COMPUTE II ISSUE 2 / JUNE/JULY 1980

Some A/D And D/A Conversion Techniques

Marvin L. De Jong
Department of Mathematics-Physics
The School of the Ozarks
Pt. Lookout, MO 65726

INTRODUCTION

The purpose of this paper is to describe some A/D conversion circuits and programs that can be used with 6502 based microcomputer systems. A digital-to-analog (D/A) converter is also described. Our motivation for this project was an NSF Short Course on the Science of Sound. The complete project was to be a circuit that would sample some waveform, from an electric guitar for example, and a program that would perform a Fast Fourier Transform (FFT). The Fast Fourier Transform program has not yet been completed, but the necessary A/D circuit and driver programs have been completed and are herein described. A digital-to-analog converter allows the sampled waveform to be displayed on an oscilloscope, producing a much improved storage oscilloscope over our original “storage scope” described in THE BEST OF MICRO, Volume 1, page 30, and Volume 2, page 61. Some results of our experiments are also included.

The analog-to-digital converter is based on the AD570, an 8-bit A/D converter sold by Analog Devices, Route 1 Industrial Park, P.O. Box 280, Norwood, MA 02062. Its nominal conversion time is 25 microseconds, allowing a maximum sampling rate of 40,000 kHz. (The time necessary to read the converter and store the data will reduce this rate.) The AD570 requires no external components, and can be operated either in a bipolar or a unipolar mode. We chose it because it is inexpensive, relatively fast, and easy to interface.

The D/A converter we used is a Signetics NE5018. It is also easy to interface because it has input latches. It can be operated with few external components, but it is not an exceptionally fast converter. A better choice would have been the Analog Devices 565, but this would have required an 8-bit latch.

The circuits shown here interface to the expansion connectors on the KIM-1 or the AIM 65. With little modification they could be connected to a SYM-1. The application connector is left free for other devices. In particular, we had hoped to do our mathematics for the FFT program with an AM9511 Arithmetic Processor Unit interfaced to the I/0 ports on the application connector. In any case, Appendix A suggests a circuit for interfacing the converters to a 6522 Versatile Interface Adapter.

Description Of The Circuit

The complete A/D, D/A, and oscilloscope trigger circuitry is shown in Figure 1. This circuit was used to interface the converters to an AIM 65 microcomputer, and all the necessary connections are available at the expansion connector, including the DS9 device select pulse. The same circuit could be used with a SYM-1 if the DS18 device select pulse, available on the SYM-1 expansion connector, were used. In that case the addresses used to activate the various circuits would be $1800 through $1803. In Figure 1 you will notice that addresses $9000 through $9003 produce pulse on the Y₀ through Y₃ outputs on the 74LS138. For example, a STA $9000 instruction produces a negative one microsecond pulse on Y_o. This pulse is applied to the CLEAR input on the 74LS74 flip-flop and it will cause the Q output to go to logic zero. A logic one to logic zero transition on the B/C pin of the analog-to-digital converter (AD570) starts a conversion. Approximately 25 microseconds later the data is ready at the outputs of the AD570. These outputs are connected to an octal, three-state, buffer-driver (81LS97). A LDA $9001 instruction activates the 81LS97 and puts the data on the microcomputer's data bus. The trailing edge of the same device select pulse that enables the 81LS97 clocks the 74LS74 back into its “Q high” logic state. This completes one analog-to-digital conversion.

The analog input voltage is applied to pin 13 of the AD570. The 15 ohm resistor can be omitted if a slight loss of precision is of no concern. With the bipolar offset switch open, as shown in Figure 1, voltages between -5 V and +5 V will be converted to a binary number between $00 and $FF respectively. A binary output of $80 corresponds to pin 13 being at zero volts. If the bipolar output switch is closed, the AD570 will read voltages between 0 V and + 10 V. The AD 570 will also work with a negative supply voltage of -12 V rather than the -15 V shown in Figure 1. Although a “data ready” signal is available on the AD570, we chose to use software to wait for the conversion to be completed. One final note on the AD570: the input impedance for the analog input is only about 5 k ohm. Consequently it makes a very poor voltmeter unless a high impedance (a voltage follower circuit, for example) amplifier is placed between the analog input mode it simply provides a high impedance buffer for the AD570. The AD521 is a differential amplifier with a differential input impedance of about 3 × 10⁹ ohms. Pin 3, the - input, need not be grounded but can be connected directly to the input voltage source.

