LM1971

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SNAS104B – FEBRUARY 1995 – REVISED APRIL 2013

APPLICATION INFORMATION

SERIAL DATA FORMAT

The LM1971 uses a 3-wire serial communication format that is easily controlled by a microcontroller. The timing

for the 3-wire set, comprised of DATA, CLOCK, and LOAD is shown in Figure 15. As depicted in Figure 15, the

LOAD line is to go low at least 150 ns before the rising edge of the first clock pulse and is to remain low

throughout the transmission of the 16 data bits. The serial data is composed of an 8-bit address, which must

always be set to 0000 0000 to select the single audio channel, and 8 bits for attenuation setting. For both

address data and attenuation setting data, the MSB is sent first with the address data preceding the attenuation

data. Please refer to Figure 16 to confirm the serial data format transfer process.

Table 1 shows the various Address and Data byte values for different attenuation settings. Note that Address

bytes other than 0000 0000 are ignored.

μPOT SYSTEM ARCHITECTURE

The

μPot's digital interface is essentially a shift register where serial data is shifted in, latched, and then

decoded. Once new data is shifted in, the LOAD line goes high, latching in the new data. The data is then

decoded and the appropriate switch is activated to set the desired attenuation level. This process is continued

each and every time an attenuation change is made. When the µPot is powered up, it is placed into the Mute

mode.

μPOT DIGITAL COMPATIBILITY

The

μPot's digital interface section is compatible with TTL or CMOS logic. The shift register inputs act upon a

threshold of two diode drops above the ground level (Pin 3) or approximately 1.4V.

Table 1. Attenuator Register Set Description

Address Register (Byte 0)

MSB LSB

A7–A0

0000 0000

Channel 1

0000 0001

Ignored

0000 0010

Ignored

Data Register (Byte 1)

Contents

Attenuation (dB)

MSB LSB

D7–D0

0000 0000

0.0

0000 0001

1.0

0000 0010

2.0

0000 0011

3.0

:::::

::

0001 0000

16.0

0001 0001

17.0

0001 0010

18.0

0001 0011

19.0

:::::

::

0011 1101

61.0

0011 1110

62.0

0011 1111

96 (Mute)

0100 0000

96 (Mute)

:::::

::

1111 1110

96 (Mute)

1111 1111

96 (Mute)

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