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Fujitsu Microelectronics Europe Application Note MCU-AN-300215-E-V12 F²MC-16FX FAMILY 16-BIT MICROCONTROLLER ALL SERIES A/D CONVERTER APPLICATION NOTE A/D CONVERTER Revision History Revision History Date 2006-07-28 2007-03-01 2007-09-04 Issue First Version; MWi V1.1, Reviewed the document and updated with review findings, MPi V1.2, Updated the entire documents with review findings from PHu, MPi This document contains 28 pages. MCU-AN-300215-E-V12 -2- © Fujitsu Microelectronics Europe GmbH A/D CONVERTER Warranty and Disclaimer Warranty and Disclaimer To the maximum extent permitted by applicable law, Fujitsu Microelectronics Europe GmbH restricts its warranties and its liability for all products delivered free of charge (e.g. software include or header files, application examples, target boards, evaluation boards, engineering samples of IC’s etc.), its performance and any consequential damages, on the use of the Product in accordance with (i) the terms of the License Agreement and the Sale and Purchase Agreement under which agreements the Product has been delivered, (ii) the technical descriptions and (iii) all accompanying written materials. In addition, to the maximum extent permitted by applicable law, Fujitsu Microelectronics Europe GmbH disclaims all warranties and liabilities for the performance of the Product and any consequential damages in cases of unauthorised decompiling and/or reverse engineering and/or disassembling. Note, all these products are intended and must only be used in an evaluation laboratory environment. 1. Fujitsu Microelectronics Europe GmbH warrants that the Product will perform substantially in accordance with the accompanying written materials for a period of 90 days form the date of receipt by the customer. Concerning the hardware components of the Product, Fujitsu Microelectronics Europe GmbH warrants that the Product will be free from defects in material and workmanship under use and service as specified in the accompanying written materials for a duration of 1 year from the date of receipt by the customer. 2. Should a Product turn out to be defect, Fujitsu Microelectronics Europe GmbH´s entire liability and the customer’s exclusive remedy shall be, at Fujitsu Microelectronics Europe GmbH´s sole discretion, either return of the purchase price and the license fee, or replacement of the Product or parts thereof, if the Product is returned to Fujitsu Microelectronics Europe GmbH in original packing and without further defects resulting from the customer’s use or the transport. However, this warranty is excluded if the defect has resulted from an accident not attributable to Fujitsu Microelectronics Europe GmbH, or abuse or misapplication attributable to the customer or any other third party not relating to Fujitsu Microelectronics Europe GmbH. 3. To the maximum extent permitted by applicable law Fujitsu Microelectronics Europe GmbH disclaims all other warranties, whether expressed or implied, in particular, but not limited to, warranties of merchantability and fitness for a particular purpose for which the Product is not designated. 4. To the maximum extent permitted by applicable law, Fujitsu Microelectronics Europe GmbH´s and its suppliers´ liability is restricted to intention and gross negligence. NO LIABILITY FOR CONSEQUENTIAL DAMAGES To the maximum extent permitted by applicable law, in no event shall Fujitsu Microelectronics Europe GmbH and its suppliers be liable for any damages whatsoever (including but without limitation, consequential and/or indirect damages for personal injury, assets of substantial value, loss of profits, interruption of business operation, loss of information, or any other monetary or pecuniary loss) arising from the use of the Product. Should one of the above stipulations be or become invalid and/or unenforceable, the remaining stipulations shall stay in full effect © Fujitsu Microelectronics Europe GmbH -3- MCU-AN-300215-E-V12 A/D CONVERTER Contents Contents REVISION HISTORY ............................................................................................................ 2 WARRANTY AND DISCLAIMER ......................................................................................... 3 CONTENTS .......................................................................................................................... 4 1 INTRODUCTION.............................................................................................................. 6 1.1 Key Features........................................................................................................... 