AN2979 Application note Implementing a simple ADC using the STM8L101xx comparator Introduction This application note gives a simple method for implementing an A/D converter with a minimum amount of external components: one resistor and one capacitor. The pratical application example described in this document uses the STM8L101xx microcontroller comparator. February 2010 Doc ID 15651 Rev 1 1/12 www.st.com Contents AN2979 Contents 1 2 Application description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1 Comparator features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.2 ADC implementation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Software description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.1 Application flowcharts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 3 Hardware description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 4 Measurements and calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 5 2/12 4.1 Typical measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 4.2 Precision of the measured value . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 4.3 How to get a better accuracy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 4.3.1 Hardware solution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 4.3.2 Software solution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Doc ID 15651 Rev 1 AN2979 Application description 1 Application description 1.1 Comparator features The STM8L101xx microcontroller embeds two zero-crossing comparators sharing the same current bias and the same voltage reference. This voltage reference can be: ● internal (comparison to ground) ● or external (comparison to a reference pin voltage) Each comparator is connected to 4 channels which can be used to generate an interrupt, a timer input capture or a timer break. Their polarity can be inverted. Note: For external comparison be aware that the STM8L101xx comparator maximum input value is VDD -1.25 V with VDDmax = 3.6 V 1.2 ADC implementation Each comparator can be used to implement an ADC. This technique is based on a simple principle: the signal to be measured is connected to the non-inverted input and the reference signal is an external signal connected to the inverting input. In the demonstration software, the comparator used is COMP2 and the channel used as the signal to be measured is the channel 3 (pin PD2). The reference signal is generated by charging a capacitor through a resistor. While the voltage across the capacitor is being charged, it follows an exponential curve. This exponential equation has been implemented in the software. The time taken by the capacitor voltage to rise above the voltage value to be converted is used in the charge equation to retrieve the digital conversion value. Charge equation: V meas = V DD × ⎛ 1 – exp ⎛ – --t-⎞ ⎞ ⎝ ⎝ T⎠ ⎠ Where: ● Vmeas is the value to be mesaured ● VDD is the input voltage ● t is the time measured by timer2 when the comparator detects that the input voltage is above the reference voltage ● T is the RC constant (here R=10 kΩ and C=100 nF so T= 1 ms) The capacitor is charged and discharged using the timer 2 PWM on channel1/ PB0. A timer is programmed to generate a 2 ms PWM with a duty cycle of 0.25. This 2-ms period permits an ADC implementation using the full voltage range that the comparator tolerates. The capacitor charging curve is shown in the following figure. Doc ID 15651 Rev 1 3/12 Application description Figure 1. AN2979 Capacitor charging curve PWM output Capacitor charging curve VDD(V) 3.3 3 2.5 2 1.5 1 0.5 1 2 3 4 5 6 7 8 9 T(ms) The figure below shows the connection of the comparator to the required external components. Figure 2. Comparator connections PD2 PA6 PB0/PC4 + COMP_OUT - R=10 k7 C=100 nF AI 1. Condition: VDD=3.3 V, R=10 kΩ, C=100 nF Note: 4/12 When using the STM8L101-EVAL evaluation board you need to connect the PB0 pin to the PC4 pin. This way you use the resistor and capacitor already present on the board (no hardware needed). Pin PC4 is configured as input floating to avoid any conflict. Doc ID 15651 Rev 1 AN2979 2 Software description Software description The software provided with this application note describes a way of implementing an A/D converter (using a timer and analog comparator interrupts). The application uses four peripherals: ● CLK: the clock enables and provides the correct clock frequency for the peripherals. ● COMP: the comparator detects when the input voltage is above the reference voltage ● TIM2: Timer 2 provides the 2 ms PWM with a duty cycle of 0.25 generating the capacitor charge and discharge. Timer 2 Capture/compare interrupt routine handles the conversion. ● GPIO: the general purpose I/O handles the I/O used. ● SPI: used to communicate with the LCD. A generic file param.h contains the parameter values that can be modified in order to re-use the application and adapt it easily to other conditions. At the start, the LCD displays “STM8L ADC using COMP”. Then it continously displays the measured values. Figure 3. Application architecture & description main.c param.c Application layer stm8l10x.h API layer stm8l10x.h stm8l10x.h param.h stm8l10x tim stm8l10x.h stm8l10x comp stm8l10x cl stm8l10x api Standard peripheral drivers stm8l10x tim stm8l10x.comp stm8l10x cl stm8l10x api STM8L101x microcontroller Note: ai15372 The software can be compiled with Cosmic and Raisonance compilers. It contains projects for STVD and Raisonance IDE. A calibration of some parameters can be optionally set if “#define Calibration” is uncommented in the param.h file. Doc ID 15651 Rev 1 5/12 Software description 2.1 AN2979 Application flowcharts Figure 4. Main loop flowchart Start Clock configuration Configure the system clock to provide a master clock frequency fMASTER = 16 MHz Enable TIM2 peripheral clock GPIO configuration Configure PC4 as input floating Configure PB0 as output push pull for the PWM A/D converter initialization LCD configuration Optional Calibration Start A/D conversion Comparator configuration Enable comparator interface Configure COMP2 channel3 With external reference and polarity high Connect Comp2 output to the TIM2 input capture1 TIM configuration TIM2 to generate the PWM Enable TIM2 capture / compare interrupt enable Enable general interrupts SPI configuration Display on LCD Configure COMP2 channel4 Compare to the reference value Enable TIM2 Display on LCD End ai15766 Figure 5. Get conversion value flowchart Start VIN > VREF Get timer value Get the conversion value Clear TIM2 pending bit Display on the LCD End ai15767 6/12 Doc ID 15651 Rev 1 AN2979 3 Hardware description Hardware description Figure 6. Circuit diagram PB0 STM8L10x R=10 kΩ PC4 VDD PD2 (COMP2_CH3) PA6 (COMP_REF) !$# INPUT VSS C=100 nF AI STM8L101 evaluation board settings: ■ Remove jumper JP2 to connect an external input voltage instead of the potentiometer input. With the potentiometer RV1, the value is limited to 0.8 V and the signal is noisy (due to the LCD). So the display on the LCD screen would not be stable. ■ Connect the external ADC voltage input directly to PD2. ■ Ground together the evaluation board and the external ADC signal. ■ An accurate 3.3 V MCU voltage can be tuned on the evaluation board using RV4. Doc ID 15651 Rev 1 7/12 Measurements and calibration AN2979 4 Measurements and calibration 4.1 Typical measurements The following values are given for information only. Table 1. 