e011070.PDF

GENERAL INTEREST

MIDI to Voltage

Converter

with 16 independent voltage sources

Design by J. Scherpenisse

Although MIDI (Musical Instrument Digital Interface) was originally

developed for controlling musical instruments, it can also be used for

numerous other purposes. In the circuit presented here, MIDI signals

are used to set a number of DC voltages. Up to 16 output voltages can

be independently adjusted.

Elektor Electronics

1/2001

GENERAL INTEREST

One PIC

with many converters

The circuit is built around a Microchip

PIC16F84 microcontroller and 1 to 16 Linear

Technology LTC1452 D/A converters (see Fig-

ure 1 ). There is thus a separate D/A converter

for each output, instead of a single D/A con-

verter multiplexed over several outputs. The

major advantage of this approach is that it

dispenses with the multiplexer and sample &

Nowadays, every computer has a

sound card, and the sound card

always has a MIDI interface. How-

ever, in most cases the MIDI inter-

face is not used. Most people control

their musical instruments using

MIDI commands (if they actually

have any musical instruments…).

A microprocessor forms an excellent

starting point for building a circuit

that responds to MIDI commands. In

this case, the author has developed

a circuit that allows the values of

several DC voltages to be set in a

simple manner. These could be used

for measurement applications, for

example. From 1 to 16 channels are

possible, with each channel having

its own D/A converter and an

adjustable output voltage ranging

from 0 to 5 V.

CLK

IC5

IC20

DIN

CLK

VOUT

REF

DOUT

CLK

DIN

DOUT

REF

VOUT

LTC1452

220 Ω

IC2

IC6

IC19

DIN

CLK

VOUT

REF

DOUT

1N1418

CLK

DOUT

REF

VOUT

DIN

6N138

LTC1452

IC7

IC18

DIN

CLK

VOUT

REF

DOUT

CLK

DIN

DOUT

REF

VOUT

LTC1452

4x 10k

IC8

IC17

27p

DIN

CLK

VOUT

REF

DOUT

2345

CLK

MCLR

RB0

RB1

RB2

RB3

RB4

RB5

RB6

RB7

DOUT

REF

VOUT

DIN

015

013

011

014

012

010

IC1

CLK

LTC1452

RA0

RA1

PIC16

RA2

RA3

IC9

IC16

F84

RA4

DIN

CLK

VOUT

REF

DOUT

OSC2

OSC1

CLK

DOUT

REF

VOUT

DIN

LTC1452

4µ7

16V

8765

1234

27p

IC10

IC15

10MHz

DIN

CLK

VOUT

REF

DOUT

CLK

DIN

DOUT

REF

VOUT

LTC1452

IC4

REF

IC11

IC14

TL061

DIN

CLK

VOUT

REF

DOUT

CLK

DOUT

REF

VOUT

DIN

LTC1452

IC3

1N4001

IC12

IC13

7 805

DIN

CLK

VOUT

REF

DOUT

9...12V

18mA

CLK

DIN

DOUT

REF

VOUT

REF

470µ

25V

10µ

16V

LTC1452

000158 - 11

Figure 1. The core of the circuit consists of a PIC16F84 and up to 16 D/A converters.

1/2001

Elektor Electronics

GENERAL INTEREST

the last (16 th ) converter are sent out

first and the data for the first con-

verter are sent out last, it is possible

to omit one or more converters with-

out making any changes to the

remaining hardware or software.

The LTC1452 converter has a resolu-

tion of 12 bits, of which only 7 bits

are used here. This results in 128

steps, which means that the step

size is 39 mV when the maximum

output voltage is 5 V. Standard MIDI

controller values have seven data

bits. However, it is possible to use

14-bit controllers with MIDI; this

requires two controller addresses,

hold circuits. The latter, in particular, are

rather sensitive with regard to correct timing

of the sample and hold signals.

The LTC1452 has a serial shift register at its

input, which is followed by a latch register

that is coupled to the actual converter. The

shift register also has a serial output, so sev-

eral converter ICs can simply be connected in

series.

With a supply voltage of +5 V, the LTC1452

output can directly deliver a voltage between

0 and 5 V. For higher output voltages or heavy

loads, a buffer must be added.

