AN1775

Freescale Semiconductor Order this document by AN1775/D Rev. 1 AN1775 Freescale Semiconductor, Inc... Expanding Digital Input with an A/D Converter By Brad Bierschenk Freescale Applications Engineering Austin, Texas Introduction This application note documents a method of extending digital input using the analog-to-digital converter (ADC) of a microcontroller unit (MCU). Many MCU applications require digital input and arbitration. For example, determining which key of a keypad was pressed. This is commonly done by arranging switches in a matrix configuration, connecting to a series of digital input pins, and reading a digital input data register to determine which key was pressed. While this method is easily implemented, it does require the use of an MCU’s parallel port pins. Some applications require all available bidirectional or input-only pins for other purposes. In such a case, an alternate method of arbitrating keypresses is desired. By using the ADC of an MCU connected to a resistor ladder, user input can be more efficiently processed. © Freescale Semiconductor, Inc., 2004. All rights reserved. © Motorola, Inc., 1998 AN1775 Rev. 1 For More Information On This Product, Go to: www.freescale.com Freescale Semiconductor, Inc. Application Note Background Dedicated Input A microcontroller typically receives user input through digital input pins. The simplest implementation is a single switch directly connected to a digital input pin. This is easy to realize, but is not the most efficient use of resources, with one pin dedicated to one input. One port data bit represents the state of one switch. Another method uses a keypad, a common element in embedded systems. These are ordinarily arranged in a matrix, as shown in Figure 1. In this case, the byte value of an entire port data register can be polled to determine which key was pressed. This is more efficient, as a 4 x 4 keypad can interface 16 keys with eight input pins. Freescale Semiconductor, Inc... Matrix Input Px0 Px1 Px2 Px3 Px4 Px5 Px6 Px7 Figure 1. Resistor Matrix Keypad Using Parallel Port Pins AN1775 Rev. 1 For More Information On This Product, Go to: www.freescale.com Freescale Semiconductor, Inc. Application Note Implementation ADC Alternative In many cases, input pins are at a premium. One can't always freely assign input pins to the function of user input. A more efficient use of microcontroller resources can be devised. One common feature of many Freescale MCUs is the analog-to-digital converter, or ADC. The ADC of a Freesca;e MCU usually features four to eight channels of analog input, which is compared with a reference voltage and converted to an 8-bit digital value. When a resistor ladder is connected to an analog input through switches in each segment, the conversion result can be used to arbitrate an input. This allows many keys to be interfaced with one input pin, with only a little more software overhead. Figure 2 shows such an implementation. VDD Freescale Semiconductor, Inc... MCU ADC INPUT PIN VIn Figure 2. Resistor Ladder Keys Using an Analog Input Pin Implementation ADC Operation An MCU ADC typically has 8-bit precision. This means there are 28, or 256, distinguishable A/D inputs, including 0. The analog inputs are converted to a binary number, which represents the magnitude of the input voltage in relation to a reference voltage. AN1775 Rev. 1 For More Information On This Product, Go to: www.freescale.com Freescale Semiconductor, Inc. Application Note The range of an ADC is the difference between its high and low reference voltages. This means an analog input between VREFH and VREFL will convert to an 8-bit number, with VREFL converting to $00 and VREFH converting to $FF. The resolution, defined as the range divided by the precision, defines the analog step that a change in one least significant bit (LSB) represents. In the case of a 5-volt, 8-bit ADC, the resolution is 5/255 (volts), or 19.6 mV. This means that a change in one LSB in the ADC data register reflects a change of about 20 mV at the analog input. Consider a resistor ladder connected to an ADC input, as shown in Figure 3. Because this arrangement is a voltage divider, each segment in the ladder can alter the voltage at the input when grounded. If switches are provided at each segment, one can selectively ground that segment, altering the composition of the divider, and thus altering the voltage presented to the ADC pin. In this way, software can determine which switch in the ladder was selected by reading the resulting A/D data value. The resistor R0 acts as a pullup to maintain VDD on the analog input line while no keys are active. Thus, a conversion value of $FF indicates that no key has been pressed. VDD Freescale Semiconductor, Inc... Resistor Ladder Voltage Divider R0 ADC INPUT R2 VIn K1 K2 R3 K3 R4 K4 R5 K5 R6 K6 R7 R8 K7 K8 Figure 3. Digital Input Alternative AN1775 Rev. 1 For More Information On This Product, Go to: www.freescale.com Freescale Semiconductor, Inc. Application Note Implementation Considerations Using this method, one can theoretically connect 255 input switches to one ADC pin. However, there are many potential sources of inaccuracies, which make it impractical to connect so many key inputs. One should account for some error padding. One inaccuracy is provided by the resistors themselves. Resistors are categorized according to their variance from a labeled value. The application should be tolerant to the precision of the resistors being