December 2007

Rev 1

1/18

AN2583

Application note

Understanding and applying the

M41T00AUD real-time clock with audio

Introduction

Drawing on the fact that the squarewave generator built in to many ST serial RTCs can be

used as a tone generator, users have been employing ST real-time clocks in printer-fax

scanner units for several years. In these applications, the squarewave output is used to

create the audible beeps these all-in-one (AIO) machines make when a key is pressed or an

error occurs.

The beep signals drive an amplifier circuit which in turn drives a speaker. Furthermore, other

tone sources from within the AIO also go to the speaker, so they are summed into the

amplifier along with the squarewave signal. One of these sources is the phone line. It is

audible while the fax is dialing and negotiating the connection.

Additionally, it is necessary to control the gain of the amplifier and to limit the bandwidth of

the audio signal, so some form of volume control and filtering is required.

As a result, ST developed the M41T00AUD which combines a real-time clock with an audio

section. Referring to Figure 1 on page 2, the clock is based on ST's M41T00 and includes

several enhancements such as a trickle charge circuit for charging backup capacitors and a

voltage reference for precisely controlling the backup switchover threshold.

The audio section starts with an input amplifier for summing multiple signals of varying

levels. Next is an 8 kHz low-pass filter, then a 16-step, digitally controlled gain stage with

3 dB steps. Finally, the output section is a bridged amplifier capable of driving 300 mW into

an 8

Thus, the M41T00AUD is a highly integrated circuit perfect for All-In-One printer

applications, as well as other consumer devices, where high functionality and low cost are

key requirements.

www.st.com