Analog Voltage Input

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

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

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

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 VREF is a reference voltage pin for converting voltages between AVss and

VREF 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

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,

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)

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 */

} }

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

• 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-low-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