AVR401: 8-bit Precision A/D Converter 8-bit Microcontroller Features • • • • • Very Low Cost High Precision Auto-calibration Eliminates Component Inaccuracy Measures Voltages for 0 to VCC Maximum Conversion Time: 1.1 ms Application Note Introduction This application note describes how to perform a kind of dual slope A/D conversion with an AVR microcontroller. The converter is very low cost, requiring only six discrete components in addition to the AVR. Five of the controller pins are used (see Figure 1). This example is based on the AT90S1200 device, but any AVR device with a comparator can be used. Figure 1. A/D Converter VCC RB PB7 T AT90S1200 (AIN0) PB0 (AIN1) PB1 C PB2 Rref 1 Vin (Vref) Rin Rref 2 PB3 Theory of Operation The capacitor is charged with a constant current supplied by the transistor. The capacitor voltage will rise linearly. To discharge the capacitor, the AIN0-pin is set to output with a “0” applied. A reference voltage at VCC/2 is supplied by the resistor network R ref1 and R ref2. When the PB1 and PB2-pins are configured as inputs, the reference is turned off, and the voltage level at the AIN1-pin will be the input voltage Vin. By setting the pins as outputs and applying a “0” and a “1”, the level at the AIN1pin will be VCC/2 (if the resistors are of equal size). The input resistor Rin have to be at least 100 times higher than the reference resistors Rref1 and Rref2 to avoid measurement errors. Rev. 0953C–AVR–02/03 1 The algorithm used for the conversion is as follows: 1. Turn on the reference. 2. Charge the capacitor until the reference voltage is reached. Measure the time needed for this, Tref. 3. Turn off the reference and discharge the capacitor 4. Charge the capacitor until the input voltage is reached. Measure the time needed for this, Tin. The conversion cycle is shown in Figure 2. The time measurement is performed by the Timer/Counter, which is expanded to nine bits by using the Timer/Counter Overflow Interrupt. Figure 2. Conversion Cycle Calculation Suppose that VCC is 5 volts. The relationship between the input voltage and the reference voltage is given by: Equation 1 V ref × T in V in = --------------------------T ref The ideal output from the conversion is an 8-bit number, where 0 volts corresponds to zero and 5 volts is 255. The reference voltage V CC/2 thus corresponds to 128. The equation can be re-written as: Equation 2 T in × 128 V in = -----------------------T ref However, with inaccuracy in the reference resistors, the reference voltage may vary slightly. To compensate for this, a calibration can be performed by applying a known voltage at the input, and compare this to the reference. If the applied calibration voltage is exactly 2.5 volts, the reference voltage can be found by the equation: Equation 3 2 T ref × 128 T ref × V cal - = --------------------------V ref = ----------------------------T cal T cal AVR401 0953C–AVR–02/03 AVR401 The calibration cycle is executed by holding the PB7-pin high during Power-up. The calibration voltage is then applied, and the PB7-pin is set low. This starts calibration, and once performed, the value of the reference voltage is stored in EEPROM. During normal operation, the reference value is read from EEPROM, and the input voltage is calculated using Equation 1. Configuration Example As the resulting output is to be eight bits, the timer should be of at least nine bits to maintain the resolution. The components should be chosen so that the nominal time charging the capacitor up to VCC is about 256 timer steps. In that way, inaccuracy in the component values and temperature changes are allowed, without causing the charging time to be longer than the maximum timer period, or too short, giving lower resolution. To achieve sufficient accuracy, a prescaler factor of eight or higher should be used. The AT90S1200 Timer/Counter0 is of only eight bits, so the ninth bit must be handled in software. The following example illustrates how the component values can be found. First, decide which crystal frequency to operate at. With a 4 MHz crystal, the clock period is 250 ns. By setting the prescaler to CK/8, the Timer is incremented every 2 µs. The maximum timer period with nine bits is 512 x 2 µs = 1,024 µs. From this, we set 2 x TREF to 512 µs. The charging of a capacitor whit a constant current is described by the equation: Equation 4 I ∆ V = ---- × ∆ t C We can find the required current when the capacitor size, the time and the voltage difference is known: Equation 5 ∆V × C I = -----------------∆t The capacitor will be charged up to VCC = 5 V, and with a 220 µF capacitor, the transistor must supply a current of 2.15 mA. The RB value is dependent upon the transistor’s hFE. For a BC558A pnp