Analog Voltage Input

h t t p : / / r e s o u r c e . r e n e s a s . c o m Page 114 Chapter 9 Analog Voltage Input Although parallel data described in the previous chapter are used for inputting and outputting digital voltages, this chapter explains the A/D converter used for inputting analog voltages. A/D conversion is used for inputting from temperature sensors. In this chapter, you should fully understand the methods for checking and clearing the status flag (ADF) when A/D conversion has been completed or in other cases since they are frequently used. Note: The following is a negative logic signal: 9.1 A/D Converter Configuration In a microcomputer-applied system, processing may require handling of information input as analog voltages by an analog sensor (sensor which generates analog voltages according to physical values such as temperature and pressure). Since the microcomputer, however, is composed of digital circuits, it is incapable of handling analog voltages as they are. Analog voltages, therefore, must be converted into numeric data of several bits before processing. This conversion from analog to digital is accomplished by the A/D converter, which is a peripheral function designed to convert analog voltages externally input into digital numeric data. The H8/3048 A/D converter employs the successive comparison method and has the following characteristics: Input voltage range 0V to 5V (range of analog input voltages which can be converted into digital numeric data.) You can use desired voltages between 0V as the minimum and 0V to 5V as the maximum. You can obtain high-precision A/D conversion results, however, by setting the maximum voltage as high as possible. Resolution 10 bits (refers to how many bits of digital numeric data voltages are to be converted into.) 10-bit resolution means that you can obtain conversion results in two to the tenth power (1024) steps. If you do not need so many steps, use the required bit count from the most significant bit from the 10 bits. For example, you can obtain conversion results h t t p : / / r e s o u r c e . r e n e s a s . c o m Page 115 in two to the eighth power (256) by using the upper 8 bits or to the fifth power (32) by using the upper 5 bits. Conversion time 266 or 134 states (time required for conversion.) For 134-state conversion (shorter conversion time), one conversion takes 6.7 microseconds in the case of 20MHz operation. In this case, the sampling measurement of signals having a period of 6.7 microseconds x 2 = 13.4 microseconds or longer (frequency of 75kHz or lower) is possible based on the sampling theorem. Since the A/D converter is a circuit for handling analog electrical signals, noise appears as conversion errors. If you want to suppress conversion errors, take sufficient measures against them. Figure 9.1: A/D Converter Block Diagram The following explains the A/D converter configuration in the H8/3048. It has 12 external input pins. AN0 to AN7 are designed to input analog voltages to be A/D converted. These 8 input pins are switched to perform A/D conversion one by one. AVcc is a power supply pin and AVss is a ground pin. Since they are separated from other power supply or ground pins, the A/D h t t p : / / r e s o u r c e . r e n e s a s . c o m Page 116 converter will not function unless power is supplied to them. If you want to suppress conversion errors, sufficient measures are also required in this case, too. V REF is a reference voltage pin for converting voltages between AVss and V REF into 10 bits (1024 steps). A/D conversion is started after the trailing edge has externally been input to the ADTRG pin. A/D conversion can also be started by an instruction. The analog voltage input in the selected analog input pin is converted into 10-bit binary data using the successive comparison method and stored in the AD data registers (ADDRA to ADDRD). There are four 16-bit A/D data registers from A to D and conversion results are stored in one of them depending on which analog input pin has been selected. The A/D conversion results are read using the MOV instruction. The A/D converter has two more registers for controlling other settings such as the operating mode, which are described in the following section. 9.2 A/D Converter Registers Table 9.1 shows the A/D converter register configuration. Table 9.1: A/D Converter Register Configuration Each register is described below. (1) A/D control status register (ADCSR) Figure 9.2 shows the A/D control status register (ADCSR), which selects the channel to be A/D converted, instructs the start of A/D conversion and judges its end. h t t p : / / r e s o u r c e . r e n e s a s . c o m Page 117 Figure 9.2: A/D Control Status Register (ADCSR) Although all 8 bits of this register are capable of reading, indicated by "R" under each bit, certain conditions apply to one of them regarding writing, indicated by "W" in parentheses with "*" attached. The bit marked "W" in parentheses with "*" attached is generally called the "status flag", which requires some precautions. The status flag bit also exists in other internal I/O registers, to which common precautions apply. As for this flag, press the link button shown above ("How to use the status flag") to completely master the use before proceeding. (2) A/D control register (ADCR) Figure 9.3 shows the A/D control register (ADCR), which has only one significant bit (bit 7). If this trigger enable bit (TRGE) is not changed from its default value of 0, A/D conversion will not be started even if the trailing edge is input to the ADTRG pin. In this case, conversion can be started by instruction only (setting the ADST of the ADCSR to 1). If the TRGE is set to 1, on the other hand, A/D conversion can be started by inputting the trailing edge to the ADTRG pin. In this case, however, A/D conversion can also be started by instruction. h t t p : / / r e s o u r c e . r e n e s a s . c o m Page 118 Figure 9.3: A/D Control Register (ADCR) Note: Do not set bit 0 of the ADCR to 1. (3) A/D data registers A to D (ADDRA to ADDRD) The 16-bit A/D data registers are designed to store A/D conversion results and located at two consecutive