The circuits of Figures 1 - 3 provide a complete A/D and D/A system that can be used for a large variety of applications including voltage measurements, temperature measurements, and the storage scope application described here.

A/D and D/A Converter Software

The program in Example 1 was designed to work with the AIM 65 or any other system that has a 6522 VIA available for timing purposes. The addresses used to start the conversion, read the A/D converter, load the D/A converter, and trigger the oscilloscope are $9000 through $9003, respectively. If a device select other than the DS9 is used to enable the 74LS138 decoder, then these addresses must be changed accordingly. For example, if the DS18 select on the SYM-1 is used, then these addresses become $1800 through $1803, respectively. Since the KIM-1 does not have a 6522, we wrote another program that will work for it, and this program is listed in Example 2.

The program in Example 1 has a maximum sampling rate of one sample every 32 microseconds, or 31,250 Hz. It allows the AD570 exactly 28 microseconds to make a conversion if the T1 timer is loaded with $0000. If you have an AD570 that is slightly faster, try taking out the NOP instruction at $0F3A. If your AD570 is slightly slower, insert some extra NOP instructions after $0F3A. Change the various branch offsets accordingly. You can tell if you are giving the AD570 enough time by examining the data it returns.

The program in Example 1 has the following features. It continuously samples the analog voltage at the input of the A/D converter. When the conversion result exceeds a preassigned level stored in TRIG (location $0000), the program proceeds to sample another 255 points on the wave-form at a rate determined by the numbers stored in TIMLO (location $0001) and TIMHI (location $0002). The 256 data points are stored in page two of memory. Once the data have been obtained, the program proceeds to read the data out, one point at a time, to the D/A converter for the purpose of displaying the values on an oscilloscope. Each time the 256 points are output to the D/A converter, a trigger pulse is also supplied. Since the conversion time is 32 microseconds with this program, there is no use loading the T1 timer with a number less than 32 unless you wish to sample at the maximum rate. In that case, put $00 in location $0001. In the program in Example 1, T1 is in its free running mode, so its interrupt flag, IFR6, will be set every N + 1 microseconds, where N is the 16-bit number loaded into T1 from locations $0001 and $0002. Be sure to load the locations TRIG, TIMLO, and TIMHI before running the program. The program comments should explain how the program works. The first two instructions may produce a dummy conversion, but their real function is to put the 74LS74 flip-flop in a condition with Q at logic one. The program consists of three main loops. The AGN loop continuously samples the incoming data, and the program branches out of this loop to the MORE loop when the incoming voltage exceeds the trigger level. In the MORE loop another 255 points are produced. When this data has been gathered, the program branches to the OUT loop to output the 256 points to the D/A converter.

Example 1. A/D and D/A driver program for 6522 based timing.

