AN1072 Measuring VDD Using the 0.6V Reference Author: chosen for this feature, although there are other capable devices. The ADC Block Diagram for the PIC16F690 is shown in Figure 1. Tom Perme Microchip Technology Inc. To measure VDD, VDD should be selected as the reference to the ADC via VCFG, and the 0.6V reference selected as the input using the channel select bits, CHS<3:0>. A measurement of the 0.6V input is taken with the ADC, and the result represents 0.6 Volts as a percentage of VDD. As VDD increases, the resulting number will decrease and vice versa. This yields a direct “1/x” relationship between VDD and the produced digital value as seen in Equation 1. In short, given a specific VDD, the digital value is always the same. Working backwards, if the digital value is known, VDD may be calculated. INTRODUCTION This application note describes how to measure the voltage supplied to a PIC® microcontroller, VDD. The device used in preparation of this application note was the PIC16F690. The ability to measure VDD lends itself to battery applications where VDD is likely to fall over time. In this application note, an example program is provided with routines to measure VDD. The 0.6V input’s ADC result may be expressed in 9 bits over all operating voltages and tolerances. So, when using the 10-bit ADC the result will be placed in the ADRESH and ADRESL registers with RightJustification to treat the value as a 16-bit integer. THEORY OF OPERATION Select Microchip PIC microcontrollers contain a 0.6V or 1.2V internal reference that is selectable as an input to the ADC module. This provides a fixed reference to allow measurement of VDD. The PIC16F690 was FIGURE 1: PIC16F690 ADC BLOCK DIAGRAM VDD VCFG = 0 VREF VCFG = 1 RA0/AN0/C1IN+/ICSPDAT/ULPWU RA1/AN1/C12IN0-/VREF/ICSPCLK RA2/AN2/T0CKI/INT/C1OUT RA4/AN3/T1G/OSC2/CLKOUT RC0/AN4/C2IN+ RC1/AN5/C12IN1RC2/AN6/C12IN2-/P1D(1) ADC RC3/AN7/C12IN3-/P1C(1) RC6/AN8/SS (2) 10 GO/DONE RC7/AN9/SDO(2) ADFM RB4/AN10/SDI/SDA(2) RB5/AN11/RX/DT(2) 0 = Left Justify 1 = Right Justify ADON 10 CVREF VSS VP6 Reference ADRESH ADRESL CHS © 2007 Microchip Technology Inc. DS01072A-page 1 AN1072 IMPLEMENTATION Seen below, Equation 1 describes how an analog voltage is converted to a digital number. For the 10-bit ADC, n = 10. This formula was used to calculate the digital values for the table of Appendix A: “ADC Results Table”. There are two considerations to keep in mind when attempting to measure and use the measurement of VDD. First, the ADC result is in counts, not in voltage. The relationship between counts and voltage is defined by Equations 1 and 2, and the values are tabulated in Appendix A: “ADC Results Table”. EQUATION 1: n 2 –1 VP6COUNT = 0.6V -------------V DD Second, any variation of VP6 will shift the table, and the values will not represent the voltage expected. There is no way to know exactly what voltage the reference VP6 is before using the device. However, by performing a few calibrations, the value of the 0.6V reference can be removed from the measurement computation. This will greatly increase the accuracy over the straightforward approach of Equation 2. Simultaneously, it will yield values in the form of a 16-bit number as 480 for 4.80V for the useable output, which makes the use of measured values more convenient. To convert a digital value back to an analog voltage is simple mathematically. Solve Equation 1 for VDD, and the result is Equation 2. EQUATION 2: n V DD 2 –1 = 0.6V -------------------------------VP6COUNT The cost to do this calibration is added complexity. To perform the calibration, a known and stable voltage VDD must be applied, and then take a measurement of the 0.6V input. This value will be