used (typically, 1 percent or 5 percent). Also, a calculated resistor value might not be a commonly available value, so the user should plan for a range of resistor values. Typically, an 8-bit ADC is accurate within two least significant bits. This should be accounted for as well. The best way to allow for these tolerances is to assign to each key switch a range of resulting A/D data register values. If an analog input falls within a particular range, one can determine that the key was pressed. By adjusting the range of ADC results which represent a given keypress, the user can change the error margin for the application. To ensure the best ADC accuracy, the full range of the converter should be used. In cases where the high reference is not variable, it is typically fixed at the operating voltage. A spreadsheet is a good way to determine resistor values and A/D result ranges. An example of computing values for Figure 3 are shown in Table 1. Resistor Precision Freescale Semiconductor, Inc... ADC Accuracy AN1775 Rev. 1 For More Information On This Product, Go to: www.freescale.com Freescale Semiconductor, Inc. Application Note Table 1. Spreadsheet Calculations VDD = # Keys = Voltage Steps = Key pressed No Key 1 2 5 8 0.625 (VDD/# Keys) VIn 5 0 1.25 2.5 3.75 Req 0 0 3333 10000 30000 Rn 10000 0 1429 1904 2667 4000 6667 13333 40000 R0 = 10000 (Pullup) (Hex) Vinmin 4.6875 0 0.3125 0.9375 1.5625 2.1875 2.8125 3.4375 4.0625 Vinmax 5 0.3125 0.9375 1.5625 2.1875 2.8125 3.4375 4.0625 4.6875 ADDRmin EF 0 F 2F 4F 6F 8F AF CF (Hex) ADDRmax FF F 2F 4F 6F 8F AF CF EF Converter Resolution = 0.01961 (Vdd/255) 0.625 1429 1.875 6000 3.125 16667 4.375 70000 Freescale Semiconductor, Inc... 3 4 5 6 7 8 Some notes on the spreadsheet: • • • • VIn is determined by decrementing the high voltage reference by the voltage step for each segment in the ladder. (See Figure 4.) Rn is the resistor value of the current key segment needed to form desired equivalent resistance Req. Req represents the equivalent resistance of ladder, including the current resistor (Rn) and excluding the pullup resistor (R0). Vinmin and Vinmax are the minimum and maximum voltages that can be arbitrated as a particular key. In this case, VIn +/–Vstep/2 was used. ADDRmin and ADDRmax are the ADC data register value range used to represent a given keypress. This range can be narrowed or widened to affect precision. In this case, the ranges were maximized, so no conversion result is undefined. This particular spreadsheet did not use resistor precision, but this could be considered to further pad the error. • • AN1775 Rev. 1 For More Information On This Product, Go to: www.freescale.com Freescale Semiconductor, Inc. Application Note Implementation VDD R0 REQ VIn REQ(N-1) RN Figure 4. Voltage Divider Constants First, VDD (or VREFH, the high reference for the converter) and the number of keys are determined. The converter resolution is found by dividing VDD by the converter precision. The number of voltage steps needed is VDD divided by the number of key switches. An ADC should always be operated at its full range. The pullup resistor, R0, typically, should be chosen between 4.7 k (to limit current) to 22 k (to limit time constant). Once these constant values are decided, a spreadsheet can be used to determine the resistor values needed in each segment of the divider ladder, according to the desired input voltages. When using an ADC, always use the entire range of the converter (VREFH – VREFL). This is the reason for determining the voltage steps first (VDD – n*step value) and then calculating the necessary resistor values to achieve these voltages through the divider. For each segment in the ladder, the nth segment’s necessary resistor value needs to be calculated (Rn in the spreadsheet). When a key is pressed, the equivalent resistance of the included segment resistors forms a voltage divider with the pullup resistor. Given the voltage desired at the ADC input (VIn), the user can determine the equivalent resistance needed to achieve that voltage by: Req(n) = (VIn*R0)/(VREFH – VIn) The resistor that will form the needed equivalent resistance with the other resistors in the ladder can be determined, as: Rn = Req(n) – Req(n – 1) Freescale Semiconductor, Inc... Variables AN1775 Rev. 1 For More Information On This Product, Go to: www.freescale.com Freescale Semiconductor, Inc. Application Note The only exception is key 1, which connects VSS to the ADC input and needs no resistor. By assigning a range of ADC conversion values to each key, the user can provide a fair amount of error padding. Considerations include resistor tolerance, ADC accuracy, and parasitic time constants. The range of conversion values for a particular keypress can be narrowed to improve the accuracy of the application. Or, for “quick and dirty” arbitration, keep the range as wide as possible. Freescale Semiconductor, Inc... Using a Single Resistor Value The method presented here used different resistor values to produce equal voltage intervals. Another method would be to use the same resistance value for all segments in the divider. The disadvantage of using the same resistor values is that it greatly diminishes the effective range of conversions. Also, if resistances are kept equal, the voltage step between switches approaches the resolution of the ADC. Therefore, the error margin diminishes as more keys are added. There are several ways to implement such an application. One thing to choose is