heinz, because it is fully compatible with the Intel syntax, allows nested include directives, and macros, and is very well documented. This tool is available for free download at http://plit.de/asem-51/.

To install the tools for creating and testing 8051 software, perform the following steps:

1.Locate the subfolder ASM51 of the folder SOFTWARE on the accompanying CD, and copy it to your hard drive, with all its content. This contains all the software examples described in this chapter.

2.Download the archive ASEM51 from http://plit.de/asem-51/ and unzip all the files to a temporary folder.

3.Copy the executable ASEMW.EXE to the hard disk in the folder ASM51.

4.Follow the instructions from Chap. 9 to create a new DOS window, having the working directory C:\ASM51, and a shortcut on your desktop linking to it.

The assembler is invoked from the command line by typing ASEMW sourcefile. The extension of the source file is assumed to be .A51. If another extension is

used, this must be specified in the command line, e.g. ASEMW sourcefile.asm.

If you are not familiar with MS-DOS™ programs, consult Appendix A15 for the description of a software utility that allow invoking the assembler from a Windows™ application. This utility also contains a text editor for creating and editing the assembler source files.

The assembler generates two output files, both having the same name as the source file, and the extensions .LST and .HEX. The list file, sourcefile.lst contains the detailed listing of the program and information on the errors occurred during the assembly process. The hex file, sourcefile.hex contains the actual executable code in Intel Hex format. The ASEM-51 package also contains a utility program to convert this type of file to plain binary files, called HEXBINW.EXE, but this is seldom necessary because most EPROM programmers directly handle Intel hex files.

The accompanying CD contains several examples of 8051 assembler source files, which can be used to verify that the assembler is correctly installed.

11.3.2 Writing and Testing Simple 8051 Programs

ASEM-51 accepts INCLUDE directives. This means that the user programs can be organized according to the structure described in the previous chapters for HC11 and AVR microcontrollers.

The simplest program (inout.asm) reads the status of the external input port and writes this value to the external output port. This is a good and simple method to test the hardware, and to get in touch with the software tools. Here is the listing of this program:

The next example is a bit more complicated. It uses the timer0 overflow interrupt to implement a user programmable software timer, which is used to determine the time intervals between the moments when the output port toggles its status. Compare this example with those presented in Chap. 9 and 10 and note the similarities and the differences. Besides the syntax differences (like $include instead of .include, ) another significant difference is that, in this case, the symbolic names of the special function

150 11 8051 Development Board

registers are automatically recognized by the assembler, and it is not necessary to define them in a special include file.

;save new value ;out to port ;loop forever ;timer0 routines

MAP.ASM contains, as usual, the application specific variables and constants.

time_st1 is the value that, multiplied by the timer quantum, determines the actual time until the software timer expires. In this example, time_st1 is 50 and the timer quantum is 10 ms, which leads to 500 ms total time between the moment when the software timer is loaded, and the moment when it reaches zero.

The actual software timer is a RAM location s_timer1, and port_v is another RAM location used to store the value to write in the external output port. It is initialized with the value 055h. Finally icnt1 is an 8-bit software counter initialized with the value 100. Here is the whole initialization sequence located in the file INIT.ASM:

The subroutine init_timer0, called here, is located in the file TIMER.ASM.

Note that the initialization sequence does not contain initialization of the stack pointer SP. There is no need for this because the SP of 8051 is hardware initialized with the value 07h.

The module TIMER.ASM contains the subroutine init_timer0, and the interrupt service routine for timer0 overflow:

Remember that in operating mode 2, timer0 is reloaded by hardware, at every overflow, with the content of th0. For the given value of the frequency of the oscillator, th0 is initialized so that the overflow interrupts from timer0 occur every 0.1 ms.

The interrupt service routine is listed below:

The quantum of the software timer is determined by the interval between two consecutive overflow interrupts from timer0, and by the value loaded in icnt1. In this example, the interval between interrupts is 0.1 ms, and the value loaded in icnt1 is 100, and the resulting timer quantum is 10 ms.

The software timer s_timer1 is decremented by the interrupt service routine every 10 ms, only if its value at the moment of the interrupt is non-zero. The main program must load the desired number of quanta in s_timer1, then check the moment when s_timer1 reaches zero. In this example, the software timer expires after 50 × 10 ms = 500 ms.