stored in the part’s memory, and then used later when the voltage VDD is unknown, call it Vu for unknown voltage at those times.This procedure for calibration and use is shown in Figure 2: These equations define the relationship between VDD and VP6COUNT. The table in Appendix A: “ADC Results Table” lists the analog VDD voltages and the corresponding digital value for 0.6V in the left two columns. The third column shows the analog value that the digital number actually represents after rounding (Equation 2). Once the ADC module is properly configured, taking a reading will produce the value VP6COUNT in ADRESH and ADRESL, and using the above relationship, VDD can be calculated or action taken on the value. FIGURE 2: EQUATION DEVELOPMENT WITH DESCRIPTION AND EXAMPLE Equation Development Description Example, VP6 = 0.59V, VREF = 4.00 VP6 VP6CALVAL = ------------- • 1023 VREF Measure 0.6V with known voltage VREF 0.59V -------------- • 1023 = 150 4.00V VP6 VuCount = K • VP6CALVAL ⎛ = ----------- • 1023⎞ ⎝ ⎠ Vu For unknown voltages, assume form Unknown Result = K * known CALVALUE (=ADC result) For example, measure 5.38V. 0.59 VuCount = 112 ⎛ = ---------- • 1023⎞ ⎝ 5.38 ⎠ VP6 VP6 K • ------------- • 1023 = ----------- • 1023 V R EF Vu Substitute in 1st equation; set equal and solve for K — V RE F K = ------------- Simplify to find K — V RE F VuCount = ------------- • VP6CALVAL Vu Substitute K into assumed form — ( V R E F • VP6CALVAL ) Vu = -----------------------------------------------------------VuCount Solve for Vu. Known calibration values on top, and measured ADC result on bottom. Vu Modify numbers for convenience of data Vref • 100⎞ ⎛ ⎛ ⎞ ⎛ ⎞ -------------------------- • VP6CALVAL ⁄ VuCount • 2 storage VREF*100 = 16-bit value for voltage Vu = ⎠ ⎠ ⎝⎝⎝ ⎠ 2 Divide by 2, multiply by 2 avoids unnecessary 24-bit math. DS01072A-page 2 ( 4.00 • 150 ) Vu = ------------------------------ = 5.35 112 ( 400 • 150 ) Vu = ---------------------------- = ( 535 ) 112 (as integer in microcontroller) © 2007 Microchip Technology Inc. AN1072 So, after taking a measurement of the unknown voltage VDD which produces ADC result VuCount, and knowing the VP6CALVAL taken during calibration with known VREF, the voltage may be found by the last two equations. The next to last equation is the mathematically proper equation, and the last equation is a form which makes the numbers more easily usable with 8 and 16bit integer arithmetic. Note: The routines to calibrate and output data in the form of VDD * 100 are provided in the source code of this application. USING THE SOFTWARE Performing the Calibration When to calibrate and where to store the calibration data is ultimately left to the end user’s implementation. The provided source code was designed such that holding a button down while power is first applied will enter calibration mode, the calibration values are stored in EEPROM, and then normal program flow begins. This calibration must be performed only once, since the value is stored in EEPROM. If the calibration value were stored in volatile memory, the calibration would need to be performed at least once each time the device became powered. To perform the calibration with the supplied source code, there are two steps. First, when entering calibration mode, the device must be supplied proper voltage, and the voltage should be stored as a constant in program memory by the following line: constant VREFPLUS = d'400' A voltage of 4.00V is recommended for VDD during calibration when the full operating voltage of the part is to be used. If the part will only operate over a narrow range of voltage, calibrating the part in the middle of that range would be best, and the value above