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Audio Serial Interface

Audio Data Formats and Timing

PCM1770 , PCM1771

SLES011E – SEPTEMBER 2001 – REVISED MARCH 2007

The audio serial interface for the PCM1770 and PCM1771 devices consists of a 3-wire synchronous serial port.

It includes terminals 1 (LRCK), 2 (DATA), and 3 (BCK). BCK is the serial audio bit clock, and it clocks the serial

data present on DATA into the audio interface serial shift register. Serial data is clocked into the PCM1770 and

PCM1771 devices on the rising edge of BCK. LRCK is the serial audio left/right word clock. It latches serial data

into the serial audio interface internal registers.

Both LRCK and BCK of the PCM1770 device support the slave and master modes, which are set by FMT

(register 3). LRCK and BCK are outputs during the master mode and inputs during the slave mode.

In slave mode, BCK and LRCK are synchronous to the audio system clock, SCKI. Ideally, it is recommended

that LRCK and BCK be derived from SCKI. LRCK is operated at the sampling frequency, f

. BCK can be

operated at 32, 48, or 64 times the sampling frequency.

In master mode, BCK and LRCK are derived from the system clock and these terminals are outputs. BCK and

LRCK are synchronous to SCKI. LRCK is operated at the sampling frequency, f

. BCK can be operated at 64

times the sampling frequency.

The PCM1770 and PCM1771 devices operate under LRCK synchronized with the system clock. The PCM1770

and PCM1771 devices do not need a specific phase relationship between LRCK and the system clock, but do

require the synchronization of LRCK and the system clock. If the relationship between the system clock and

LRCK changes more than ±3 BCK during one sample period, internal operation of the PCM1770 and PCM1771

devices halts within 1/f

, and the analog output is kept in last data until resynchronization between system clock

and LRCK is completed.

The PCM1770 device supports industry-standard audio data formats, including standard, I

S, and left-justified.

The PCM1771 device supports the I

S and left-justified data formats. Table 2 lists the main features of the audio

data interface. Figure 22 shows the data formats. Data formats are selected using the format bits, FMT[2:0] of

control register 3 in the case of the PCM1770 device, and are selected using the FMT terminal in the case of the

PCM1771 device. The default data format is 24-bit, left-justified, slave mode. All formats require binary 2s

complement, MSB-first audio data. Figure 23 shows a detailed timing diagram for the serial audio interface in

slave mode. Figure 24 shows a detailed timing diagram for the serial audio interface in master mode.

Table 2. Audio Data Interface

AUDIO-DATA INTERFACE FEATURE CHARACTERISTIC

Audio data interface format (PCM1770) Standard, I

S, left-justified

(PCM1771) I

S, left-justified

Audio data bit length 16-, 20-, 24-bit selectable

Audio data format MSB-first, 2s-complement

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