Microcontrollers in Practice (Mitescu M., 2005)

if direct = 0 PC = PC + rel

Index n may take values in the range [0,7]. Index i may take values in the range [0,1].

A.12 An Example of 8051 Operating with External Bus

Figure A12.1 shows an example of a typical structure of 8051 operating with an external bus.

Fig. A12.1. 8051 operating with external bus

The EA\ signal is grounded, which means that the MCU ignores the internal program memory, if any.

A.13 Programming the Internal Memory of 8051

The waveforms of the signals involved in the process of programming the internal memory of 8051 are shown in Fig. A13.1.

ALE/PROG

VPP

H

EA/VPP

L

Fig. A13.1. Waveforms for the signals involved in programming 8051 memory

The information in this section concerns those versions of 8051 microcontrollers that have internal EPROM or flash memory. Both EPROM (87C51) and flash (89C51) versions are programmable following the same principles. Only the value of the programming voltage Vpp differs. Note that any attempt to program the internal memory using inappropriate Vpp may cause permanent damage to the chip. See the specific data sheets for the exact requirements for Vpp.

Table A13.1 presents the status of the control signals for each particular operation. Besides these signals, during any program/verify operations, RST must be HIGH and PSEN\ must be LOW.

Table A13.1. Status of the control signals for program/read memory operations

Legend:

1 – Only applicable for flash versions (89xxx series).

2 – Chip erase operations require 10 ms PROG pulse. H-L-H indicates a neagative pulse of 100 s, except the ERASE MEMORY operation, where this pulse must be 10 ms wide.

The signature bytes are two to four data bytes, read from addresses specific for each device. For example, AT89C51 reports three signature bytes, read from the addresses 30h, 31h, 32h. The first signature byte indicates the manufacturer (1Eh for Atmel Corporation), the second byte identifies the device (51h for AT89C51), and the third signature byte indicates the value of the programming voltage Vpp (FFh for 12 V, 05h for 5 V).

The lock bits are non-volatile control bits, accessible only in programming mode, similar to those described in Appendix A9 for AVR microcontrollers. The effect of programming the lock bits of AT89C51 is described in Table A13.2.

Table A13.2. The effect of the lock bits for AT89C51

Legend

U – Unprogrammed bit (1)

P – Programmed bit (0)

See the accompanying CD for details on how to build a programmer for 8051 microcontrollers.

A.14 SPI Seven-Segment Display Units

This section presents schematics and suggestions on how to use two types of sevensegment display units, both designed to be connected to the SPI interface of a microcontroller.

The first implementation, presented in Fig. A14.1, is multiplexed, i. e. at a certain time moment only one digit is displayed. The software must refresh the data permanently, by switching the active digit fast enough so that the human eye integrates the successive images into a single, complete perception.

The SPI interface of the MCU is brought to the connector SV1. An additional connector SV2 has been provided so that the ISP interface can be used without the need to remove the display unit.

Fig. A14.1. Schematic of a multiplexed seven-segment display

Data from the SPI is received in the shift register 4094 (IC1) and must be prepared by the software as follows:

•The least significant four bits of each byte contain the BCD value of the digit to be displayed.

•The next three bits contain the address of the active digit.

•The most significant bit, if set to 1, activates the decimal point of the corresponding display digit.

4094 receives the serial data sent by the MCU through the MOSI line and shifts it into the internal shift register with the clock SCK. When the serial transfer completes, the software must generate a positive pulse on the LD line so that the data received this way is presented on the output lines Q1–Q8. The SPI must be programmed to send data with the most significant bit first.

A.15 Description of the Software Utility ASMEDIT

Some users, familiar with the Windows™ environment, might find it difficult to work with MS-DOS™ type applications, which must be invoked from the command line in a DOS window. Since most freeware assemblers available, including those recommended in this book for HC11 and 8051, are DOS applications, we wrote a small utility program called ASMEDIT.EXE with the following features:

•It contains a simple built-in text editor, which allows the user to create assembler source files, save them to disk, or open existing files for editing.

•The user can choose an assembler, define command line options and switches, if required, then invoke the assembler and run it on the selected source file. When the assembly process completes, ASMEDIT brings the list file generated by the assembler in the main window for viewing.

•ASMEDIT also contains a built-in terminal program capable of using one of the computer’s COM ports to communicate with the development boards described in this book through the RS232 interface.

The main window presented by ASMEDIT when launched is shown in Fig. A15.1. The following pull-down menus are available:

Fig. A15.1. Snapshot of the main window of ASMEDIT

File. This menu presents options for Open, Save, Save as, and Create New source file. If you select the option Open file, ASMEDIT filters only files with the extensions

.ASM and .A51.

View. This menu allows the user to select the active window between the source file and the list file. It is also possible to adjust the font size.

Assembler. This pull down menu has only two menu options: Assemble, which runs the assembler for the selected source file, and Select assembler. The dialog window presented in this case is shown in Fig. A15.2. Select the desired executable

244 Appendices

file by clicking the Browse button, then fill in the command line options, and switches, according to the syntax required by the assembler. Make sure that the assembler is set to generate a list file. For example, the case of the assembler ASHC11 by Peter Gargano, referred in Chap. 9, requires the switch ––LIST to generate the list file.

Terminal. This pull-down menu displays a list of options concerning the builtin terminal of ASMEDIT. The user can select the communication port (default is COM2), set the communication speed and clear the terminal window. In addition to this, the terminal menu contains two options that are useful when using the 8051 development board described in Chap. 11. If the option Send is activated, ASMEDIT looks in the folder where the active source file is located for a file with the same name and the extension .HEX, loads the file and sends it to the development board. The option Send&Go automatically launches a G<aaaa> command to the board, when download completes, where <aaaa> is the starting address of the program, extracted from the HEX file.

Fig. A15.2. Snapshot of the menu option Select Assembler

The output files generated by the assembler are placed in the same folder as the specified source file. If the source file uses the INCLUDE directive, the include files must be present in the same folder, or the full path where they are located must be indicated in the source file, e. g.

INCLUDE C:\ASHC11\INCLUDE\68HC11F1.DEF