should be changed. Once a voltage for calibration has been decided, the second step is to run the calibration routine. After supplying proper voltage as specified above, make the following call to store the calibration data to EEPROM. call VDD * 100, and may be used for a trip point or to display the voltage. The 16-bit value is readily convertible to BCD formatting to display on an LCD for example. ACCURACY By calibrating the device, the maximum tolerance for error is reduced. The average value of the 0.6V reference is removed from calculations, and only its variation over temperature and voltage primarily affect any error in measurements. Round-off errors from using the ADC module always exist, but these errors are typically small in comparison. Table 1 shows measurements taken for a single PIC16F690 for example purposes. This example shows why 4.00V was chosen for the VDD used to calibrate the device. Calibrating at 4V will cut the error due to voltage variation of the 0.6V reference roughly in half. This is seen by the negative error readings below 4 volts and positive error readings above 4 volts. It also puts an exact measurement on the calibration voltage. TABLE 1: VDD Applied MeasureVDD Output % Error 2.50 248 -0.8% 2.75 272 -1.1% 3.00 298 -0.7% 3.25 324 -0.3% 3.50 348 -0.6% 3.75 374 -0.3% 4.00 400(2) 0.0%(2) 4.25 428 0.7% 4.50 452 0.4% 4.75 478 0.6% 5.00 506 1.2% 5.25 532 1.3% 5.50 556 1.1% Mean Error 0.7% StoreCalibData The routine StoreCalibData takes a measurement of the 0.6V input and then stores the ADC value to the EEPROM within the device as described by the first equation of Figure 2. The calibration is now complete. Measuring VDD With Software When the device has proper values in the calibration registers, the software can be used by calling MeasureVdd. Two bytes will form the result in Vdd_H and Vdd_L. These two bytes are a 16-bit value of the form © 2007 Microchip Technology Inc. EXAMPLE CALIBRATED ACCURACY(1) Max Error 1.3% Note 1: 2: This example is not representative of all manufactured parts, and is used to illustrate the methods shown. See Appendix B. The variation of the 0.6V reference with respect to voltage does not have a specified tolerance, but a general characterization can still give an idea for expected tolerance. The following data are not specifications of the part, but are here to provide a general idea of the behavior. DS01072A-page 3 AN1072 For a given part, the absolute value of the 0.6V reference is ensured within the bounds specified by its data sheet. Given a specific part, the 0.6V reference will fluctuate as VDD changes (e.g., 0.605V at VDD = 2.0V, 0.595V at VDD = 5.5V). The increase or decrease in the 0.6V reference above or below its value as measured during calibration will result in error in the measurement. Table 2 shows rough values that can be expected for the range of voltage on the 0.6V reference. It shows these with differing degrees of confidence. TABLE 2: Ensure calibration voltage constant is correct constant VREFPLUS = d'###' Supply VREF and run calibration call StoreCalibValue Issue call in main program call MeasureVdd 0.6V VARIATION OVER VDD Standard Deviation % Devices Included Typical Range (ΔV) ±1σ 68.2% 14 mV ±3σ 99.7% 19 mV ±6σ 99.9% 28 mV This data is a characterization from many PIC16F690 parts, but it is not a specification. It is very likely all devices’ VP6 will vary across VDD to within the maximum amount shown. For example, the voltage could change 28 mV from 0.650V to 0.622V over VDD of 2 to 5.5V. Appendix B: “Comparison of Calibrated Method versus Table Look-up Method for Same Part” shows an example part with the confidence intervals applied to its mean as well as a