addresses in the memory. Figure 9.4 shows the A/D data register A (ADDRA) as an example. Although the A/D data registers B (ADDRB) to D (ADDRD) have different analog input pins and addresses, the use is completely the same. Although the ADDRA to ADDRD are 16-bit registers, conversion results are stored in the upper 10 bits. To use only the upper 8 bits of the conversion results, read them in byte-size units. To use all 10 bits of the conversion results, read them in word-size units and handle them by shifting them to the right by 6 bits or by other means. Figure 9.4: A/D Data Register A (ADDRA) 9.3 Sample Use of A/D Converter This section shows a program using an A/D converter, which is assumed to operate under the following conditions: • The AN0 analog voltage is A/D converted in single mode (results are stored in the ADDRA). • The conversion time is 134 states. • The ADTRG pin is not used. • The A/D conversion complete interrupt is not used. • The main routine obtains A/D conversion results in 8-bit units to store them in R1H and calls the OUTPUT subroutine. Then, the above operation is repeated (the contents of the OUTPUT subroutine are not shown). h t t p : / / r e s o u r c e . r e n e s a s . c o m Page 119 C Language Sample #include "iodefine.h" /* include I/O address define header file */ /* define variavle */ unsigned char data ; /* stack area size */ #pragma stacksize 0x100 /* Power ON Reset function */ __entry(vect=0) void main(void) { AD.CSR.BYTE = 0x08 ; /* single mode , 134 clock , not use interrupt */ while(1) { /* endless loop */ AD.CSR.BIT.ADST = 1 ; /* start A/D convert */ while(AD.CSR.BIT.ADF == 0) ; /* wait A/D convert end */ AD.CSR.BIT.ADF = 0 ; /* clear ADF */ data = AD.DRA >> 8 ; /* data <- ADDRA */ } } 1. You want to convert the voltage externally input into 9-bit (512-step) digital numeric values using the A/D converter. What do you do? (A) Since the resolution of the A/D converter is fixed at 10 bits, 9-bit conversion results cannot be obtained. (B) Use the ADCSR to change the resolution setting from the default of 10 bits to 9 bits. (C) Use the upper 9 bits from the 10-bit A/D conversion results. h t t p : / / r e s o u r c e . r e n e s a s . c o m Page 120 Answer: (C) When you need a resolution lower than 10 bits, you can simply use the required bit count from the most significant bit from the conversion results. (B) This setting is not available in the H8/3048. 2. How many methods can you use to start conversion by the A/D converter? (A) Only one method to externally input a signal. (B) Only one method to use an instruction. (C) Two methods to externally input a signal and use an instruction. Answer: (C) The H8/3048 is capable of starting conversion with two methods. 3. Where are the A/D conversion results of analog voltages input to the AN3 pin stored? Answer: ADDRD Refer to "Figure 9.1: A/D Converter Block Diagram" in 9.1 to understand the relationship between the analog input pin and the ADDR. 4. How is the ADF changed after the first A/D conversion regarding the selected input channel is completed? Answer: The ADF is changed to 1 The ADF flag is designed to indicate that the first A/D conversion has been completed. 5. You want to obtain A/D conversion results regarding four input channels from AN0 to AN3 in scan mode. How do you set the lower 3 bits of the ADCSR? Answer: B'011 Understand how the ADCSR is used by referring to "Figure 9.2: A/D Control Status Register (ADCSR)" in 9.2. <A/D converters> Write a program to use an A/D converter as you have learned in Chapter 9 and run it on the training board. Work out through the following steps: • Complete the exercise source program by filling out its blanks. • Make sure that the program runs successfully on the training board. h t t p : / / r e s o u r c e . r e n e s a s . c o m Page 121 • If the program will not run as specified in the exercise, consult the sample answer and make necessary changes to it before rerunning it. A trimmer is connected to the AN4 pin of the training board. Fully turn the trimmer clockwise to input the VREF voltage to the AN4 pin, or fully turn it counterclockwise to input the AVss voltage to the AN4 pin. Further, eight LEDs are connected by way of the 74LV574 placed at address H'FFFF12. The following correspondence exists between the value written to each bit of address H'FFFF12 and the resulting on/off status of the LED: 0 . LED on 1 . LED off SW28 is connected to the ADTRG pin. Press SW28 to input a low- level voltage to the ADTRG pin; release it to input a high-level voltage to the ADTRG pin. Convert the analog voltage input to the AN4 pin from analog to digital at a 134-state conversion speed in scan mode and display the conversion result as an 8-bit binary value on the eight LEDs. As you turn the trimmer to vary the analog input voltage, the LED display should vary accordingly. If you have successfully written a program that meets the requirements defined above, modify it this time so it will carry out a single run A/D conversion each time SW28 is pressed and display the conversion result as an 8-bit binary value on the eight LEDs. Even when you turn the trimmer, the LED display will not change unless you press SW28. • Configure the A/D converter for scan mode, 134-state, and AN4 selection before starting the conversion process. • The program should loop by waiting until each run of A/D conversion completes, or ADF equals 1, and reading the conversion result for output on the LEDs. • Write the display data resulting from the conversion process to address H'FFFF12. Allowing for the correspondence between the value of 0 or 1 written to address H'FFFF12 and the resulting on/off status of the LED, it is necessary to invert the conversion result and write it to address H'FFFF12. h t t p : / / r e s o u r c e . r e n e s a s . c o m Page 122 . Press SW28 to input a low- level voltage to the ADTRG pin; release it to input a high-level voltage to the ADTRG pin. Convert the analog voltage input to the. characteristics: Input voltage range 0V to 5V (range of analog input voltages which can be converted into digital numeric data.) You can use desired voltages between