AN2583 Datasheet, PDF (1/18 Pages) STMicroelectronics – M41T00AUD real-time clock with audio

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 Ω load (with V_CC at 3.3 V).
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.
December 2007
Rev 1
1/18
www.st.com