comparison of the calibration routine versus using the ADC result alone. In terms of the output of the calibrated VDD measurement, this means that by using 4.00V to calibrate the device, the error is split roughly in half as shown earlier in Table 1. For the one standard deviation case, there will be about 7 mV above and 7 mV below the average 0.6V value for a tolerance of 1.2%. Table 3 shows each case. TABLE 3: VDD may be measured with reasonable accuracy, and the user must perform only three actions with the source code to include it in a project. To summarize, the actions are below. CALIBRATED RESULT TOLERANCES Standard Deviation Measured VDD Estimated Tolerance ±1σ ± 1.2% ±3σ ± 1.6% ±6σ ± 2.4% Even using the largest tolerance for error with six standard deviations, the estimated tolerance of the calibrated output is roughly ±2.4% assuming a 0.6V average value. Compared to the direct method, where the tolerance of the 0.6V is the tolerance of the output, this is a great increase in accuracy. Please see the related source code for the routines required to measure VDD. An application using the PICkit™ 2 Low Pin Count Demo Board (DS51556) was used with an LCD to display VDD along with other pertinent data such as the calibration value and the 10-bit ADC result, VuCount. MEMORY USAGE Memory usage for the minimal calibration and measurement routines are shown as “Cal & Meas.” Memory usage for the example program which displays VDD on an LCD is shown as “LCD Example.” TABLE 4: MEMORY USAGE Prog. Words RAM Bytes EEPROM Cal & Meas 209 23 1 LCD Example 472 29 1 Program GLOSSARY OF TERMS Acronym Description VDD Supply Voltage VP6COUNT ADC Output Value VP6 Actual Voltage of 0.6V reference. VP6CALVAL ADC Value of VP6 at Calibration VDD Vu Unknown Voltage VuCount Unknown ADC result VREF VDD used for Calibration CONCLUSIONS Measuring VDD can be used for a number of purposes such as system monitoring, low battery detection, or calibrating ADC measurements based on the true voltage. DS01072A-page 4 © 2007 Microchip Technology Inc. AN1072 APPENDIX A: ADC RESULTS TABLE Table provided for convenient look-up of ADC results given a VDD voltage under nominal conditions. TABLE A-1: VDD ADC RESULTS APPLIED DIGITAL 0.6V (VP6COUNT) VDD REPRESENTED 2.50 2.52 2.54 2.56 2.58 2.60 2.62 2.64 2.66 2.68 2.70 2.72 2.74 2.76 2.78 2.80 2.82 2.84 2.86 2.88 2.90 2.92 2.94 2.96 2.98 3.00 3.02 3.04 3.06 3.08 3.10 3.12 3.14 3.16 3.18 3.20 3.22 3.24 3.26 3.28 3.30 3.32 3.34 3.36 3.38 3.40 3.42 245 243 241 239 237 235 233 232 230 228 227 225 223 222 220 218 217 215 214 212 211 209 208 207 205 204 202 201 200 199 197 196 195 193 192 191 190 189 188 186 185 184 183 182 181 180 179 2.50 2.52 2.54 2.56 2.59 2.61 2.63 2.64 2.67 2.69 2.70 2.72 2.75 2.76 2.79 2.81 2.82 2.85 2.86 2.89 2.91 2.93 2.95 2.96 2.99 3.00 3.03 3.05 3.07 3.08 3.11 3.13 3.14 3.18 3.19 3.21 3.23 3.24 3.26 3.30 3.31 3.33 3.35 3.37 3.39 3.41 3.42 © 2007 Microchip Technology Inc. VDD APPLIED DIGITAL 0.6V (VP6COUNT) VDD REPRESENTED 3.44 3.46 3.48 3.50 3.52 3.54 3.56 3.58 3.60 3.62 3.64 3.66 3.68 3.70 3.72 3.74 3.76 3.78 3.80 3.82 3.84 3.86 3.88 3.90 3.92 3.94 3.96 3.98 4.00 4.02 4.04 4.06 4.08 4.10 4.12 4.14 4.16 4.18 4.20 4.22 4.24 4.26 4.28 4.30 4.32 4.34 4.36 4.38 178 177 176 175 174 173 172 171 170 169 168 167 166 165 164 163 163 162 161 160 159 158 157 157 156 155 154 154 153 152 151 150 150 149 148 148 147 146 145 145 144 143 143 142 141 141 140 139 3.44 3.46 3.48 3.50 3.52 3.54 3.56 3.58 3.61 3.63 3.65 3.67 3.69 3.72 3.74 3.76 3.76 3.78 3.81 3.83 3.86 3.88 3.90 3.90 3.93 3.95 3.98 3.98 4.01 4.03 4.06 4.09 4.09 4.11 4.14 4.14 4.17 4.20 4.23 4.23 4.26 4.29 4.29 4.32 4.35 4.35 4.38 4.41 DS01072A-page 5 AN1072 VDD APPLIED