6 2 THE ANALOGUE/DIGITAL CONVERTER....................................................................... 7 2.1 Block Diagram......................................................................................................... 7 2.2 Registers................................................................................................................. 8 2.2.1 Control Status Register (ADCS) .................................................................. 8 2.2.2 Date Register (ADCR)................................................................................. 9 2.2.3 Setting Register (ADSR) ............................................................................. 9 2.2.3.1 Sampling/Conversion Time ...................................................... 10 2.2.3.2 Start/Stop Channels................................................................. 10 2.2.4 ADC Extended Configuration Register (ADECR) ....................................... 11 2.2.5 Analog Input Enable Register (ADER0 – ADERn) ...................................... 11 3 POWER SUPPLY OF A/D CONVERTER ...................................................................... 13 3.1 Power consumption............................................................................................... 13 3.2 Noise consideration............................................................................................... 13 4 ANALOGUE INPUT AND RELATED EXTERNAL CIRCUITS ....................................... 15 4.1 External circuits for analogue input........................................................................ 15 4.2 Input Leakage current consideration...................................................................... 16 5 SAMPLING TIME CONSIDERATION ............................................................................ 18 6 LATCH-UP RELATED TO AVCC/VCC AND LARGE INPUT SIGNAL .............................. 20 6.1 AVCC > VCC ............................................................................................................ 20 6.2 AVCC < VCC............................................................................................................. 20 6.3 UAIN > AVCC or VCC .................................................................................................. 21 6.4 Conclusion ............................................................................................................ 21 7 INPUT IMPEDANCE ...................................................................................................... 22 7.1 Recharging and discharging the Sampling capacitor ............................................. 22 8 ADC EXAMPLE ............................................................................................................. 24 8.1 ADC with interrupts ............................................................................................... 24 9 ADDITIONAL INFORMATION ....................................................................................... 26 MCU-AN-300215-E-V12 -4- © Fujitsu Microelectronics Europe GmbH A/D CONVERTER Contents LIST OF FIGURES ............................................................................................................. 27 LIST OF TABLES............................................................................................................... 28 © Fujitsu Microelectronics Europe GmbH -5- MCU-AN-300215-E-V12 A/D CONVERTER Chapter 1 Introduction 1 Introduction This application note describes the functionality of the Analog/Digital Converter (ADC) and gives some examples. 1.1 Key Features • Minimum conversion time per Channel: 1.7 Hs at 20 MHz of CLKP1 (for 4.5V J AVCC J 5.5V) • 8-bit or 10-bit conversion resolution • Sequential channel conversion, once (Single Mode), continuous (Continuous Mode) and converts one channel, stops and waits for the next activation (Stop Mode) • Interrupt generation after conversion selectable • Interrupt can trigger DMA to transfer conversion result to memory • Triggered by software, external Pin (ADTG) or Reload Timer 1 MCU-AN-300215-E-V12 -6- © Fujitsu Microelectronics Europe GmbH A/D CONVERTER Chapter 2 The Analogue/Digital Converter 2 The Analogue/Digital Converter THE BASIC FUNCTIONALITY OF THE ANALOGUE/DIGITAL CONVERTER 2.1 Block Diagram Figure 2-1 shows the internal block diagram of the ADC. Figure 2-1: ADC Block Diagram x is last/highest ADC channel available on the particular device. © Fujitsu Microelectronics Europe GmbH -7- MCU-AN-300215-E-V12 A/D CONVERTER Chapter 2 The Analogue/Digital Converter 