LM1972

SNAS094D – APRIL 1995 – REVISED MARCH 2013

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The mute function is obtained during power-up of the device or by sending any binary data of 01111111 and

above (to 11111111) serially to the device. The device may be placed into mute from a previous attenuation

setting by sending any of the above data. This allows the designer to place a mute button onto his system which

could cause a microcontroller to send the appropriate data to a

μPot and thus mute any or all channels. Since

this function is achieved through software, the designer has a great amount of flexibility in configuring the

system.

DC INPUTS

Although the

μPot was designed to be used as an attenuator for signals within the audio spectrum, the device is

capable of tracking an input DC voltage. The device will track DC voltages to a diode drop above each supply

rail.

One point to remember about DC tracking is that with a buffer at the output of the

μPot, the resolution of DC

tracking will depend upon the gain configuration of that output buffer and its supply voltage. It should also be

remembered that the output buffer's supply voltage does not have to be the same as the

μPot's supply voltage.

This could allow for more resolution when DC tracking.

SERIAL DATA FORMAT

The LM1972 uses a 3-wire serial communication format that is easily controlled by a microcontroller. The timing

for the 3-wire set, comprised of DATA-IN, CLOCK, and LOAD/SHIFT is shown in Figure 3. Figure 22 exhibits in

block diagram form how the digital interface controls the tap switches which select the appropriate attenuation

level. As depicted in Figure 3, the LOAD/SHIFT 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 each set of 16 data bits. The serial data is

comprised of 8 bits for channel selection and 8 bits for attenuation setting. For both address data and attenuation

setting data, the MSB is sent first and the 8 bits of address data are to be sent before the 8 bits of attenuation

data. Please refer to Figure 20 to confirm the serial data format transfer process.

Table 1. LM1972 Micropot Attenuator

Register Set Description

MSB:

LSB

Address Register (Byte 0)

0000 0000

Channel 1

0000 0001

Channel 2

0000 0010

Channel 3

Data Register (Byte 1)

Contents

Attenuation Level dB

0000 0000

0.0

0000 0001

0.5

0000 0010

1.0

0000 0011

1.5

: : : : :

: :

0001 1110

15.0

0001 1111

15.5

0010 0000

16.0

0010 0001

16.5

0010 0010

17.0

: : : : :

: :

0101 1110

47.0

0101 1111

47.5

0110 0000

48.0

0110 0001

49.0

0110 0010

50.0

: : : : :

: :

10

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