Файл: Microcontroller Programming. Thi Micro Chip PIC (Julio Sanchez, 2007).pdf

Chapter 16

Analog to Digital and Realtime Clocks

Digits are a human invention; nature does not count or measure using numbers. We measure natural forces and phenomena using digital representations, but the forces and phenomena themselves are continuous. Time, pressure, voltage, current, temperature, humidity, gravitational attraction, all exist as continuous entities which we measure in volts, pounds, hours, amperes, or degrees, so as to better understand them and to be able to perform numerical calculations.

In this sense, natural phenomena occur in analog quantities. Sometimes they are digitized so as to facilitate measurements and manipulations. For example, a potentiometer in an electrical circuit allows reducing the voltage level from the circuit maximum to ground, or zero level. In order to measure and control the action of the potentiometer, we need to quantify its action by producing a digital value within the physical range of the circuit; that is, we need to convert an analog quantity that varies continuously between 0 and 5 volts, to a discrete digital value range. If, in this case, the voltage range of the potentiometer is from 5 to 0 volts, we can digitize its action into a numeric range of 0 to 500 units, or measure the angle or rotation of the potentiometer disk in degrees from 0 to 180. The device that performs either conversion is called an A/D or ana- log-to-digital converter. The reverse process, digital-to-analog, is also necessary, although not as often as A/D. In this chapter we explore A/D conversions in PIC software and hardware.

The second topic of this chapter is the measurement of time in discrete (albeit, digital) units. In this context we speak of “realtime” as years, days, hours, minutes, and so on. So a realtime clock measures time in hours, minutes, and seconds, and a realtime calendar measures it in years, months, weeks, and days. Since time is a continuum that escapes our comprehension, we must divide it into measurable chunks that can be manipulated and calculated. However, not all time units are in proportional relation with one another. There are 60 seconds in a minute and 60 minutes in an hour, but 24 hours in a day and 28, 29, 30, or 31 days in a month. Furthermore, the months and the days of the week have traditional names. Finally, the Gregorian calendar requires adding a 29th day to February on any year that is evenly divisible by 4. The device or software to perform all of these time calculations is referred to as a realtime clock. In this chapter we discuss the use of realtime clocks in PIC circuits.

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Most analog-to-digital converters in PIC applications, either internal or external, are of the successive approximation type. The successive approximation algorithm performs a conversion on one bit at a time, beginning with the most significant bit and ending with the least significant bit. To determine each bit in the range, the value of the input signal is tested to see if it is in the upper or lower portion of this range. If in the upper portion, the conversion bit is a 1, otherwise it is a 0. The next most significant bit is then tested in the lower half of the remaining range. The process is continued until the least-significant bit has been determined.

16.1 A/D Integrated Circuits

Several popular integrated circuits are used to perform as A/D converters, among them the ADC0831, the LTC1298, and the MAX 190 and MAX 191. The variations consist in the resolution and interfacing of the different ICs. Of these, the ADC0831, from National Semiconductor, is an 8-bit resolution, serial interface A/D quite suited to applications for small, mid-range PICs such as the 16F84. The input range of the

0831 is 0 to 5 volts, which matches the TTL voltage levels used in PIC circuits. The 0831 pin diagram is shown in Figure 16-3.

Figure 16-3 ADC0831 Pin Diagram

The ADC0831 uses three control lines, labeled DO (data out), CLK (clock), and _CS (chip select) in Figure 16-3. Interfacing the ADC0831 requires three I/O lines. Of these, two can be multiplexed with other functions or with other ADC0831. Actually, only the chip-select (CS) pin requires a dedicated line. This allows for several ADCs to be multiplexed on the CLK and DO lines as long as each one has its own CS connection to the microcontroller. In this case, the controller determines which device is being read by the port to which CS line is connected.

The input voltage range of the ADC0831 is determined by the Vref (positive voltage reference line) and Vin- (negative voltage reference line) pins. Vref is used to set the maximum level and Vinthe minimum. Since the ADC0831 has an 8-bit range, the voltage reading that matches the Vref value is read as 255 and the one that matches the Vinvalue is read as 0. The minimum difference between the voltage limits is of 1 volt.

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Figure 16-4 ADC0831 Demonstration Circuit

16.1.1 ADC0331 Sample Circuit and Program

A simple circuit to illustrate the action of an analog-to-digital converter consists of connecting a potentiometer with the positive voltage reference line, as shown in Figure 16-4. In the circuit the potentiometer was selected so as to produce a voltage range between 0 and +5 volts. Vref was wired to the circuit’s +5 V source and Vinwas wired to ground. The potentiometer variable line was connected to the ADC0831 Vin+ line and the other ADC lines to the corresponding 16F84 Port-B pins.

The sample program is named ADF84, and can be found in the book’s online software. The ADF84 program uses the ADC0831 to convert the analog voltage from the potentiometer, in the range +5 to 0 volts, into a digital value in the range 0 to 255. The value read is then displayed on the LCD. The initialization routine defines