2.2 Registers 2.2.1 Control Status Register (ADCS) This register controls the A/D converter and indicates its status. Bit No. 15 Initial Value Name Explanation BUSY* Busy Flag and Stop* 0 Interrupt Flag* 0 Value 10-Bit Mode 0 0 1 Read: No A/D Conversion Write: Force Conversion Stop Read: A/D Conversion ongoing Write: No effect Read: No A/D Data Write: Clear Flag Read: A/D Data and Interrupt Write: No effect Interrupt disabled Interrupt, if A/D Data Read: No A/D Conversion Pause Write: Clear Bit Read: Pause occurred Write: No effect ADC Activation by Software ADC Activation by Software and ADTG Pin ADC Activation by Software and Timer ADC Activation by Software, ADTG Pin, and Timer Always read; Write: no effect Start and Restart A/D Conversion Reserved Bit , always write “0” to it Single Mode 1; Reactivation during Conversion allowed Single Mode 2; Reactivation during Conversion not allowed Continuous Mode; Reactivation during Conversion not allowed Stop Mode; Reactivation during Conversion not allowed 10-Bit Conversion Mode 8-Bit Conversion Mode - Undefined X 0 Reserved Bit , always write “0” to it - Reserved 0 0 Reserved Bit , always write “0” to it 0 1 0 14 INT* 1 13 INTE Interrupt enable 0 12 PAUS A/D Converter Pause 0 0 1 0 1 0, 0 0, 1 11, 10 STS1,0 Start Source Select 0, 0 1, 0 1, 1 9 STRT 8 - Start Conversion Undefined 0 1 0 0 X 0, 0 7, 6 Operation MD1, 0 Mode Select 0, 1 0, 0 1, 0 1, 1 5 4 … 1 0 Operation S10 Table 2-1: ADCS * These bits return “1” during Read-Modify-Write instruction. MCU-AN-300215-E-V12 -8- © Fujitsu Microelectronics Europe GmbH A/D CONVERTER Chapter 2 The Analogue/Digital Converter 2.2.2 Date Register (ADCR) This register stores digital value generated as a result of conversion. This register contains the last converted value and is rewritten every time the conversion ends. Bit No. 15 … 10 9 … 0 Name D9 … D0 Explanation Value Operation - 0 These bits always return “0” Data Bits - These bits contain A/D data after successful conversion Table 2-2: ADCR 2.2.3 Setting Register (ADSR) This registers sets the sampling and conversion time, also sets the starting and ending channel for conversion. Bit No. Name Explanation Initial Value 15 … 13 ST2 … ST0 Sampling Time Setting 0, 0, 0 12 … 10 CT2 … CT0 Compare Time Setting 0, 0, 0 Value 0, 0, 0, 0, 1, 1, 1, 1, 0, 0, 0, 0, 1, 1, 1, 1, 0, 0, 1, 1, 0, 0, 1, 1, 0, 0, 1, 1, 0, 0, 1, 1, Operation 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0, 0, 0 9 … 5 ANS4 … ANS0 Starting Channel Setting 0, 0, 1 0, 0, 0 0, 1, 0 . . . 1, 1, 1 0, 0, 0 4 … 0 ANE4 … ANE0 Ending Channel Setting 0, 0, 1 0, 0, 0 0, 1, 0 . . . 1, 1, 1 4 peripheral clock cycles 6 peripheral clock cycles 8 peripheral clock cycles 12 peripheral clock cycles 24 peripheral clock cycles 36 peripheral clock cycles 48 peripheral clock cycles 128 peripheral clock cycles 22 peripheral clock cycles 33 peripheral clock cycles 44 peripheral clock cycles 66 peripheral clock cycles 88 peripheral clock cycles 132 peripheral clock cycles 176 peripheral clock cycles 264 peripheral clock cycles AN0 (ADECR_ADSEL = 0) AN32 (ADECR_ADSEL = 1) AN1 (ADECR_ADSEL = 0) AN33 (ADECR_ADSEL = 1) AN2 (ADECR_ADSEL = 0) AN34 (ADECR_ADSEL = 1) ... AN31 (ADECR_ADSEL = 0) AN0 (ADECR_ADSEL = 0) AN32 (ADECR_ADSEL = 1) AN1 (ADECR_ADSEL = 0) AN33 (ADECR_ADSEL = 1) AN2 (ADECR_ADSEL = 0) AN34 (ADECR_ADSEL = 1) ... AN31 (ADECR_ADSEL = 0) Table 2-3: ADSR Note: Always use word access to write to this register. © Fujitsu Microelectronics Europe GmbH -9- MCU-AN-300215-E-V12 A/D CONVERTER Chapter 2 The Analogue/Digital Converter Please refer the datasheet for the highest ADC channel number available on the device. 2.2.3.1 Sampling/Conversion Time The sampling time must be equal to or greater than 0.5 Hs. The Conversion time must be at least 1 Hs. Therefore an overall A/D conversion time could be 1.5 Hs, but this depends on the used peripheral clock. Assume a peripheral clock of 16 MHz (62.5 ns cycle time). The sample cycle number then has at least to be 8 (0.5 Hs) and the conversion cycle number can be 22 (1.375 Hs). Therefore the overall A/D conversion time is 1.875 Hs. All these settings are valid for 4.5 V J AVCC J 5.5 V. Please see the hardware manual for external impedance considerations. Please consider that the sampling/conversion time settings also depend on the settings in the peripheral Clock 1 divider. CKFCRH_PC1D[3:0] CLKS1 Peripheral Clock 1 Divider (div-1 to div-16) CLKP1 to ADC Figure 2-2: ADC Clock 2.2.3.2 Start/Stop Channels • Use start channel = end channel for conversion of only one channel. • If start channel is greater then end