TC9455,88F/N

2002-02-05

4

Operation

1.

Oscillator circuit

The TC9455F/N and TC9488F/N generate an internal system clock using the CR oscillator circuit.

Connecting a capacitance (C) of 68 pF and a resistance (R) of 5.6 kΩ sets the oscillator frequency to approx.

4 MHz and the sampling frequency to 32 kHz.

Varying the external capacitance enables the oscillator frequency and sampling frequency for the system

clock to be adjusted. Varying the system clock enables the delay time to be adjusted.

Figure 1 shows the CR oscillator circuit.

1.1

How to determine oscillator frequency (f)

f [MHz] = 1532/(C [pF] × R [kΩ])

= 1532/(68 × 5.6)

≈ 4.0 MHz (Note 2)

Note 2: The oscillator frequency is a typical value (Ta =

25°C). Set the resistance to between 4.7 kΩ and

6.8 kΩ.

1.2

How to determine sampling frequency (fs)

fs [kHz] = f/128

= 4.0/128

≈ 32 kHz

2.

Microphone amps

The IC incorporates microphone amps with auto level control (ALC).

The microphone amp gain is typically 36dB.

When a signal from 0 to −46dB is input, the ALC circuit can control the output signal level.

Changing the external capacitor connected to the ALC circuit to a resistor sets the ALC circuit to off.

Figure 2 shows the microphone amp circuit.

When C = 4.7 µF, the attack and release times are as follows:

Attack time = Approx. 29 ms

Release time = Approx. 1.6 s

Setting the capacitance (C) to a smaller value enables the

attack and release times to be adjusted. When the ALC circuit

is not used, change the capacitor (*1) to a resistor (approx. 10

kΩ), and connect to GND line.

At power on, while the capacitor is being charged, no sound is

output. So, use a capacitance of 4.7 µF or less.

Figure 1

CR Oscillator circuit

Figure 2

Microphone amp with ALC