Файл: The Microcontroller Idea book (Jan Akelson, 1997).pdf

Powering Up

You’re now ready to power up the target system. Turn on its power supply, and press the SPACE bar at the host’s keyboard. You should see this BASIC-52 sign-on message and prompt:

*MCS-51(tm) BASIC V1.1*

READY

Figure 3-6 shows the sign-on message and a simple program, using Windows’ Terminal accessory for communications.

Troubleshooting

If you don’t see the prompt, it’s time to troubleshoot. Getting the system to boot up the first time can be the most challenging part of a project, especially when serial communications are involved. Here are some things that may help you isolate the cause of the problem:

•Try again by pressing and releasing S1 and pressing the space bar. If you are using a 32K RAM for U7, BASIC-52 requires about 1 second to perform its memory check after a reset, before it will respond to the space bar. With an 8K RAM, the delay is a few tenths of a second (proportionately longer with slower crystals).

•Double-check the easy things. Are the communications parameters correct? Did you select the correct serial port? Are all ICs inserted?

•Verify that pin 9 of U2 goes high, then low, when you press and release S1.

•Check the power and ground pins of all ICs for proper voltages.

•Connect a logic probe to pin 10 of U2. When you press the space bar, you should see the logic level toggle as U2 receives the ASCII code for a space (20h). If not, you probably have a problem in the setup of your communications software or in the serial cabling.

•Verify that pin 30 of U2 is toggling (at 1/6 the crystal frequency, if you have an oscilloscope to measure). This indicates that the oscillator circuit is functioning.

•Verify that pins 21-28 and 32-39 of U2 toggle as BASIC-52 performs its memory check immediately after powering up or rebooting.

•If all else fails, recheck your wiring for missing or misrouted wires. Sometimes there’s no alternative but to go through the schematic connection by connection, checking each with an ohmmeter.

Chapter 3

Basic tests

When your system boots, you’re ready for some basic tests. The BASIC-52 programming manual is a useful reference at this point.

In some ways, BASIC-52 is similar to BASIC compilers like Microsoft’s QuickBASIC. Many of the keywords and syntax rules are similar. But BASIC-52 is closer to older interpreted BASICs like GW-BASIC or BASICA. You can type a statement or command and execute it immediately when you press ENTER, or you can type a series of statements and run them later as a program. When a line begins with a line number, BASIC-52 treats it as a program line rather than as a command to execute immediately.

Here are some quick tests and experiments you can do:

Memory Check

Type

PRINT MTOP

to learn the amount of external data memory that BASIC-52 detected on boot-up. With an 8K RAM, MTOP should be 8191, and with 32K, it should be 32,767. If you prefer hexadecimal notation, type

PH0. MTOP

(In PH0., be sure to include the period and use a zero, not the letter “O”.)

Crystal Frequency

The special operator XTAL represents the value of the timing crystal that clocks the 8052-BASIC. The default value is 11059200, or 11.0592 Mhz. You can verify this by typing

PRINT XTAL

Most BASIC-52 statements don’t use the XTAL operator, so it doesn’t matter if the value isn’t accurate. Exceptions are the real-time clock, programming commands, PWM output, and LPT output. For these, XTAL should match your crystal’s frequency. To set XTAL for a 12Mhz crystal, type

XTAL=12000000

To verify, type

Powering Up

PORT 1 Bit Values:

Bit 0 = 1

Bit 1 = 1

Bit 2 = 1

Bit 3 = 1

Bit 4 = 1

Bit 5 = 1

Bit 6 = 1

Bit 7 = 1

If a port pin is open, or unconnected, its internal pull-up resistor will cause it to read as 1. If you connect a jumper wire from a port pin to ground, or bring the pin low by driving it with a logic low output, it should read 0. Line 10 in Listing 3-1 brings all of Port 1’s bits high, which enables them to be used as inputs.

Writing to Port 1

You can control the bits of Port 1 by writing to them. Listing 3-2 allows you to set or clear individual bits. Here’s an example of what happens when you run the program:

Enter a bit to set or clear (0-2, 4-7) :7

Enter 1 to set, 0 to clear :0

Enter a bit to set or clear (0-2, 4-7) :3

Do not change bit 3!

The program doesn’t allow you to change bit 3 (P1.3), because the 8052-BASIC circuit requires this bit to be high when accessing external memory (assuming that you’ve included U3B in your circuit). If you do clear bit 3 accidentally, you’ll crash the system and will have to reboot.

Listing 3-1. Displays the value of each bit in Port 1.