4.2 Measured value after conversion and margin error for VDD=3.3 V Value to be measured (V) Measured value after conversion (V) Error (%) 0.25 0.204 18.4% 0.5 0.419 16.2% 0.8 0.704 12% 1 0.903 9.7% 1.25 1.14 8.8% 1.5 1.39 7.33% 1.75 1.628 6.97% Precision of the measured value When using the RC charge equation, the precision of the measured value depends on the accuracy of the capacitor C and the resistor R. Accuracy example: if C=100 nF with 10% accuracy: Cacc1 =90 nF or Cacc2 =110 nF if R=10 kΩ with 2% accuracy: Racc1=9.8 kΩ or Racc2=10.2 kΩ Calculation of the RC constant: 3 –9 T = R × C = ( 10 × 10 ) × ( 100 × 10 ) = 1 ms 3 –9 T ACC1 = R ACC1 × C ACC1 = ( 9.8 × 10 ) × ( 90 × 10 ) = 882 μs 3 –9 T ACC2 = R ACC2 × C ACC2 = ( 10.2 × 10 ) × ( 110 × 10 ) = 1.122 ms Charge equations: V meas = V DD × ⎛ 1 – exp ⎛ – --t-⎞ ⎞ ⎝ ⎝ T⎠ ⎠ ⎛ ⎛ t ⎞⎞ V measacc1 = V DD × ⎜ 1 – exp ⎜ – -----------------⎟ ⎟ ⎝ T acc1⎠ ⎠ ⎝ ⎛ ⎛ t -⎞ ⎞ V measacc2 = V DD × ⎜ 1 – exp ⎜ – ---------------⎟⎟ ⎝ ⎝ T acc2⎠ ⎠ 8/12 Doc ID 15651 Rev 1 AN2979 Measurements and calibration The following figure shows the impact of the capacitor and resistor accuracy. Figure 7. Charge equation depending on RC accuracy V 3 2.5 Vmeas_max = 2.05 2 y = 3.3(1-exp(-x/0.882)) 1.5 y = 3.3(1-exp(-x)) y = 3.3(1-exp(-x/1.122)) 1 tmax = 0.970ms tacc1max = 0.856ms 0.5 tacc2max = 1.09 ms 0.5 tacc1max 1 1.5 2 2.5 ms tmax tacc2max 1. In blue: Vmeasacc1 ---- In red: Vmeas ---- In green: Vmeasacc2 2. Condition: VDD=3.3 V Note: The results are not only impacted by the marging error due to hardware components but also by the accuracy of the MCU clock. In this example, the device works at a HSI frequency of 16 MHz at 25 °C, that is, with a clock accuracy of 1% (refer to the datasheet). The temperature variation also impacts the accuracy of the measured value as the microcontroller, resistors and capacitors are temperature dependent. 4.3 How to get a better accuracy 4.3.1 Hardware solution The first solution is to choose components with a high accuracy value (this may imply a higher cost). 4.3.2 Software solution The second solution is to calibrate the RC constant in the software using VDD as a reference. Implementation description In order to perform the calibration, you need to uncomment the “#define calibration” in the “param.h” file. Doc ID 15651 Rev 1 9/12 Measurements and calibration AN2979 The calibration is then performed by firware and the RC constant is updated. This constant is used in the charge equation. On the STM8L101-EVAL evaluation board, the COMP2 channel 4 (PD3) is connected to VDD via a resistor bridge. Figure 8. Evaluation board calibration schematic 0" 2 K7 34 - , X 6$$ 0 $ # / - 0 ? # ( 0# 0 $ # / - 0 ? # ( 0! 6 33 # / - 0?2%& K7 #ALIBRATION INPUT !$# INPUT K7 # N& AI As VDD=3.3 V, PD3 is a fixed value equal to 0.4 V In the file “param.h” you need to define the “expected_value” equal to 0.4 ( this value needs to be updated if a different resistor bridge is used). The specific function “Calibration()” configures the COMP2 Channel 4 and compares the value measured on this channel with the expected value. This function then updates the RC constant that will be used afterwards. When the calibration is performed, the COMP2 configuration changes to use the Channel 3 (PD2). The following table lists a few measurement examples with a calibration performed at VDD=3.3 V, R=10 kΩ and C=100 nF. Table 2. 10/12 Accuracy measurement when the calibration is performed Value to be measured (V) Measured value after conversion (V) Error (%) 0.25 0.233 6.8% 0.5 0.475 5% 0.8 0.792 1.25% 1 0.99 1% 1.2 1.211 0.92% 1.5 1.512 0.8% 1.75 1.784 1.9% Doc ID 15651 Rev 1 AN2979 5 Revision history Revision history Table 3. Document revision history Date Revision 12-Feb-2010 1 Changes Intitial release. Doc ID 15651 Rev 1 11/12 AN2979 Please Read Carefully: Information in this document is provided solely in connection with ST products. STMicroelectronics NV and its subsidiaries (“ST”) reserve the right to make changes, corrections, modifications or improvements, to this document, and the products and services described herein at any time, without notice. All ST products are sold pursuant to ST’s terms and conditions of sale. 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