The Microcontroller Idea book (Jan Akelson, 1997)

Clocks and Calendars

If you want an alarm frequency other than daily, hourly, or on the minute, there are a couple of ways to achieve it. For an alarm every 10 minutes, you could generate an interrupt once per minute and ignore 9 out of 10 interrupts.

If this seems wasteful, you can update the alarm minutes to the next desired value after each interrupt. Using the example of an interrupt every 10 minutes, you would set the mask bits for when minutes match (0-1-1), and start out by storing 0 in register 3, which will cause an interrupt to occur on the hour. When the interrupt occurs, you would add 10 to register 3 to schedule the next interrupt for 10 minutes after the hour. By continuing to add 10 to register 3 after each interrupt, and returning to 0 on a count of 60, you end up with an interrupt every 10 minutes.

As Table 10-1 shows, many of the DS1286’s registers have multiple functions. In register 9, bits 0-4 store the current month, bit 6 enables the square-wave output, bit 7 enables the clock, and bit 5 is always 0. For situations like this, you can create mask bytes and use BASIC-52’s logical operators to read and write to selected bits while ignoring other bits in a register. For example, assume that you want to store a month in bits 0-4 of the 1286’s register 9, without affecting the settings of bits 5-7.

To do so, follow these steps:

(1) Read the current value of the byte. In our example, with a current month of December, the clock enabled, and the square wave disabled, register 9 will hold these values:

0101 0010

(2) Create a mask byte by setting all bits to be masked, or unchanged, to 1, and clearing the other bits. To alter only the month’s value, bits 5-7 are masked:

1110 0000

(3) Logically AND the current value with the mask byte, with this result:

0100 0000

(4) Place the new month’s value in bits 0-4 of the byte. To change the month to June (6th month), logically OR the above byte with this:

0000 0110

which results in:

0100 0110

Chapter 10

(5) Save the result in the original location (register 9). Bits 5-7 are unchanged from the original, while bits 0-4 have been changed from 12 (December) to 6 (June).

Listing 10-3 shows how to use the DS1286 in a BASIC-52 system. It’s a long program, but accomplishes a lot. If you don’t need the alarm or another function, you can shorten the program by editing out the code for it.

The program begins with a menu that asks you select the desired function: set-up and initialize, display the time and date, or set the alarm. To initialize the clock/calendar, follow the prompts in the subroutine beginning at line 200, and enter the information requested. The program then uses the subroutine at line 3000 to convert the information to BCD, and stores the result in the appropriate register of the DS1286. When all of the information has been entered, line 470 starts the clock by bringing bit 7 of register 9 low.

Line 1000 begins the subroutine to display the current time and date. Before reading from the DS1286, the program clears TE (transfer enable, register B, bit 7). This freezes the registers at their current values, and allows you to read the complete time and date information without errors.

If you don’t freeze the registers, if one of them updates in the middle of a series of read operations, you could end up with an invalid time or date. For example, if you read the hour just before 10:00, and read the minutes just after 11:00, you will think that it is 10:00 when it is really 11:00. Freezing the registers ensures that you will read the value of all of the registers as they were when TE went low. Freezing the registers does not stop the clock, however. The DS1286 continues to keep track of the time, and when you bring TE high again, the chip updates the registers to the current time and date.

After the program freezes the registers, it reads the values from the DS1286, uses a subroutine at line 3100 to convert them from BCD to decimal, and displays the results. Finally, the program sets TE to update the registers.

A subroutine at line 2000 handles the third function of the program, setting the alarm. To use this routine, you must wire pin 1 of the DS1286 (INTA) to pin 13 of the 8052-BASIC (INT1). A menu asks you what type of alarm you would like, prompts you for additional information, and stores the appropriate values in the DS1286’s registers 3, 5, and 7.

Line 2240 configures INTA as a low-going pulse. An endless loop at lines 2250-2270 then waits for an interrupt. On interrupt, the program displays the word ALARM and the current time and date. You could place any program code in the interrupt routine. For example, you could read sensor data, or write to a port to cause a stepping motor to increment.

Chapter 10

Listing 10-3 (page 2 of 4).

450 PRINT “Press any key when ready to start the clock” 460 A=GET : IF A=0 THEN GOTO 460

470 XBY(WT+9)=(XBY(WT+9)).AND.7FH

480 RETURN

Clocks and Calendars

Listing 10-3 (page 3 of 4).

1210 REM get month

1220 X=XBY(WT+9).AND.3FH

1230 GOSUB 3100

1240 PRINT “Date = ”,X,"/",

1250 REM get day of month

1260 X=XBY(WT+8)

1270 GOSUB 3100

1280 PRINT X,"/",

1290 REM get year

1300 X=XBY(WT+0AH)

1310 GOSUB 3100

1320 PRINT X

1330 REM get day of week

1340 X=XBY(WT+6)

1350 GOSUB 3100

1360 PRINT “Day of week = ”,X

1370 REM set TE when reads are done

1380 XBY(WT+0BH)=XBY(WT+0BH.OR.80H)

1390 RETURN

2000 REM set alarm

2010 PRINT “Select alarm type:”

2020 PRINT “Once per minute (1)”

2030 PRINT “When minutes match (2)”

2040 PRINT “When hours and minutes match (3)”

2050 PRINT “When hours, minutes, and days match (4)”

2060 INPUT AF

2070 REM clear all 3 alarm mask bits

2080 XBY(WT+3)=XBY(WT+3).AND.7FH

2090 XBY(WT+5)=XBY(WT+5).AND.7FH

2100 XBY(WT+7)=XBY(WT+7).AND.7FH

2110 REM set alarm mask bits as needed

2120 IF AF<4 THEN XBY(WT+7)=XBY(WT+7).OR.80H

2130 IF AF<3 THEN XBY(WT+5)=XBY(WT+5).OR.80H

2140 IF AF=1 THEN XBY(WT+3)=XBY(WT+3).OR.80H

2150 IF AF>1 THEN INPUT “Minute? ”,M

2160 IF AF>2 THEN INPUT “Hour? ”,H

2170 IF AF=4 THEN INPUT “Day? ”,D

Chapter 10

Listing 10-3 (page 4 of 4).