channel, conversion starts form start channel to the highest available channel, then from channel 0 to the end channel. For products with less than 32 A/D converter channels, the conversion results can be incorrect. To prevent this, do not set ANS larger than ANE. • It is not possible to have a start channel < 32 and an end channel Q 32. If this is needed, please use the following sequence: o Set ADECR:ADSEL = 0 to convert channels from required start channel to channel 31. o Set ADECR:ADSEL = 1 and convert channel 32 to the required end channel. MCU-AN-300215-E-V12 - 10 - © Fujitsu Microelectronics Europe GmbH A/D CONVERTER Chapter 2 The Analogue/Digital Converter 2.2.4 ADC Extended Configuration Register (ADECR) If there are more than 32 ADC channels available, the ADECR switches between upper and lower ADC channel set. The deselected set cannot be accessed then. It also selects A/D converter’s high and low reference voltage, if available. Bit No. 7 … 3 2, 1 Name Explanation Initial Value Value - Reserved Bits X 0 LSEL,HSEL Reference Voltage ADSEL 0 Read value is undefined; Write “0” to these Bits. Low reference voltage AVRL/AVRH2 AVSS 0,0 0,1 1,0 1,1 0 1 0,0 ADC Channel Set Select Operation 0 High reference voltage AVRH AVRH AVSS AVRH AVSS AVRL/AVRH2 Select ADC Channels 0 - 31 Select ADC Channels Q 32 Table 2-4: ADECR It should be noted that the reference voltage switch is available on selected devices. In the devices where the reference voltage switch is unavailable, bits LSEL and HSEL are unavailable and high reference voltage pin is AVRH and low voltage reference pin is AVSS always. 2.2.5 Analog Input Enable Register (ADER0 – ADERn) The ADER0 - ADERn selects whether a corresponding pin is used as an analog input or a digital I/O channel. The following formula shows the relation between total number of ADC channels and the highest ADER register: n= x 1 8 where, x is the total number of ADC channels available on the device n is the highest ADER register The following table shows ADER0. Bit No. 7 … 0 Name Explanation ADE7 … ADE0 ADC Input Selection Initial Value 0 Value Operation 0 Digital I/O Port enabled Analog Input enabled 1 Table 2-5: ADER0 The following table shows ADER1. Bit No. 7 … 0 Name Explanation ADE15 … ADE8 ADC Input Selection Initial Value 0 Value Operation 0 Digital I/O Port enabled Analog Input enabled 1 Table 2-6: ADER1 © Fujitsu Microelectronics Europe GmbH - 11 - MCU-AN-300215-E-V12 A/D CONVERTER Chapter 2 The Analogue/Digital Converter The following table shows ADERn. Bit No. 7 … 0 Name ADE(X-1) … ADE(X-8) Explanation ADC Input Selection Initial Value 0 Value Operation 0 Digital I/O Port enabled 1 Analog Input enabled Table 2-7: ADERx Hence if total number of ADC channels available on a particular device is 24 then the highest ADER register is ADER2 and bits of the same would be ADE16 to ADE23. MCU-AN-300215-E-V12 - 12 - © Fujitsu Microelectronics Europe GmbH A/D CONVERTER Chapter 3 Power supply of A/D converter 3 Power supply of A/D converter ELECTRICAL POWER CONSIDERATIONS 3.1 Power consumption The power consumption (IR, IA) of the ADC increases in case a conversion is in progress (ADCSH_BUSY = 1). While the ADC is halted (ADCSH_BUSY = 0), only leakage current (IRH, IAH) flows. The following diagrams reflect this behaviour: ADCSH_BUSY [Bit-value] 1 ADCSH_BUSY=1 0 t Reference Voltage Current [mA] (AVRH, AVRL) IR IRH t Power Supply Current [mA] AVCC IA IAH t Figure 3-1: Power consumption and operating status of ADC Note: Please refer to the datasheet in order to get the absolute values of IR, IRH, IA and IAH. 3.2 Noise consideration Fujitsu microcontroller has implemented an embedded 10-bit Successive Approximated Register (SAR) ADC. Due to the high resolution, the digital bit stream from the ADC output is sensitive to the environment noise. For example, 1LSB corresponds to only 4.9mV for UREF=5V. Hence, the noise introduced from the external circuits must be considered and should be reduced to the minimum as possible. © Fujitsu Microelectronics Europe GmbH - 13 - MCU-AN-300215-E-V12 A/D CONVERTER Chapter 3 Power supply of A/D converter The reference voltage UREF, which is equal to AVRH-AVRL (depends on ADECR setting if reference voltage switch is available), is connected to the weighted capacitor array and the resistor array of the ADC. The noise coupled to AVR will not be rejected by ADC. This noise will be added to the UREF directly, introducing an error with a ratio of UNoise/UREF. For example, to keep the error caused by this kind of noise below 0.1LSB, the noise level of UREF must be kept within 0.49mV. As a result, the