The interrupt routine in the processor scans

the MIDI input signal to see if any ‘control-

change’ messages with controller numbers

between 48 and 64 (for example)

appear on channel 1. If such a mes-

sage is found, the ‘value’ parameter

(the third byte of the message) is

stored in a table. In the time

between interrupts, the main pro-

gram loop continuously sends the

contents of this table, bit by bit, to

the converter ICs via output port B1

of the processor. Following each data

bit, a shift pulse is generated on port

B2. After all data bits for the 16 con-

verters have been output, a latch

pulse is sent via B2. This transfers

the data in all 16 converters to their

internal latches. Since the data for

MIDI in brief

For true connoisseurs of music electronics, no further explanation of MIDI is necessary.

However, a brief explanation of the structure and function of the control-change messages

is in order for readers who are not fully familiar with MIDI.

MIDI (Musical Instrument Digital Interface) is a protocol that transfers data from a trans-

mitter (MIDI out) to a receiver (MIDI in) via a serial connection. In principle, MIDI is a sys-

tem for unidirectional data traffic. Due to their musical (synthesiser) origins, these signals

were designed as messages that enable or disable a musical note, change a synthesiser set-

ting, modify a volume level or apply a frequency shift to all notes of a synthesiser (‘pitch

blend’).

A MIDI message is usually made up of three 8-bit bytes. The ﬁrst byte of each message

(the status byte) indicates the message type and the MIDI channel for which the message is

intended. There are 16 MIDI channels, so several synthesisers or several sounds within a

single synthesiser can be separately controlled by a single computer using a single MIDI

connection.

One group of messages is called ‘control change’. These messages regulate controllers

within a device (such as a synthesiser) for functions such as pitch blend, volume and porta-

mento. When other types of devices (such as effects generators and reverb or delay units)

are controlled via MIDI, control change messages can be used to modify delay times or

reverb settings. A number of control change messages are predeﬁned for a large number of

synthesisers and accessory equipment.

The second byte of a control change message can address up to 127 controllers (the

MSB of the second byte is always ‘0’). The third byte of the message contains the value that

is sent to the controller in question. Here again the values range between 0 and 127, since

the MSB of the third byte is also ‘0’.

Only the ﬁrst byte of a message has a ‘1’ in the MSB position. This means that the status

byte can always be distinguished from the following bytes.

COMPONENTS LIST

Resistors:

R1,R3 = SIL array 4 x 10k

Ω

R2 = 33k Ω

R4 = 470

Ω

R5 = 220 Ω

R6,R7 = 10k

Ω

Capacitors:

C1 = 4

F7 16V radial

C2,C3 = 27pF

C4,C7-C10 = 100nF

C5 = 470 µ F 25V radial

C6 = 10

F 16V radial

Semiconductors:

D1 = 1N4148

D2 = 1N4001

IC1 = programmed PIC16F84-

10/p, order code 000158-41 ,

see Readers Services page

IC2 = 6N138

IC3 = 7805

IC4 = TL061

IC5-IC20 = LTC1452CN8

The control change messages used in this circuit thus appear as follows:

10110000

0ccccccc

0vvvvvvv

Miscellaneous:

K1 = 5-way DIN socket, PCB

mount, receptacles at 180 °

K2 = 20-way boxheader, angled

pins

K3 = mains adapter socket, PCB

mount

S1,S2 = 4-way DIP switch

X1 = 10MHz quartz crystal

Disk, contains source code and hex

code ﬁles, order code 000158-

11 , see Readers Services page

0 0 0 0 = channel 1

0 1 1 = control change

1 = status byte

controller number

controller value

000158 - 12

A MIDI message is always transmitted at a bit rate of 31.25 kbit/s, so one bit takes 32 µs.

Each byte is preceded by a start bit (always ‘0’) and followed by a stop bit (always ‘1’).

The idle value between bytes is also ‘1’.

A 3-byte message thus contains 3 × 10 = 30 bits and thus takes 30 × 32 µs to send

(approximately 1 ms in total).

In terms of hardware, MIDI connections are always current loops, with optocouplers

used in the receiver to avoid ground loops.

Elektor Electronics

1/2001

GENERAL INTEREST

Program structure

The processor used in this circuit comes from the Microchip Inc. PIC series ( http://www.microchipinc.com ).

The PIC16F84 has 1 k words of 14-bit ﬂash program memory, 68 bytes of data memory, 1 timer, 13 I/O ports and several interrupt

options. The package has only 18 pins.

The software for the PIC16F84 is written in the CVSAM language from TechTools. This assembler language can actually be regarded as

a collection of macros that translate single source code statements into one, two, three or sometimes even four true processor instruc-

tions. The new instructions that arise in this manner closely resemble standard instructions for ‘big’ microprocessors and are very easy to

use. The actual instruction set of the PIC processors is also understood and correctly assembled by the CVSAM. In some cases, using the

original instruction set is slightly more efficient. This can be a consideration with real-time applications and in case of interrupts that must

be handled as quickly as possible. On account of the readability of programs written in CVSAM, the author has chosen this language in

preference to the Microchip assembler (MPASM).

