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| AVR401: 8-bit Precision A/D Converter Features * * * * * Very Low Cost High Precision Auto-calibration Eliminates Component Inaccuracy Measures Voltages for 0 to VCC Maximum Conversion Time: 1.1 ms 8-bit Microcontroller 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 T PB7 (AIN0) PB0 C (AIN1) PB1 PB2 Rref 1 Vin Rin Rref 2 PB3 (Vref) AT90S1200 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: V ref x T in V in = -------------------------T ref Equation 1 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: T in x 128 V in = -----------------------T ref Equation 2 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: T ref x 128 T ref x V cal V ref = ----------------------------- = --------------------------T cal T cal Equation 3 2 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 s. The maximum timer period with nine bits is 512 x 2 s = 1,024 s. From this, we set 2 x TREF to 512 s. The charging of a capacitor whit a constant current is described by the equation: I V = --- x t C Equation 4 We can find the required current when the capacitor size, the time and the voltage difference is known: Equation 5 V x C I = ----------------t The capacitor will be charged up to VCC = 5 V, and with a 220 F 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 A. 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: V CC + V BE 4.9V R B = ------------------------------- = ------------------- = 430k IB 11.4A Equation 6 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 k for Rin and 1 k 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 O U T 200 P U 150 T V 100 A L 50 U E 0 0 1 2 3 4 5 INPUT VOLTAGE 250 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 Code Size Execution Cycles Register Usage Value 24 words Depends on the reference voltage. Low Registers High Registers Global :None :2 :1 4 AVR401 0953C-AVR-02/03 AVR401 Table 2. "reference" Register Usage Register R17 R18 R20 TH - High part of timer. temp Input Internal Output Tref - Holds the time to reach the reference voltage. 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 Code Size Execution Cycles Register Usage Value 19 words Depends on the input voltage Low Registers High Registers Global :2 :None :1 Table 4. "input" Register Usage Register Input Internal Output TinH - High part of the input voltage charge time. TinL - Low part of the input voltage charge time. R14 R15 R20 temp 6 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 Code Size Execution Cycles Register Usage Value 2 words 9 - including the reti instruction Low Registers High Registers Global :None :None :1 "mpy9u" 9 x 8 Bit Multiplication This routine performs a 9 x 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 Code Size Execution Cycles Register Usage Value 11 words 83 Low Registers High Registers Global Flags :3 :None :1 :C Table 7. "mpy9u" Register Usage Register Input mc9u - Multiplicand mp9u - Multiplier Internal Output R0 R1 R2 C-flag R20 m9uL - Result Low byte m9uH - Result High byte Multiplier, ninth bit temp - Used as Loop Counter Result, 17th bit 8 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 Code Size Execution Cycles Register Usage Value 18 words 209 min, 292 max. Low Registers High Registers Global Flags :6 :None :1 :C Table 9. "div17u" Register Usage Register R1 R2 C-flag R3 R4 R5 R6 Input didL - Low part dividend didH - High part dividend 17th bit of dividend divL - Low part divisor divH - High part divisor remL - Low part reminder remH - High part reminder Internal Output dresL - Low part result dresH - High part result Example Program 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 Code Size Register Usage Value 43 words - Conversion routines only (not mpy9u and div17u) 147 words - Complete application note Low Registers High registers Pointers Timer/Counter 0 Interrupt 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 Interrupt Usage Peripheral Usage 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 Corporate Headquarters 2325 Orchard Parkway San Jose, CA 95131 TEL 1(408) 441-0311 FAX 1(408) 487-2600 Atmel Operations Memory 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 1150 East Cheyenne Mtn. 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Blvd. Colorado Springs, CO 80906 TEL 1(719) 576-3300 FAX 1(719) 540-1759 Scottish Enterprise Technology Park Maxwell Building East Kilbride G75 0QR, Scotland TEL (44) 1355-803-000 FAX (44) 1355-242-743 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 literature@atmel.com Web Site http://www.atmel.com (c) Atmel Corporation 2003. Atmel Corporation makes no warranty for the use of its products, other than those expressly contained in the Company's standard warranty which is detailed in Atmel's Terms and Conditions located on the Company's web site. The Company assumes no responsibility for any errors which may appear in this document, reserves the right to change devices or specifications detailed herein at any time without notice, and does not make any commitment to update the information contained herein. No licenses to patents or other intellectual property of Atmel are granted by the Company in connection with the sale of Atmel products, expressly or by implication. Atmel's products are not authorized for use as critical components in life support devices or systems. ATMEL (R) and AVR (R) are the registered trademarks of Atmel. Other terms and product names may be the trademarks of others. Printed on recycled paper. 0953C-AVR-02/03 0M |
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