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When I rst started using the analog-to-digital converter (ADC) built into the PIC16C7x devices, I felt like the feature was very complex and dif cult to work with. When you read through the Microchip datasheets on the ADC that is built into the different PIC microcontroller part numbers, you will nd that there are multiple 20-page descriptions of the ADC. Each of these descriptions is slightly different in terms of register locations and bit operations depending on the part number and its features (such as the number of I/O pins that can provide ADC input), but they all work essentially the same way. In this section I want to give you a brief overview of the basics of ADCs, along with the important concepts that you will have to know to use them in your applications. At the end of this section I have provided some sample code to help guide you in using the ADC successfully in your own applications. Some time ago, it was easy to tell which PIC microcontroller part numbers had an ADC built into them; they were the ones with a 7 as the character after the C or F of the part number. Now, many different PIC microcontrollers have a built-in ADC that will indicate an analog voltage level from 0 to Vdd with 8- or 10-bit accuracy, and you will have to look at their datasheets to understand which ones have this capability. The ADC inputs usually are situated in the PORTA I/O pins and can be used as either digital I/O or analog inputs. The actual bit accuracy, utilization of pins, and operating speed are a function of the PIC microcontroller part number and the clock speed at which
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The equivalent to the PIC microcontroller ADC input circuitry.
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the PIC microcontroller runs. When a pin is con gured for analog input, it follows the models shown in Fig. 16.26. Rs in the Vsource circuit is the in-line resistance of the power supply. In order to get reasonable times for charging the ADC s holding capacitor, this value should be less than 10 k . If you look through the ADC documentation, you will nd that the time required for the holding capacitor to load the analog voltage and to stabilize is
Tack = 5 ms + [(temp 25C) x 0.05 ms/C] + (3.19C x 10**7) x (8k + Rs)
which works out to anywhere from 7.6 to 10.7 s at room temperature. I usually avoid this calculation altogether and assume that at least 12 s is required for the holding capacitor voltage to stabilize to the input voltage. Once the voltage is stabilized within the holding capacitor, a test for each bit is made; 9.5 cycles are required to do an 8-bit conversion. The bit conversion cycle time (known as TAD) can be anywhere from 1.6 to 6.4 s and can use either the PIC microcontroller s instruction clock or a built-in 250-kHz RC oscillator. To get a valid TAD time using the PIC microcontroller s instruction clock, a 2, 8, or 32 prescaler is built into the ADC. For example, a 4-MHz clock using the divide by 8 prescaler will have a 2- s TAD time, which is acceptable for the ADC. If the divide by 2 counter were used, the TAD would be 500 ns, which is much too fast for the ADC to work correctly. The built-in 250-kHz oscillator is used to carry out the ADC conversion when the PIC microcontroller is asleep. Microchip recommends that the PIC microcontroller be put to sleep during the ADC conversion for maximum accuracy (and minimum internal voltage or current upsets). If the PIC microcontroller is put to sleep, then the minimum conversion time is much longer than what is possible using the built-in clock because the PIC microcontroller has to restart when the ADC completion interrupt has been received. The minimum conversion time is de ned as the total time required for the holding capacitor to stabilize at the input voltage and for the ADC operation to complete. Assuming that a 12- s holding time could be implemented along with a 15- s ADC conversion time, the maximum time is about 27 s, or 37,000 ADC samples per second can be implemented.
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