The optocoupler is connected to port B0, which is the standard interrupt pin. The start bit of each received byte thus triggers an interrupt.

The interrupt routine polls port B0 two times in rapid succession, in order to eliminate any noise pulses that may be present. If the start bit is

recognised, the port B0 interrupt is disabled and a new interrupt is enabled, namely that of the timer. The timer is then started, so that an

interrupt occurs again after approximately 32 µs. In the subsequent routine, the value at the B0 input is stored and the timer is restarted. In

this manner, the eight bits of the byte are read in. After this, the B0 interrupt is re-enabled, to allow the following byte to be received.

After each byte has been received, a test is made to see what type of byte it is. If it is status byte for a control change message, a switch

is set to cause the following byte to be interpreted as a controller number. Following that byte, the switch is set to yet another value, so

that the byte following the controller number byte will be seen as a controller value. Should a status byte be received at an incorrect time

(when a data byte is expected), the bytes received up to that point are ignored and the cycle starts again from the beginning.

Once all data for a controller have been received, the table is updated with the current values.

As long as no interrupt routine is active, the main program loop looks after sending the data in the table to the D/A converters. The

cycle time of the main loop is approximately 620 µs.

IC5

IC20

IC2

IC6

IC19

IC7

IC18

IC8

IC17

IC1

IC9

IC16

IC10

IC15

IC4

IC11

IC14

IC12

IC13

Figure 2. The single-sided printed circuit board for this circuit holds all components and connectors.

1/2001

Elektor Electronics

GENERAL INTEREST

and thus two control-change messages. This

capability is not (yet) implemented. Conse-

quently, in this version only seven bits are

sent for each DAC, and they must be padded

with ﬁve null bits and ﬁve extra shift pulses

per converter.

The reference voltages for the converters are

set to a value of 2.5 V, using the voltage

divider R7/R6 and the buffer IC4. This yields a

maximum output voltage of 5 V. The output

signals from all the converters are available

on connector K1.

Four DIP switches are connected to port A of

the processor. These can be used to set the

controller numbers. The binary value of port

A (four bits, with A0 = LSB) is shifted three

bits to the left when the supply voltage is

switched on. This yields steps of 8, thus 0, 8,

16, …, 120. This value is then used as the

starting value for the controller numbers to

which the circuit will respond. If all switches

are closed, the circuit starts with controller

numbers beginning at zero. If the switch for

RA0 is open (‘1’), the controller numbers

begin with 8.

The MIDI channel number can be set using

the DIP switches connected to ports

RB4–RB7. This port is also read on power-up.

If all four switches are closed (‘0000’), MIDI

channel 1 is selected. When all four

switches are open (‘1111’), MIDI

channel 16 is selected.

The MIDI signal is fed to port RB0 of

the PIC via DIN socket K1. Optocou-

pler IC2 provides electrical isolation

between the MIDI source and the

circuit, as is common with MIDI

equipment.

A 10-MHz crystal with two capaci-

tors is provided to generate the clock

signal for the PIC.

There is also a 5-V regulator on the

circuit board, so a simple unregu-

lated mains adapter that provides

9–12 V (20 mA max.) can be used to

power the circuit.

of wire bridges, especially around

the converter ICs. Mount as many

converters as you need, starting

with the ﬁrst converter (IC5).

A programmed PIC for this circuit is

available from Readers Services

under order number 000158-41. If

you want to program your own

16F84, you can download the source

code ﬁle from the Elektor Electronics

Internet site ( http://www.elektor-

electronics.co.uk ), but it is also avail-

able on a floppy disk under order

number 000158-11 for those few

readers left with no access to the

Internet.

Other than this, anyone with a bit of

soldering experience should not

have any trouble assembling the cir-

cuit. All connectors are located at

one edge of the board, so it can eas-

ily be ﬁtted into a suitable enclosure.

A standard MIDI interconnect cable

can be used to connect the circuit to

the equipment that supplies the con-

trol data.

Construction

The printed circuit board layout

shown in Figure 2 has been devel-

oped for this circuit. The board is not

available from Readers Services, but

since the layout is single sided, it

should not be too difficult for you to

make the board yourself. When

mounting the components, pay

attention to the fairly large number

(000158-1)

Plik z chomika:

Inne pliki z tego folderu:

Inne foldery tego chomika: