CS48L32 datasheet(35/192 Pages) CIRRUS

35 / 192 page

DS1219F4

Cirrus Logic

CS48L32

4.2 Input Signal Path

The signal-detection algorithm is illustrated in Fig. 4-12.

Figure 4-12. Ultrasonic Signal Detection

The ultrasonic demodulator function is enabled by setting USn_EN. The frequency band and signal gain are selected using

USn_FREQ and USn_GAIN respectively.

The output from the ultrasonic demodulator is a frequency-modulated image of the selected input frequency range. The

folding frequency that characterizes the frequency modulation is set according to the USn_FREQ setting—see Table 4-6.

The relationship between input and output frequencies is described in Eq. 4-1.

Equation 4-1. Ultrasonic Demodulator Characteristic

Note that, depending on the input frequency range and the folding frequency, FFOLD, the modulated output in respect of

certain input frequencies may overlap others. This effect arises if the folding frequency lies within the input frequency

range, with the result that two different input frequencies will each be modulated to the same output frequency. This effect

is limited to the outer edges of the input frequency range in all cases. Amplitude response across the input frequency range

is flat to within 1.5 dB in all cases.

The demodulated ultrasonic outputs can be selected as input to the digital mixers or signal-processing functions within the

digital core by setting the respective x_SRCn fields as described in Section 4.3.1.

The sample rate for the demodulated ultrasonic output is configured using USn_RATE—see Table 4-21. The selected

sample rate must be one of the SYSCLK-related rates, and must be equal to the output rate set by USn_FREQ (see

Table 4-6). Note that sample-rate conversion is required when routing the ultrasonic signals to any signal chain that is

configured for a different sample rate.

The characteristics associated with the USn_FREQ field setting are shown in Table 4-6.

Table 4-6. Ultrasonic Frequency Control

Condition

Input Frequency Band

Output Sample Rate

Demodulator Folding

Frequency (FFOLD)

Signal-Detection Rate

USn_FREQ = 010

16–24 kHz

16 kHz

80.8 kHz

USn_FREQ = 011

20–28 kHz

16 kHz

20.21 kHz

96 kHz

Threshold detect

Accumulator count

Signal detection

Accumulator value increases

if threshold detection is valid

Accumulator value is

unchanged during hold time

Accumulator value falls

after hold time expires

Status indication when accumulator reaches

signal-detect level. Interrupt on rising edge.

Signal level

measurements

Hold Time

USn_DET_HOLD

Signal-detect level

USn_DET_NUM

Decay Time

USn_DET_DCY

FOUT

FIN FFOLD

–

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