PCM1861 datasheet(24/115 Pages) TI

24 / 115 page

PCM1861, PCM1863, PCM1865

SLAS831C – MARCH 2014 – REVISED AUGUST 2014

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8.10 Zero-Crossing Detect

The PCM186x uses a zero crossing detector to make gain changes only when the incoming signal crosses its

halfway point between negative and positive swing, reducing "zipper noise".

There are two sources for the controller, the output of the ADC Modulator and the output from the digital PGA.

The Analog PGA is sampled at 4× the audio sampling rate to detect the zero crossing. The digital PGA is

sampled at a similar rate.

The process for changing gain in the PCM186x is as follows:

1. Detect a zero crossing of the oversampled analog input channel.

2. Increment or Decrement the gain toward the target PGA value step by 0.5dB.

3. Repeat from (1) until arrival at the target PGA value.

4. If zero crossing does not occur for 8192 sample times (= time out), change the gain per sample.

This process does not require intervention by the user. This data serves as information only.

8.11 Digital Inputs

8.11.1 Stereo PCM Sources

The PCM186x can support Stereo PCM data on GPIO pins so that I2S sources, such as wireless modules can

have their data mixed with the incoming analog content. The clock rate of the incoming data (known as DIN)

must be synchronous with the PCM186x software programmable device main clocks. There is no integrated

sample rate converter on-chip. The DIN signal can be received on GPIO0,1,2,3. configured on GPIO_FUNC_X

(Page.0 0x10 and 0x11). The incoming data is then driven to the digital mixer running on DSP2.

The audio format can be configured separately from the output serial port using register RX_TDM_OFFSET (P0,

0x0E).

Inputs can be mixed and volume-controlled before routing to a digital amplifier. Typical uses could be the

connection to a Bluetooth module. The mixing and crossfading could be done all in the PCM186x, rather than a

hard switch in external logic. The on-chip PLL can also help create the system master clock (SCKOUT) for poorly

designed I2S Bluetooth modules that don't provide an system clock to drive the system DACs.

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PCM1861 Datasheet(PDF) 24 Page - Texas Instruments