A N 9 3
58
Rev. 1.4
4.5. Analog Output
The call progress tone provided by AOUT and discussed in this section comes from a PWM output pin on the
ISOmodem. AOUT is a 50% duty cycle, 32 kHz square wave, pulse-width modulated (PWM) by voice band audio,
such as call progress tones.
The PWM signal should be processed by a high-pass filter (R2, R3, R4, C2,C3 and C4), and, with the aid of a
bridge mode amplifier, provides low-cost 100 mW to 250 mW power with a 3 to 5 V supply. See the circuit in
Figure 24. A slightly more expensive amplifier (LM4862) is available and, while still pin-compatible, provides twice
as much power.
Figure 24. PWM Audio Processing and Amplifying Circuit
4.5.1. Interaction between the AOUT Circuit and the Required Modem Reset Time
When modifying the circuit shown in Figure 24, it is important to examine the reset timing and know that when
external reset is applied to the modem, the AOUT pin still has time to rise to VCC due to the pullup installed on it.
One has to assume that the modem has been operating prior to reset and has put AOUT into a PWM state that is
100% low.
This is important because the AOUT pin, which is shared with INT in some packages, is read by the strapping
option logic in the modem at the end of the reset time to set the operational mode as shown in "2.1.3. Reset-Strap
Options for 16-Pin SOIC Package" and "2.1.4. Reset-Strap Options for 24-Pin TSSOP Package" on page 13 and
"2.1.5. Reset Strapping Options for QFN Parts" on page 15.
The value of the capacitors and resistors in the above circuit thus has an effect on the minimum required
ISOmodem reset time.
Summary of Contents for Si2404
Page 2: ...AN93 2 Rev 1 4 ...
Page 200: ...AN93 200 Rev 1 4 Figure 31 TAM Handset and Speakerphone Voice Paths ...
Page 201: ...AN93 Rev 1 4 201 Figure 32 Si3000 Codec Gain and Signal Selection Options ...
Page 290: ...AN93 290 Rev 1 4 Figure 57 256 Band Spectral Display Figure 58 2048 Band Spectral Display ...
Page 305: ...AN93 Rev 1 4 305 Figure 76 Parallel or SPI Port Interrupt Service Flowchart ...