pin AVRH and AVRL pins must be connected with low impedance. In practice often a simple low-pass RC-filter is used for noise reduction. In this case the reference voltage supply current (see datasheet) has to be taken into account when calculating the resistor of the filter, in order to minimize the voltage drop while converting. Normally two capacitors in parallel are recommended, one filtering low frequency noise, the other one filtering high frequency noise ((10nF–1µF)||(10pF-100pF)). In most cases, this configuration suppresses the noise efficiently. If very high frequency noise appears in the environment, an additional noise filter such as a dedicated mode RC filter might be useful. The analogue power path AVCC supplies the internal voltage comparator and the analogue switches of the ADC, while the VCC path supplies all the digital parts in the microcontroller. Internal parasitic capacitors may couple noise from AVCC to the internal voltage comparator of the ADC. For this reason, also AVCC should not be connected directly to VCC but filter should be used, too. For more efficient noise filtering the same configuration as for AVRH is recommended. VCC +5 VSS AVCC AVRH 10 G 10pF 100pF 10nF 10pF 100pF 10nF 1uF 10pF 100pF 10nF 1uF 1uF Reference Voltage µC AVRL Reference Voltage AVSS Figure 3-2: A suggested connection for the power supplies MCU-AN-300215-E-V12 - 14 - © Fujitsu Microelectronics Europe GmbH A/D CONVERTER Chapter 4 Analogue input and related external circuits 4 Analogue input and related external circuits 4.1 External circuits for analogue input Usignal AVCC ~ Dclamp R1 Rclamp R2 AN x Cnoise µC Dclamp Figure 4-1: A typical external circuit for analogue input To protect the analogue pins to suffer from an over-voltage, the so-called “clamping resistor” is usually added to the input pins. The minimum value of the resistor can be chosen as Rclamp= Uovervoltage/Iclamp, Where, Iclamp is the specified maximum clamp current in the data sheet. For some applications, a large clamp resistor is sometimes unacceptable. As a compromise, an external clamping diode with low leakage current could be added between the input pin and AVCC pin. In some cases, the sensor has been biased with a voltage supply higher than the maximum allowed voltage for the microcontroller. For example, in the automotive applications, the sensors could be biased directly with the car battery, which exhibits a voltage of 12V/24V. A resistor divider consisting of R1/R2 is commonly used to tail the sensor voltage signal “seen” on the pin down to the value which is equal or smaller than AVCC/VCC (see Figure 4-1). The ratio between R1 and R2 should satisfy the following constrain: R1 R2 U Signal AVCC 1 Other factor which influences the size dimension of R1, R2 and Rclamp, is related to current consumption budget and the input signal noise suppressing. The second factor will be discussed here with more detail. The signal from the sensors could be also noisy. The noise, which has a time constant smaller than the sampling time Tsampling, is transparent to the ADC, resulting distorted output. In this case, an additional dedicated bypass capacitor together with the clamping resistor or resistor divider, works as a low pass filter. A larger capacitor will lower the AC impedance and will be more effective at shunt away the noise signal. Generally, the time constant of this low pass filter (Rclamp + R1 || R2) x Cnoise should be chosen considerably larger than the sampling time (5 to 10 times larger with a rule of thumb). However, at the same time this time constant should be also considerably smaller than the one of the sensor signal, depending on the applications. In this way, the analogue pin is able to follow the dynamic changes, which the ADC is being used to track. These, along with the dimension of R1/R2 or Rclamp must be considered when choosing the capacitor dimension to avoid rolling off any high frequency signal components of interest. © Fujitsu Microelectronics Europe GmbH - 15 - MCU-AN-300215-E-V12 A/D CONVERTER Chapter 4 Analogue input and related external circuits 4.2 Input Leakage current consideration The analogue input pins show a small leakage current, whose maximum value is about 3µA and ranged from 3µA down to 1µA depending on the temperature. The leakage current, which flows through the external resistor, introduces an undesired voltage drop. This error voltage is a function of the external resistor and the leakage current itself. The following example shows a dimension of the resistor with this factor taken into consideration. For the case of using a resistor