$0F00 	8D 	00 	90 	START 	STA 	CVNST 	Pulse 74LS74 flip-flop to be in a known
 0F03 	AD 	01 	90 	 	LDA 	A/D 	condition with Q at logic one.
 0F06 	A2 	00 	 	 	LDX 	$00 	Initialize X register to zero
 0F08 	A9 	40 	 	 	LDA 	$40 	Initialize ACR of 6522 to put T1 in
 0F0A 	8D 	0B 	A0 	 	STA 	ACR 	its free-running mode.
 0F0D 	A9 	00 	 	 	LDA 	$00 	Clear accumulator.
 0F0F 	F0 	03 	 	 	BEQ 	TEST 	Branch to start the first conversion.
 0F11 	AD 	01 	90 	AGN 	LDA 	A/D 	Read A/D converter
 0F14 	8D 	00 	90 	TEST 	STA 	CVNST 	Start a conversion.
 0F17 	8D 	02 	90 	 	STA 	D/A 	Output A/D to D/A converter.
 0F1A 	C5 	00 	 	 	CMP 	TRIG 	Compare conversion result with trigger
 0F1C 	B0 	0E 	 	 	BCS 	SAMPLE 	level. Branch to sample an additional
 0F1E 	A5 	01 	 	 	LDA 	TIMLO 	255 points if A/D exceeds trigger level.
 0F20 	8D 	04 	A0 	 	STA 	TILL 	Load 6522 with the number of micro-
 0F23 	A5 	02 	 	 	LDA 	TIMHI 	seconds between conversions.
 0F25 	8D 	05 	A0 	 	STA 	T1LH 	Start timer.
 0F28 	90 	37 	 	 	BCC 	AGN 	Branch to read A/D.
 0F2A 	AD 	01 	90 	MORE 	LDA 	A/D 	Read A/D
 0F2D 	8D 	00 	90 	SAMPLE 	STA 	CVNST 	Start sampling waveform.
 0F30 	9D 	00 	02 	 	STA 	TAB,X 	Previous result into table.
 0F33 	E8 	 	 	 	INX 	 	X keeps track of the number of conversions.
 0F34 	F0 	0C 	 	 	BEQ 	OUT 	When X = 00, 256 conversions are complete.
 0F36 	AD 	04 	A0 	 	LDA 	T1CL 	Clear T1 interrupt flag.
 0F39 	EA 	 	 	 	NOP 	 	Fill in the 25 microsecond conversion
 0F3A 	EA 	 	 	 	NOP		time with no operation instructions.
 0F3B 	2C 	0D 	A0 	LOAF 	BIT 	IFR 	Has T1 timed-out?
 0F3E 	70 	E9 	 	 	BVS 	MORE 	Yes, get another conversion.
 0F40 	50 	F9 	 	 	BVC 	LOAF 	No, wait for timer.
 0F42 	8D 	03 	90 	OUT 	STA 	SCPTRG 	Trigger scope.
 0F45 	EA 	 	 	 	NOP 	 	Use an eight microsecond pulse to
 0F46 	EA 	 	 	 	NOP 	 	trigger scope.
 0F47 	8D 	03 	90 	 	STA 	SCPTRG 	
 0F4A 	BD 	00 	02 	NEXPT 	LDA 	TAB,X 	Get some data from the table.
 0F4D 	8D 	02 	90 	 	STA 	D/A 	Output it to the D/A and the scope.
 0F50 	E8 	 	 	 	INX 	 	
 0F51 	D0 	F7 	 	 	BNE 	NEXPT 	Branch to get more data.
 0F53 	F0 	ED 	 	 	BEQ 	OUT 	
$0000 = TRIG; load with desired triggering level but not $00.
$0001 = TIMLO; low-order byte of time interval between samples (microseconds).
$0002 = TIMHI; high-order byte of time interval between samples.
$0200 = TAB: base address of table to store 256 samples.
$9000 = CNVST; a STA CVNST instruction will start an A/D conversion.
$9001 = A/D; the analog-to-digital converter is read at this location.
$9002 = D/A; write to this location to perform a digital-to-analog conversion.
$9003 = SCPTRG; write to this location to trigger the oscilloscope.

The program in Example 2 works in about the same way with the same purpose in mind, but it was used on the KIM-1. The sampling rate with this program is once every 39 microseconds, or 25641 Hz. Its speed could be improved to be about the same as the program in Example 1. In any case, the on-board timers on the KIM-1 were used to produce the necessary timing. Again, the trigger level is stored in $0000, and the time is stored in $0001. The divide-by-one timer at $1704 on the KIM-1 was used, but the other timers may also be used.

Example 2. A/D and D/A driver program for a KIM-1 interface.