transistor, hFE is in the range 125 to 250. This makes this transistor ideal for use, since any hFE value in the specified range can be used. To make sure the full range in hFE can be used, the average value, 188, is used in the calculations. The resulting base current is 11.4 µA. The transistor is turned on by applying a “0” on the corresponding pin. At this current values, the transistor base-emitter voltage is about ÷0.1 V. The base resistor is found to be: Equation 6 V CC + V BE 4.9V R B = -------------------------------- = ------------------- = 430k Ω IB 11.4 µ A The reference voltage is generated by the divider network Rref1 and Rref2. The Rin has to be much larger than these two, so that the input voltage will not influence with the reference voltage. 100 kΩ for Rin and 1 kΩ for each of Rref1 and Rref2 is suitable. 3 0953C–AVR–02/03 The transistor should be connected to a pin as long away from the comparator inputs as possible. When a pin is switched, a noise spike appear at the adjacent pins. This will cause problems when measuring low voltages, as the noise spike might trigger the comparator before the capacitor voltage has reached the measured voltage. Figure 3 shows measured linearity for a 4 MHz clocked application using the component values calculated in the above example. Figure 3. Measured Linearity LINEARITY 250 O U T 200 P U 150 T V 100 A L 50 U E 0 0 1 2 3 4 5 INPUT VOLTAGE Implementation The software consist of several subroutines. The routines “reference” and “convert” handles the charging and timing. After they complete execution, the main program must perform the calculation needed. This is done using two subroutines that performs division and multiplication, “div17u” and “mul9”. There are also two delays used by the other routines and the main program. They are used to discharge the capacitor completely and to generate a delay between each conversion. “reference” Subroutine – The routine discharges the capacitor, turns on the transistor and charges the capacitor Measures the Reference until the capacitor voltage is equal to the reference voltage. The time from the beginning of the charging and until the voltages are equal is measured. The capacitor is then disVoltage charged again. The charging time is used together with the charging time from the “convert” routine to calculate the input voltage. This routine does not have to be called every time a conversion is performed, depending on variations in ambient temperature. Especially the parameter hFE in the transistor is quite temperature dependent, so if the ambient temperature is varying, the subroutine will have to be executed frequently. In the example program, the “reference” routine is called each time a conversion is performed. Table 1. “reference” Subroutine Performance Figures Parameter Value Code Size 24 words Execution Cycles Depends on the reference voltage. Register Usage 4 Low Registers High Registers Global :None :2 :1 AVR401 0953C–AVR–02/03 AVR401 Table 2. “reference” Register Usage Register Input Internal Output R17 Tref – Holds the time to reach the reference voltage. R18 TH – High part of timer. R20 temp Figure 4. Flow Chart for “reference” Reference AIN0 as Output (Discharge Capacitor) PB2 and PB3 as Outputs (Turn on Vref) Delay Clear Timer Variables AIN0 as Input (Capacitor Ready for Charging) Turn on Transistor (Charge Capacitor) Yes ACO = 1? (Vcap > Vref?) No Timer > 0x1FF? No Yes Store Tref Stop Timer Turn off Transistor AIN0 as Output (Discharge Capacitor) PB2 and PB3 as Inputs (Turn off Vref) Return 5 0953C–AVR–02/03 “input” Subroutine – Measures the Input Voltage The routine turns on the transistor and charges the capacitor until the capacitor voltage is equal to the input voltage. Then capacitor is then discharged. The time needed to do this is measured and stored in Tin. There should be a few microseconds delay between two conversion cycles, to ensure that the capacitor is completely discharged. In the example program, this is done by calling a delay routine. Table 3. “input” Subroutine Performance Figures Parameter Value Code Size 19 words Execution Cycles Depends on the input voltage Register Usage Low Registers High Registers Global :2 :None :1 Table 4. “input” Register Usage Register Internal Output R14 TinH – High part of the input voltage charge time. R15 TinL – Low part of the input voltage charge time. R20 6 Input temp AVR401 0953C–AVR–02/03 AVR401 Figure 5. Flow Chart for “input” Input AIN0 as Input (Capacitor Ready for Charging) Clear Error Flag Clear Timer Variables Turn on Transistor (Charge Capacitor) Yes ACO = 1? (Vcap > Vin?) No Timer > 0x1FF? No Yes Set Error Flag Store Tin Stop Timer Turn off Transistor AIN0 as Output (Discharge