divider to reduce the error due to leakage current, the size of R1 || R2 + Rclamp should not be chosen too large and should be according to the following equation: R1 || R2 + Rclamp U LSB I leakage Note: ULSB = UREF / 1024 To keep the error smaller than one LSB for a leakage of 3µA, the size of R1 || R2 + Rclamp should be smaller than 1.6k . As the leakage current drops down to 1 µA, the value of R1 || R2 + Rclamp can be chosen as large as 5k . This is considering UREF of 5V. It is found in the test that the leakage current consists of two parts: one is due to the leakage current of the input ESD structure. Another leakage current appears only as the multiplexer is switched on during the sampling time, whose contribution is usually considerably larger than the one created from ESD structure. The second leakage current can be regarded as a noise during the sampling time by the bypass capacitor, which is commonly used to filter the noise from the sensor input. If this capacitor is large enough, it can absorb most of the second leakage current during the sampling time, eliminating its contribution to the error voltage. VCC Ileakage2 appears after switches ESD Ileakage1 Usignal Cnoise ~ Ileakage2 MUX Reff Cbypass within the MUX turn on Csampling ESD µC Figure 4-2: Leakage current flowing to the analogue input pin MCU-AN-300215-E-V12 - 16 - © Fujitsu Microelectronics Europe GmbH A/D CONVERTER Chapter 4 Analogue input and related external circuits Leakage current Ileakage1+Ileakage2 Ileakage1 Ileakage1 t Error voltage due to leakage current without the bypass capacitor Error voltage Error voltage due to leakage current with the bypass capacitor t No active phase Sampling phase Comparison phase Figure 4-3: Reducing the leakage current with the bypass capacitor To show the effect of the bypass capacitor on reducing the leakage current error, we take a sampling time of 5µs and a leakage current of 3µA as an example. If we want to keep the voltage drop due to the second leakage current small than 0.5 LSB, the minimum size of the bypass capacitor should be chosen as: C= © Fujitsu Microelectronics Europe GmbH 3µA × 5µs 4.9mV / 2 - 17 - 6nF MCU-AN-300215-E-V12 A/D CONVERTER Chapter 5 Sampling time consideration 5 Sampling time consideration SAMPLING TIME CONSIDERATION Fujitsu applies an embedded 10-bit successive approximation register ADC with an internal integrated sampling and hold stage. The signal will charge the sampling capacitor at first and then the voltage signal on the sampling capacitor will be evaluated by the 10-bit ADC successively. The time to charge the sampling capacitor to its final value equal to the signal level is a function of the sampling capacitor Csampling, the external resistor and the internal switch on-resistor. To reduce the error caused by the limited sampling time to an acceptable level, the sampling time should be chosen much larger than the time constant to charge the sampling capacitor. For example, if we choose a sampling time with a factor 7 of the RC constant, namely, 7 × ( Rextern + Rswitch ) × Csampling . Then the error amounts to e 7 × U REF , corresponding 0.94 × U LSB only. For MB96340 Series, the on-resistor of the transmission gate amounts to 2.25k and the sampling capacitor Csampling equals to 10.7pF at AVCC = 5V. For an external resistor of 2.25k , the sampling time should be chosen larger than 7 × (2.25k + 2.25k ) × 10.7 pF = 0.38µs by using above thumb rule. Rswitch=2.25k (max.) Usampling Rextern COMP Sequential Comparison Register ~ Usignal Csampling=10.7pF (max.) µC Figure 5-1: Block diagram for ADC in MB96340 Series MCU-AN-300215-E-V12 - 18 - © Fujitsu Microelectronics Europe GmbH A/D CONVERTER Chapter 5 Sampling time consideration Usampling Error<0.94LSB@samling time=7 Usignal Sampling time 7 Figure 5-2: Error related to sampling time For the Fujitsu microcontroller the sampling time can be set by defining the register bit ST2 to ST0 of the ADC data register (ADCRH). Please refer to the hardware manual. Please also refer Chapter 7: Input Impedance. © Fujitsu Microelectronics Europe GmbH - 19 - MCU-AN-300215-E-V12 A/D CONVERTER Chapter 6 Latch-up related to AVCC/VCC and large input signal 6 Latch-up related to AVCC/VCC and large input signal RECOMMENDATIONS TO PREVENT LATCH-UP Latch-up conditions can permanently damage the device and must be avoided. It is up to the application to assign any precautions in order to avoid any latch-up condition. 