whether to poll the ADC when desired or link a keypress to an interrupt source. Some ADCs continuously convert once enabled, allowing a new value to be available every 32 clock cycles. Others do a single conversion when a register is written to and don’t do another conversion until the register is written to again. A polling scheme can use a periodic timing source as a signal to poll. For example, the real-time interrupt (RTI) or timer overflow (TOF) interrupt can be used to scan the ADC input at a given rate. Not linking a keypress to an interrupt source can cause timing problems and might miss a keypress. Careful consideration should be given to timing and voltage error requirements to determine if this method is appropriate. Programming Considerations AN1775 Rev. 1 For More Information On This Product, Go to: www.freescale.com Freescale Semiconductor, Inc. Application Note A Brief Example with the MC68HC705P6A MCU A Brief Example with the MC68HC705P6A MCU The small code segment example that follows illustrates the software implementation of this method of keypress arbitration. The example was defined around the spreadsheet analysis example shown in Table 1. The MC68HC705P6A (P6A) MCU features a 4-channel, 8-bit A/D converter. The P6A ADC uses continuous conversion, making a new value available every 32 internal clock cycles after being turned on. Freescale Semiconductor, Inc... The software example assumes a resistor ladder is connected to AD0 (A/D channel 0, port C, pin 6). The software is not intended to be a complete application. This software starts the ADC, selects channel 0 for A/D conversions, then polls the ADC data register to determine if a key was pressed. The software uses a lookup table, with predefined maximum and minimum ADC values which represent a specific segment in the divider being grounded. Once the key has been arbitrated, the RAM variable InKey will tell an application which key was switched most recently. AN1775 Rev. 1 For More Information On This Product, Go to: www.freescale.com Freescale Semiconductor, Inc. Application Note Code Listings * * * * * * * * * * ---------------------------------------------------------------------------------EXPANDIO.ASM Written for the MC68HC705P6A microcontroller A code segment example to illustrate the use of the A/D converter for key input arbitration This simple example polls the AD0 channel, compares the conversion result to a lookup table, and determines which of 8 keys were pressed. ---------------------------------------------------------------------------------- Freescale Semiconductor, Inc... ; Memory map equates RAMSPACE EQU ROMSPACE EQU ; A/D Registers ADC EQU ADSC EQU ; ADSC Bits CC ADRC ADON $50 $100 $1D $1E ;A/D Data register ;A/D Status and control EQU EQU EQU 7 6 5 ;Conversion complete flag ;A/D RC oscillator enable ;A/D enable bit * ---------------------------------------------------------------------------------; RAM Variables * ---------------------------------------------------------------------------------ORG RAMSPACE ;Start of user RAM InKey RMB 1 ;Identifies the last key pressed ADValue RMB 1 ;Stores the last ADC result * ; ; ; * ---------------------------------------------------------------------------------Program code Simply loops, polling the A/D converter channel AD0 and determining which key was pressed ---------------------------------------------------------------------------------ORG ROMSPACE ;Start of user ROM Start: LDA #$20 ;Turn on A/D, select AD0 channel STA ADSC MainLoop: BRCLR LDA STA CLR CLRX CC,ADSC,* ADC ADValue InKey ;Wait for conversion complete ;Get the result ;Record the result ;Clear the InKey variable ;Clear the offset AN1775 Rev. 1 For More Information On This Product, Go to: www.freescale.com Freescale Semiconductor, Inc. Application Note Code Listings Freescale Semiconductor, Inc... ;Check the entries in the table, to find the ADC value range ;that corresponds to the ADC data register value. KeyLoop: LDA KeyTable+1,X ;Check high range CMP ADValue BLS Match ;Within range LDA KeyTable,X ;Check low range CMP ADVAlue BLS Match ;Within range INCX ;Point to next table record INCX ;Increment the key value, when a match is made, ;the variable will contain the key that was pressed. INC InKey BRA KeyLoop ; At this point, InKey variable holds keypress information ; One can arbitrate the key press here. For this simple example ; we just repeat the main loop Match: BRA MainLoop * ---------------------------------------------------------------------------------; Key lookup table. Holds the minimum and maximum ADC values ; which identify a particular key in the resistor ladder * ---------------------------------------------------------------------------------KeyTable: NoKey FCB $EF,$FF ;No key pressed Key1 FCB $00,$0F Key2 FCB $0F,$2F Key3 FCB $2F,$4F Key4 FCB $4F,$6F Key5 FCB $6F,$8F Key6 FCB $8F,$AF Key7 FCB $AF,$CF Key8 FCB $CF,$EF * ---------------------------------------------------------------------------------; Vector definitions * ---------------------------------------------------------------------------------ORG $1FFE ;Reset vector FDB Start AN1775 Rev. 1 For More Information On This Product, Go to: www.freescale.com Freescale Semiconductor, Inc. Application Note REQUIRED How to Reach Us: Home Page: www.freescale.com E-mail: support@freescale.com USA/Europe or Locations Not Listed: Freescale Semiconductor Technical Information Center, CH370 1300 N. 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