DIGITAL 0.6V (VP6COUNT) VDD REPRESENTED 4.40 4.42 4.44 4.46 4.48 4.50 4.52 4.54 4.56 4.58 4.60 4.62 4.64 4.66 4.68 4.70 4.72 4.74 4.76 4.78 4.80 4.82 4.84 4.86 4.88 4.90 4.92 4.94 4.96 4.98 5.00 5.02 5.04 5.06 5.08 5.10 5.12 5.14 5.16 5.18 5.20 5.22 5.24 5.26 5.28 5.30 5.32 5.34 5.36 5.38 5.40 5.42 139 138 138 137 136 136 135 135 134 133 133 132 132 131 130 130 129 129 128 128 127 127 126 126 125 125 124 124 123 123 122 122 121 121 120 120 119 119 118 118 117 117 116 116 116 115 115 114 114 113 113 113 4.41 4.44 4.44 4.47 4.51 4.51 4.54 4.54 4.57 4.61 4.61 4.64 4.64 4.68 4.72 4.72 4.75 4.75 4.79 4.79 4.83 4.83 4.87 4.87 4.90 4.90 4.94 4.94 4.98 4.98 5.02 5.02 5.07 5.07 5.11 5.11 5.15 5.15 5.19 5.19 5.24 5.24 5.28 5.28 5.28 5.33 5.33 5.38 5.38 5.42 5.42 5.42 DS01072A-page 6 VDD APPLIED DIGITAL 0.6V (VP6COUNT) VDD REPRESENTED 5.44 5.46 5.48 5.50 5.52 112 112 111 111 111 5.47 5.47 5.52 5.52 5.52 © 2007 Microchip Technology Inc. AN1072 APPENDIX B: Note: COMPARISON OF CALIBRATED METHOD VERSUS TABLE LOOKUP METHOD FOR SAME PART This example is not representative of all manufactured parts, and is used for illustrative purposes. Measuring VDD on the same part with the two different methods Calibrated Method Table Look-up Method (Both) VDD Applied MeasureVdd Output % Error VP6COUNT VDD of ADRES (Eq. 2) % Error Actual 0.6V 2.50 248 -0.8% 245 2.51 0.2% 0.599 2.75 272 -1.1% 223 2.75 0.0% 0.599 3.00 298 -0.7% 203 3.02 0.8% 0.595 3.25 324 -0.3% 187 3.28 1.0% 0.594 3.50 348 -0.6% 174 3.53 0.8% 0.595 3.75 374 -0.3% 162 3.79 1.0% 0.594 4.00* 400* 0.0%* 152 4.04 1.0% 0.594 4.25 428 0.7% 142 4.32 1.7% 0.590 4.50 452 0.4% 134 4.58 1.8% 0.589 4.75 478 0.6% 127 4.83 1.7% 0.590 5.00 506 1.2% 120 5.12 2.3% 0.587 5.25 532 1.3% 114 5.38 2.6% 0.585 5.50 556 1.1% 109 5.63 2.4% 0.586 * Calibration voltage reads exact when operating under calibration conditions. FIGURE 3: Mean Error 0.7% Mean Error 1.3% Max Error 1.3% Max Error 2.6% 0.6V VARIATION AND CONFIDENCE INTERVALS ON ΔV 0.6V Variation For Exam ple Part VP6 Voltage 0.610 0.6V Variation (Example Part) 0.600 -1 ± 1sigma σ ± 3 σ -3 sigma ± 6sigma σ -6 0.590 0.580 0.570 2.50 Mean 3.00 3.50 4.00 4.50 5.00 5.50 VDD © 2007 Microchip Technology Inc. DS01072A-page 7 AN1072 NOTES: DS01072A-page 8 © 2007 Microchip Technology Inc. Note the following details of the code protection feature on Microchip devices: • Microchip products meet the specification contained in their particular Microchip Data Sheet. • Microchip believes that its family of products is one of the most secure families of its kind on the market today, when used in the intended manner and under normal conditions. • There are dishonest and possibly illegal methods used to breach the code protection feature. All of these methods, to our knowledge, require using the Microchip products in a manner outside the operating specifications contained in Microchip’s Data Sheets. Most likely, the person doing so is engaged in theft of intellectual property. • Microchip is willing to work with the customer who is concerned about the integrity of their code. • Neither Microchip nor any other semiconductor manufacturer can guarantee the security of their code. Code protection does not mean that we are guaranteeing the product as “unbreakable.” Code protection is constantly evolving. We at Microchip are committed to continuously improving the code protection features of our products. Attempts to break Microchip’s code protection feature may be a violation of the Digital Millennium Copyright Act. If such acts allow unauthorized access to your software or other copyrighted work, you may have a right to sue for relief under that Act. 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