0300 	8D 	00 	04 	START 	STA CVNST 	Pulse 74LS74 flip-flop to be in a known
0303 	AD 	01 	04 	 	LDA A/D		condition with Q at logic one.
0306 	A2 	00 	 	 	LDX $00		Initialize X register to zero.
0308 	A9 	00 	 	 	LDA $00		Initialize accumulator to zero.
030A 	8D 	00 	04 	TEST 	STA CVNST		Start A/D conversion.
030D 	8D 	02 	04 	 	STA D/A		Previous result into D/A converter.
0310 	C5 	00 	 	 	CMP TRIG		Compare conversion result with trigger
0312 	B0 	16 	 	 	BCS SAMPLE	level. Branch to sample 256 points if
0314 	EA 	 	 	 	NOP		voltage exceeds trigger level.
0315 	EA 	 	 	 	NOP		Delay with no-operation instructions
0316 	EA 	 	 	 	NOP		until the 25 microsecond conversion
0317 	EA 	 	 	 	NOP		time is completed.
0318 	EA 	 	 	 	NOP
0319 	EA 	 	 	 	NOP
031A 	EA 	 	 	 	NOP
031B 	AD 	01 	04 	 	LDA A/D		Read A/D converter.
031E 	90 	EA			BCC TEST		Branch to start another conversion.
0320 	8D 	00 	04 	MORE 	STA CVNST		Start an A/D conversion.
0323 	9D 	00 	02 	 	STA TAB,X		Previous result into table.
0326 	E8 	 	 	 	INX		X keeps track of number of conversions.
0327 	F0 	13 	 	 	BEQ OUT		When X = 00, 256 conversions are complete.
0329 	A5 	01 	 	SAMPLE 	LDA TIME		Get time in microseconds from $0001.
032B 	8D 	04 	17 	 	STA TIMER		Store in divide-by-one timer.
032E 	EA 	 	 	 	NOP		Fill in time to make 25 microseconds
032F 	EA 	 	 	 	NOP		before reading A/D converter.
0330 	EA 	 	 	 	NOP
0331 	EA 	 	 	 	NOP
0332 	AD 	01 	04 	 	LDA A/D		Read converter.
0335 	2C 	07 	17 	LOAF 	BIT TIMER		Has timer timed out?
0338 	30 	E6 	 	 	BMI MORE		Yes, then start another conversion and
033A 	10 	F9 	 	 	BPL LOAF		store the result of the last. Otherwise
033C 	8D 	03 	04 	OUT 	STA SCPTRG	wait. Trigger the oscilloscope.
033F 	A2 	00 	 	 	LDX $00
0341 	8D 	03 	04 	 	STA SCPTRG
0344 	BD 	00 	02 	NEXPT 	LDA TAB,X		Get some data from the table.
0347 	8D 	02 	04 	 	STA D/A		Output it to the D/A and the oscillo-
034A 	E8 	 	 	 	INX		scope.
034B 	D0 	F7 	 	 	BNE NEXPT		Branch to get more data.
034D 	F0 	ED 	 	 	BEQ OUT		Return to output table again.
$0000 = TRIG; load with desired triggering level.
$0001 = TIME: load with time (in microseconds) between samples.
$0400 = CVNST; a STA CVNST instruction will start an A/D conversion.
$0401 = A/D; the analog-to-digital converter is read at this location.
$0402 = D/A; write to this location to perform a digital-to-analog conversion.
$0403 = SCPTRG; write to this location to trigger the oscilloscope.