2180 REM store alarm settings

2190 IF AF1 THEN X=M : GOSUB 3000 : XBY(WT+3)=80H+X 2200 IF AF2 THEN X=H : GOSUB 3000 : XBY(WT+5)=80H+X 2210 IF AF=4 THEN X=D : GOSUB 3000 : XBY(WT+7)=80H+X 2220 REM turn on alarm

2230 REM time-of-day alarm is edge-triggered, low-going, INTA 2240 XBY(WT+0BH)=0D8H

2250 DO

2260 ONEX1 3200

2270 WHILE 1=1

2280 RETURN

Control Circuits

11

Control Circuits

This chapter presents a variety of ways to use an 8052-BASIC system for computer control. The applications include switching power to a load, controlling a matrix of switches, selecting the gain of an op amp, and controlling speed and direction of stepping and dc motors.

Switching Power to a Load

You can use your 8052-BASIC system’s port bits to control power to all kinds of devices, including those powered by alternating current (AC), or direct current (DC) at voltages other than 5 volts. Figure 11-1 shows two port bits that control solid-state relays that switch power to AC and DC loads.

A solid-state relay is a simple, safe way to switch power to devices that require high voltages or currents. A logic voltage at the relay’s control inputs determines whether or not power is applied to the load.

In a typical solid-state relay, the control voltage is electrically isolated from the switching circuits, which contain an optoisolated triac or a similar device. Many AC solid-state relays include zero-switching circuits, which reduce noise by switching power only when the AC signal is near zero volts.

Control Circuits

the LSTTL part, and wire the desired bit to the relay’s + input, with the relay’s - input connected to GND.

Look for a relay with a control voltage of 5 volts or less, and input control current of 15 milliamperes or less. The relay’s rated output voltages and currents should be greater than those of the load you intend to switch.

Take care to work safely when you’re wiring, testing, and using circuits that control high-current or high-voltage loads. For circuits that connect to 117V line voltage and have a metal chassis, you can ground the chassis by connecting it to the safety-ground wire in a 3-wire power cord. Insulate any exposed wires and terminals with heat-shrinkable tubing. If in doubt about how to wire the power connections, get qualified help before you continue.

You can control a relay from any output port bit. Just write a 1 or 0 to the corresponding bit to switch the load on or off. If you control a solid-state relay with a port bit on an 8255 or the 8052-BASIC, you may have to add an LSTTL or HCMOS buffer (such as a 74LS244) to supply enough current to the relay’s control inputs.

Controlling a Switch Matrix

Figure 11-2 shows how you can use 9 output bits to control an 8 x 8 array of electronic switches. You can connect any of eight X inputs to any of eight Y inputs, in any combination. Possible applications include switching audio or video signals to different monitors or recording instruments, selecting inputs for test equipment, or any situation that requires flexible, changeable routing of analog or digital signals.

A Mitel MT8808 8 x 8 analog switch array simplifies the circuit design and programming. The chip contains an array of crosspoint switches, plus a decoder that translates a 6-bit address into a switch selection, and latches that control the opening and closing of the switches. Maxim is another source for switch arrays like this.

Connecting an X and Y input requires the following steps: Write the X and Y addresses to AX0-AX2 and AY0-AY2. Bring STB high. Bring DATA high to close the switch. Bring STB low to latch the data. To open a connection between an X and Y input, you do the same but bring DATA low to open the switch.

You can make and break as many connections as you want by writing the appropriate values to the chip. All previous switch settings remain until you change them by writing to the specific switch.

You can connect the switches in any combination. For example, you can connect one X input to each of the eight Y inputs, to create eight distinct signal paths. Or, you can connect all eight Y inputs to a single X input, to route one signal along eight different paths.

Chapter 11

The MT8808 is shown powered at +5V, but VDD may be anywhere from 4.5V to 13.2V. VEE is an optional negative supply that enables you to switch negative signals.

The switches do have some resistance, which varies with the supply voltage. At 5V, typical switch resistance is 120 ohms; at 12V, it drops to 45 ohms. This should cause no problems in switching standard LSTTL or CMOS signals. If you are routing an analog voltage to a low-impedance input, the switch resistance may attenuate the signal. Maximum switching frequency of the chip is 20 Megahertz.

In Figure 11-2, the switch array is controlled by Port A and bit 7 of Port C (PC.7) on an 82C55 PPI. Bringing PC.7 high opens all of the switches. If you don’t need this ability, you can tie RESET low and use PC.7 for something else.

You can use any output port bits to control the MT8808. Logic high inputs at the MT8808 must be at least 3.5V, though, so if you use NMOS or TTL outputs, add a 10K pull-up resistor from each output to +5V.

For a simple test of the switches, you can connect a series of equal resistors as shown to the X inputs. Each X input will then be at a different voltage. To verify a switch closure, measure the voltages at the selected X and Y inputs; they should match.

Figure 11-2. With the MT8808 switch array, an 8-bit output port can connect eight X inputs to eight Y inputs, in any combination.