6.1 AVCC > VCC VCC ESD D3 1 µC AVCC ESD VSS Figure 6-1: Latch-up in case AVCC > VCC Latch-up can happen if AVCC becomes larger than VCC. This might be related to the application cases that VCC is switched on later than AVCC or VCC is switched off earlier than AVCC. The ESD diode D3 becomes forward biased, introducing a possible latch-up. 6.2 AVCC < VCC PMOS VCC VCC D1 Digital Control Transmission gate VSS VSS To ADC AVSS AVCC D2 2 Figure 6-2: Latch-up in case AVCC < VCC This case happens if the analogue pin is used as a digital output pin, the output level is “H”, and at the same time, AVCC is switched off. In this case, the PMOS in the output is on and the parasitic diode D2 of the transmission gate within the analogue multiplex becomes forward biased. A quiescent current flows through PMOS and D2. In case that a latch-up does not happen, a reliable logic “1” should not be expected at the output, due to the load diode D2. MCU-AN-300215-E-V12 - 20 - © Fujitsu Microelectronics Europe GmbH A/D CONVERTER Chapter 6 Latch-up related to AVCC/VCC and large input signal 6.3 UAIN > AVCC or VCC PMOS VCC D1 Digital Control 3 UAIN Transmission gate VSS To ADC AVSS AVCC D2 3 Figure 6-3: Problem in case UAIN > AVCC or VCC If UAIN becomes larger than VCC or AVCC, then the ESD diode D1 or D2 will be forward biased. A latch-up can happen. Even if a latch-up does not always happen in this case, the input signal, which exceeds VCC or AVCC, cannot be converted by the ADC properly. 6.4 Conclusion It is strongly suggested that AVCC and VCC should be DC short circuit together to avoid any possible latch-up. With the presence of AVCC and VCC voltage, an analogue input signal, which is smaller than AVCC and VCC can be always put on the analogue pins, independent on the MCU modes. Latch-up conditions can permanently damage the device if the related specified currents are exceeded. So Latch-up conditions must be avoided under all circumstances. It is up to the application to assign any precautions in order to avoid any latch-up condition. © Fujitsu Microelectronics Europe GmbH - 21 - MCU-AN-300215-E-V12 A/D CONVERTER Chapter 7 Input Impedance 7 Input Impedance ADC BEHAVIOR ON HIGH INPUT IMPEDANCES 7.1 Recharging and discharging the Sampling capacitor Because the ADC uses a sampling capacitor the input impedance must be set to a value below 15 k to recharge or discharge this capacitor within the sampling time. Example: Assume an application uses two ADC inputs. At one pin there is a voltage of about Vcc and at the other pin Vss. The first conversion charges the internal capacitor to Vcc within the sample time (A). After this the conversion starts (B). At the second conversion the MUX switches to the other input and the capacitor is discharged to Vss (C). The second conversion starts (D). If the input impedance (Z0) is too high, a rest of charge will remain and a wrong conversion will result. Figure 7-1: Current flow in the example The left illustration shows the voltage glitch on AN0 if Z0 is low. The glitch occurs, when the sampling switch is closed. Figure 7-2: Voltage glitch at low input impedance MCU-AN-300215-E-V12 - 22 - © Fujitsu Microelectronics Europe GmbH A/D CONVERTER Chapter 7 Input Impedance In this illustration the input impedance Z0 is too high. The sampling capacitor is not discharged within the sampling time and thus a wrong voltage is converted (red circle). Figure 7-3: Voltage glitch at high input impedance Note: At high impedance input circuit, there will always be a glitch of about 0.5 Volts max even if the previous conversion was at 0 Volt. This results from the ADC internal architecture itself. For the same reason discussed in this chapter an EMI capacitors for any analogue input pins should not exceed 1 nF. © Fujitsu Microelectronics Europe GmbH - 23 - MCU-AN-300215-E-V12 A/D CONVERTER Chapter 8 ADC Example 8 ADC Example EXAMPLES FOR THE ADC 8.1 ADC with interrupts Main.c /* THIS SAMPLE CODE IS PROVIDED AS IS AND IS SUBJECT TO ALTERATIONS. FUJITSU */ /* MICROELECTRONICS ACCEPTS NO RESPONSIBILITY OR LIABILITY FOR ANY ERRORS OR */ /* ELIGIBILITY FOR ANY PURPOSES. */ /* (C) Fujitsu Microelectronics Europe GmbH */ /*---------------------------------------------------------------------------*/ void InitADC (void) { ADER0 = 0x02; /* channel 1 to analog input */ ADCSL = 0xA0; /* ADC set to 8 bit continuous mode */ ADSR = 0xFC21; /* ADSRL + ADSRH, Sampling time – 128 CLKP1 Cycles, Conversion time – 264 CLKP1 Cycles, convert only AN1 */ ADCSH = 0xA2; /* interrupt enabled, software trigger, start conversion */ } void main (void) { DDR00 = 0xFF ; InitIrqLevels(); __set_il(7); __EI(); InitADC(); /* Data direction Port 0 = Output */ /* allow all levels /* globally enable interrupts */ */ /* init AD - converter */ while(1) /* waiting for interrupt (no operation) __asm("\tnop"); */ } __interrupt void ISR_ADC (void) { PDR00 = ADCRL; /* shows voltage on Port 0 ( LEDs ) ADCSH = 0xA2; /* clear interrupt flag } */ */ The above example demonstrates to configure ADC in the continuous mode with 8 bit conversion resolution with interrupts enabled. The maximum possible sampling and conversion time is used here. After every ADC interrupt the converted data is output to the LEDs connected to Port 0. MCU-AN-300215-E-V12 - 24 - © Fujitsu Microelectronics Europe GmbH A/D CONVERTER Chapter 8 ADC Example vectors.c /* THIS SAMPLE CODE IS PROVIDED AS IS AND IS SUBJECT TO ALTERATIONS. FUJITSU */ /* MICROELECTRONICS ACCEPTS NO RESPONSIBILITY OR LIABILITY FOR ANY ERRORS OR */ /* ELIGIBILITY FOR ANY PURPOSES. */ /* (C) Fujitsu Microelectronics Europe GmbH */ /*---------------------------------------------------------------------------*/ void InitIrqLevels (void) { . . . ICR = (76 << 8) | 6; /* Priority Level 6 for ADC of MB96340 Series */ . . . } /* ISR prototype */ __interrupt void ISR_ADC (void); . . . #pragma intvect ISR_ADC 76 . . . © Fujitsu Microelectronics Europe GmbH /* ADC of MB96340 Series */ - 25 - MCU-AN-300215-E-V12 A/D CONVERTER Chapter 9 Additional Information 9 Additional Information Information about FUJITSU Microcontrollers can be found on the following Internet page: http://mcu.emea.fujitsu.com/ The software examples related to this application note is: 96340_adc 96340_adc_rlt 96340_adc_dma 96340_ppg16_rlt_adc_dma It can be found on the following Internet page: http://mcu.emea.fujitsu.com/mcu_product/mcu_all_software.htm MCU-AN-300215-E-V12 - 26 - © Fujitsu Microelectronics Europe GmbH A/D CONVERTER List of Figures List of Figures Figure 2-1: ADC Block Diagram ............................................................................................. 7 Figure 2-1: ADC Clock ......................................................................................................... 10 Figure 3-1: Power consumption and operating status of ADC .............................................. 13 Figure 3-2: A suggested connection for the power supplies ................................................. 14 Figure 4-1: A typical external circuit for analogue input ........................................................ 15 Figure 4-2: Leakage current flowing to the analogue input pin ............................................. 16 Figure 4-3: Reducing the leakage current with the bypass capacitor.................................... 17 Figure 5-1: Block diagram for ADC in MB96F340................................................................. 18 Figure 5-2: Error related to sampling time ............................................................................ 19 Figure 6-1: Latch-up in case AVCC > VCC .............................................................................. 20 Figure 6-2: Latch-up in case AVCC < VCC .............................................................................. 20 Figure 6-3: Problem in case UAIN > AVCC or VCC .................................................................. 21 Figure 7-1: Current flow in the example ............................................................................... 22 Figure 7-2: Voltage glitch at low input impedance ................................................................ 22 Figure 7-3: Voltage glitch at high input impedance............................................................... 23 © Fujitsu Microelectronics Europe GmbH - 27 - MCU-AN-300215-E-V12 A/D CONVERTER List of Tables List of Tables Table 2-1: ADCS ..................................................................................................................... 8 Table 2-2: ADCR ..................................................................................................................... 9 Table 2-3: ADSR ..................................................................................................................... 9 Table 2-4: ADECR ................................................................................................................. 11 Table 2-5: ADER0 ................................................................................................................. 11 Table 2-1: ADER1 ................................................................................................................. 11 Table 2-1: ADERx ................................................................................................................. 12 MCU-AN-300215-E-V12 - 28 - © Fujitsu Microelectronics Europe GmbH