The last program listing for the circuit in Figure 1 is a program that can be used to sample a waveform at as many points as your R/W memory will allow. Rather than use just one page of R/W memory for storing the waveform, it will use as many pages as you have available. The maximum sampling rate for this program is one sample every 43 microseconds or 23256 Hz. The program in Example 3 uses several of the same locations as the program in Example 1. The trigger level is stored in TRIG at $0000. The 16-bit number giving the number of microseconds between samples is stored in TIMLO at $0001 and TIMHI at $0002. The low-order byte of the base address of the table to store the conversion data is in location TAB at $0003. Normally this location initialized to $00. The high-order byte of the base address (page number) of the table is stored in TAB + 1 at $0004. For our experiments with the AIM 65 we used pages $02 through $0E. The page number of the last page you wish to fill with data is incremented by one and stored in location END at $0005. Thus if page $0E is the last page to be used to store data, then $0F is stored in END. Load location $0006, STARTP with the same value you put in $0004 if you wish to output all the results to the D/A for display on the oscilloscope.

The program in Example 3 samples an incoming waveform at N*256 points where N is the number of pages used to store the data. It then outputs all of these points to the D/A converter at the same rate that it sampled the waveform. If you want to output the results faster, replace the BIT IFR and BVC WAIT instructions at $0f5D with NOPs.

Example 3. N-Page A/D Conversion and Storage Program

$0F00 	8D 	00 	90 	START 	STA 	CVNST 	Pulse 7474 to be in a known condition,
 0F03 	AD 	01 	90 	 	LDA 	A/D 	with Q at logic one.
 0F06 	A0 	00 	 	 	LDY 	$00 	Initialize Y to zero for indirect indexed
 0F08 	A9 	40 	 	 	LDA 	$40 	addressing that follows.
 0F0A 	8D 	0B 	A0 	 	STA 	ACR 	Put 6522 T1 in free-running mode.
 0F0D 	A9 	00 	 	 	LDA 	$00 	Clear A.
 0F0F 	8D 	00 	90 	AGN 	STA 	CVNST 	Start a conversion.
 0F12 	8D 	02 	90 	 	STA 	D/A 	Output result to D/A converter
 0F15 	C5 	00 	 	 	CMP 	TRIG 	Compare conversion result with trigger
 0F17 	B0 	21 	 	 	BCS 	SAMPLE 	level.
 0F19 	A5 	01 	 	 	LDA 	TIMLO 	Get low-order byte of time between conversions.
 0F1B 	8D 	04 	A0 	 	STA 	TILL 	Result into T1.
 0F1E 	A5 	02 	 	 	LDA 	TIMHI 	Get high-order byte of time between
 0F20 	8D 	05 	A0 	 	STA 	T1LH 	conversions
 0F23 	AD 	01 	90 	 	LDA 	A/D 	Read A/D converter to get conversion level.
 0F26 	90 	E7 	 	 	BCC 	AGN 	Return to compare with trigger level.
 0F28 	8D 	00 	90 	DATA 	STA 	CVNST 	Start an A/D conversion.
 0F2B 	91 	03 	 	 	STA 	(TAB),Y	Result of previous conversion into table.
 0F2D 	C8 	 	 	 	INY 	 	
 0F2E 	D0 	0A 	 	 	BNE 	EQUAL 	Branch around the page number increment routine.
 0F30 	E6 	04 	 	 	INC 	TAB + 1	Increment page number
 0F32 	A5 	04 	 	 	LDA 	TAB + 1	Now compare it with the ending page
 0F34 	C5 	05 	 	 	CMP 	END 	number.
 0F36 	90 	09 	 	 	BCC 	MORE 	Fill another page.
 0F38 	B0 	14 	 	 	BCS 	NOMORE 	Table is filled, branch to output the table.
 0F3A 	EA 	 	 	SAMPLE 	NOP 	 	These NOPs equalize the time between
 0F3B 	EA 	 	 	 	NOP 	 	loading the table when no page boundary
 0F3C 	EA 	 	 	 	NOP 	 	is crossed and when a page boundary is
 0F3D 	EA 	 	 	 	NOP 	 	crossed.
 0F3E 	EA 	 	 	 	NOP 	 	
 0F3F 	A5 	05 	 	 	LDA 	TAB + 2	This is also a dummy instruction.
 0F41 	AD 	04 	A0 	MORE 	LDA 	T1CL 	Clear the T1 interrupt flag.
 0F44 	AD 	01 	90 	 	LDA 	A/D 	Read the A/D converter.
 0F47 	2C 	0D 	A0 	LOAF 	BIT 	IFR 	Has the timer timed-out?
 0F4A 	70 	DB 	 	 	BVS 	DATA 	Start another conversion.
 0F4C 	50 	F9 	 	 	BVC 	LOAF 	Wait for timer.
 0F4E 	8D 	03 	90 	NOMORE 	STA 	SCPTRG 	Trigger scope.
 0F51 	A5 	06 	 	 	LDA 	STARTP 	Reload TAB with starting page number.
 0F53 	85 	04 	 	 	STA 	TAB + 1 	
 0F55 	AD 	04 	A0 	RPT 	LDA 	T1CL 	Clear T1 interrupt flag.
 0F58 	B1 	03 	 	 	LDA 	(TAB),Y	Get data from the table.
 0F5A 	8D 	02 	90 	 	STA 	D/A 	Output it to D/A.
 0F5D 	2C 	0D 	A0 	WAIT 	BIT 	IFR 	Test T1 flag.
 0F60 	50 	FB 	 	 	BVC 	WAIT 	
 0F62 	C8 	 	 	 	INY 	 	
 0F63 	D0 	F0 	 	 	BNE 	RPT 	Get some more data for the D/A converter.
 0F65 	E6 	04 	 	 	INC 	TAB + 1 	
 0F69 	C5 	05 	 	 	CMP 	END 	
 0F6B 	90 	EA 	 	 	BCC 	RPT 	Get more data from a new page.
 0F6D 	B0 	E1 	 	 	BCS 	NOMORE 	Output the table again.