10 PORT1 = 0FFH

20 PRINT “PORT 1 Bit Values:”

30 PRINT “Bit 0 = ”,(PORT1.AND.1)

40 PRINT “Bit 1 = ”,(PORT1.AND.2)/2

50 PRINT “Bit 2 = ”,(PORT1.AND.4)/4

60 PRINT “Bit 3 = ”,(PORT1.AND.8)/8

70 PRINT “Bit 4 = ”,(PORT1.AND.10H)/10H

80 PRINT “Bit 5 = ”,(PORT1.AND.20H)/20H

90 PRINT “Bit 6 = ”,(PORT1.AND.40H)/40H

100 PRINT “Bit 7 = ”,(PORT1.AND.80H)/80H

110 END

Chapter 3

Listing 3-2. Allows you to set or clear individual bits of Port 1.

10 INPUT “Enter a bit to set or clear (0-2, 4-7) :”,X 20 IF X=3 THEN PRINT “Do not change bit 3!” : GOTO 10 30 INPUT “Enter 1 to set, 0 to clear :”,Y

40 IF Y=1 THEN PORT1=PORT1.OR.2**X

50 IF Y=0 THEN PORT1=PORT1.AND.0FFH-2**X

60 END

Run the program and follow the on-screen instructions to set or clear a bit. To monitor a port bit as you set and clear it, you can use a logic probe, voltmeter, or oscilloscope. For example, to monitor bit 0, place a logic probe on pin 1 of U1, or connect the + lead of a voltmeter to pin 1 and the - lead to ground.

Accessing Memory

Listing 3-3 allows you to read and write to external RAM. Here is an example of what happens when you run this program:

Enter 0 (read), 1 (write), or 2 (quit): 1 Free memory ranges from 397H to 1FFFH Enter an address to write to : 1000H Enter data to be written : 55H

55H has been written to address 1000H Enter 0 (read), 1 (write), or 2 (quit): 0 External RAM ranges from 0 to 1FFFH

Enter an address to read : 1000H 55H is stored in address 1000H

If you write to an address outside the range specified as free memory, you will overwrite the RAM currently in use to store your program and run BASIC-52. If you do this accidentally, your system may crash and you’ll have to reset the system and re-enter the program.

If you prefer decimal numbers to hex notation, change each PH0 in the program to PRINT. (PH0. includes a period; PRINT does not.)

Real-time Clock

Listing 3-4 demonstrates BASIC-52’s real-time clock by displaying an on-screen 60-second timer.

For the timer to be accurate, you must set XTAL to match the timing crystal your system uses.

Further Experiments

Feel free to continue experimenting with BASIC-52 programs, using the programming reference as a guide. You can do quite a bit with just these circuits.

Listing 3-4. Real-time clock.

Chapter 3

Listing 3-5. This program uses BASIC-52’s GET instruction to detect when the user has pressed a key.

Exiting Programs

Some programs, such as Listing 3-3’s, continue to run until the user requests to end it. In BASIC-52, there are several ways to detect that the user wants to stop a program.

Set a User Variable

In Listing 3-3, the program displays a menu of choices on the host computer’s screen. The program continues to run until the user selects QUIT by entering 2, which sets the variable RW to 2 and causes the DO...WHILE loop and the program to end.

Use GET

Sometimes, selecting a menu option isn’t convenient or appropriate. Listing 3-5 reads and displays the value of PORT1 once per second until the user presses any key at the host computer. The program uses BASIC-52’s GET operator to detect a keypress. GET stores the ASCII code of a keypress at the host computer. Setting a varialble equal to GET (line 50) causes GET to reset to 0. You can detect a keypress by reading GET periodically. If GET

Listing 3-6. This program will end only when the user presses CONTROL+C.

10 CLOCK 1:TIME=0:SEC=0

20 DO

30 ONTIME 1,100

40 WHILE 1=1

50 END

100 TIME=TIME-1

110 PH0. PORT1

120 RETI

Powering Up

Listing 3-7. This program ends when INT1 (pin 13) is brought low and causes an interrupt routine to execute.

doesn’t equal zero, it means that a key was pressed. In Listing 3-5, when GET no longer equals 0, the program ends.

Wait for CONTROL+C

You can always end a program by pressing CONTROL+C at the host’s keyboard. The only exceptions are runaway programs that have crashed the system and force you to reboot. Listing 3-6 is an expanded version of Listing 3-5. It continues to read and display PORT1 in an endless loop (DO...WHILE 1=1), until you press CONTROL+C.

Detect a Switch Press

A final method will end a program without any input from the host’s keyboard. You can use this in stand-alone projects that don’t connect to a host computer at all. Listing 3-7 ends when the 8052-BASIC’s pin 13 (INT1) goes low, which causes an interrupt routine to execute. Bring the pin low by jumpering it briefly to GND, or connect a pushbutton switch as described in Chapter 7.