Capacitor) Return 7 0953C–AVR–02/03 “T0_int” Interrupt Service Routine The only function for this routine is to increment the TH variable, so a 16-bit Timer is created. Only nine bits are used. Table 5. “T0_int” Interrupt Performance Figures Parameter Value Code Size 2 words Execution Cycles 9 – including the reti instruction Register Usage “mpy9u” 9 × 8 Bit Multiplication Low Registers High Registers Global :None :None :1 This routine performs a 9 × 8 bit multiplication. The 9-bit multiplier must be stored in the Carry Flag (MSB) and the “mp9u” Register. The multiplicand is stored in the “mc9u” Register. The answer is placed in “C:m9uH:m9uL”. The registers used for the result are the same as those used for the input to the division routine. The routine is based on the “mpy8u” multiplication routine described in application note AVR 200. Table 6. “mpy9u” Subroutine Performance Figures Parameter Value Code Size 11 words Execution Cycles 83 Register Usage Low Registers High Registers Global Flags :3 :None :1 :C Table 7. “mpy9u” Register Usage Register Input R0 mc9u – Multiplicand R1 mp9u – Multiplier Internal m9uL – Result Low byte R2 C-flag R20 8 Output m9uH – Result High byte Multiplier, ninth bit Result, 17th bit temp – Used as Loop Counter AVR401 0953C–AVR–02/03 AVR401 “div17u” 17/16 Bit Division This routine performs a 17/16 bit division. The 17-bit dividend must be stored in the (C:didH:didL) variable, where the Carry Flag is most significant. The divisor is stored in the (divH:divL) variable. The result is placed in (resH:resL) and the reminder in (remH:remL). The routine is based on the “div16u” multiplication routine described in application note AVR 200. Table 8. “div17u” Subroutine Performance Figures Parameter Value Code Size 18 words Execution Cycles 209 min, 292 max. Register Usage Low Registers High Registers Global Flags :6 :None :1 :C Table 9. “div17u” Register Usage Register Internal Output R1 didL – Low part dividend dresL – Low part result R2 didH – High part dividend dresH – High part result C-flag Example Program Input 17th bit of dividend R3 divL – Low part divisor R4 divH – High part divisor R5 remL – Low part reminder R6 remH – High part reminder The included example program performs repeated conversions. First, the charging time for the reference is measured, then for the input voltage. The result is output to Port D and Port B pin 4 (MSB). The result is inverted before it is output, so active low LEDs can be connected to show the result. This conversion cycle is repeated in an endless loop. To perform a calibration the PB7-pin must be initially in high state and the capacitor has to be discharged. Afterwards the user should apply 2.5 volts at the input before setting the PB7-pin low. The calibrated Vref is stored in EEPROM, where it is fetched at every normal Power-up. 9 0953C–AVR–02/03 Performance Figures Table 10. Overall Performance Figures Parameter Value Code Size 43 words - Conversion routines only (not mpy9u and div17u) 147 words - Complete application note Register Usage Low Registers High registers Pointers Interrupt Usage Timer/Counter 0 Interrupt Peripheral Usage Timer/Counter0 Analog Comparator Port B, pin 0 to 3 and pin 7 Port D, all pins (example program only) Port B, pin 4 (example program only) :9 :5 :None The calibration routine can be skipped if only relative values are measured. The reference voltage is then assumed to be 128, which will also make the calculations easier. The reference network can be substituted with a voltage reference to achieve even better accuracy. It is then possible to measure variations in V CC by connecting it via a voltage divider network to the input. 10 AVR401 0953C–AVR–02/03 AVR401 11 0953C–AVR–02/03 Atmel Headquarters Atmel Operations Corporate Headquarters Memory 2325 Orchard Parkway San Jose, CA 95131 TEL 1(408) 441-0311 FAX 1(408) 487-2600 Europe Atmel Sarl Route des Arsenaux 41 Case Postale 80 CH-1705 Fribourg Switzerland TEL (41) 26-426-5555 FAX (41) 26-426-5500 Asia Room 1219 Chinachem Golden Plaza 77 Mody Road Tsimhatsui East Kowloon Hong Kong TEL (852) 2721-9778 FAX (852) 2722-1369 Japan 9F, Tonetsu Shinkawa Bldg. 1-24-8 Shinkawa Chuo-ku, Tokyo 104-0033 Japan TEL (81) 3-3523-3551 FAX (81) 3-3523-7581 2325 Orchard Parkway San Jose, CA 95131 TEL 1(408) 441-0311 FAX 1(408) 436-4314 RF/Automotive Theresienstrasse 2 Postfach 3535 74025 Heilbronn, Germany TEL (49) 71-31-67-0 FAX (49) 71-31-67-2340 Microcontrollers 2325 Orchard Parkway San Jose, CA 95131 TEL 1(408) 441-0311 FAX 1(408) 436-4314 La Chantrerie BP 70602 44306 Nantes Cedex 3, France TEL (33) 2-40-18-18-18 FAX (33) 2-40-18-19-60 ASIC/ASSP/Smart Cards 1150 East Cheyenne Mtn. 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