We used this program to see how the waveform from a plucked guitar string varied with time, but we couldn't help connecting a microphone to the AD521 and using the program to output this speech sound to an audio amplifier. The results are quite good, even though we made no attempt to include low-pass filters in either the input or output circuits. The word spoken into the microphone and output to an audio amplifier is intelligible and one can easily identify the person who made the sound. We had enough storage capability on the AIM 65 to store one three-syllable word. If you want a project, you might try improving the circuit and program to do a better job with speech.

Results

In Figure 4 we show a photograph of the results of sampling a 1000 Hz sine wave at a rate of about 25,000 Hz. The photograph shows 256 points on the sine wave. Since we did not have a camera for our oscilloscope, the pictures were taken through a Celestron 5” telescope, placed about 25 ft. from the oscilloscope. Figure 5 shows the scope trace expanded to show just one cycle of the same waveform in Figure 4. Figure 6 shows 256 points of a 100 Hz sine wave sampled about once every 40 microseconds, while Figure 7 shows 256 points on a 10 Hz sine wave sampled once every 2000 microseconds. Figure 8 is the waveform of the A string of an electric guitar just after being plucked. The sampling rate in this case was about one sample every 85 microseconds. Finally, in Figure 9 we show a sampled version of a 2500 Hz sine wave. Clearly the system still does a pretty good job of reconstructing a 2500 Hz sine wave, but the information in frequencies much above 5000 Hz will be lost. Hopefully these pictures are worth a thousand words.

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Appendix A. Interfacing The Converters To A 6522 VIA

The AD570 analog-to-digital converter and the NE5018 converter can easily be interfaced to a 6522, eliminating the need for most of the control logic shown in Figure 1. AIM 65 and SYM-1 users may wish to use the 6522 accessed at the application connector and the circuit shown in Figure 10. Note that only the data and control connections are shown in Figure 10. The other circuitry, mainly a few resistors and capacitors, can be found in Figure 1, as are the necessary power connections. This circuit eliminates the 74LS138, the 74LS74, the 81LS97, and the various NAND, NOR, and INVERTER chips. The CA2 pin on the 6522 could be used as an output to trigger the oscilloscope. Below find a short assembly language program that will collect 256 conversions and store them. This program has not been tried.

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