9.3.11.1 Power-On, Attach, and Play Back Sequence

The PCM2912A is ready for setup when the reset sequence has finished and the USB bus is attached. After a connection has been established, the PCM2912A is ready to accept USB audio data. While waiting for the audio data (that is, in an idle state), the analog output is set to bipolar zero (BPZ).

When receiving the audio data, the PCM2912A stores the first audio packet, which contains 1-ms audio data, into the internal storage buffer. The PCM2912A starts playing the audio data when the subsequent Start of Frame (SOF) packet is detected, as shown in Figure 32.

Figure 32. Initial Sequence

9.3.11.2 Play, Stop, and Detach Sequence

When the host finishes or aborts the play back process, the PCM2912A stops playing after last audio data has played, as shown in Figure 33.

Figure 33. Play, Stop, and Detach

9.3.11.3 Record Sequence

Figure 34 illustrates how the PCM2912A records the audio into the internal memory after receiving the SET_INTERFACE command.

Figure 34. Record Sequence

9.3.11.4 Suspend and Resume Sequence

The PCM2912A enters a suspend state when it sees a constant idle state on the USB bus after approximately
5 ms. When the PCM2912A enters the suspend state, the SSPND flag (pin 29) is asserted. The PCM2912A wakes up immediately after detecting the non-idle state on the USB bus. Figure 35 illustrates these actions.

Figure 35. Suspend and Resume

Table 1. Device Descriptor

Table 2. Configuration Descriptor

Table 3. String Descriptor

9.5.1.1 Device Configuration

Figure 36 illustrates the USB audio function topology. The PCM2912A has three interfaces. Each interface is constructed with some alternative settings.

Figure 36. USB Audio Function Topology

9.5.1.2 Interface #0

Interface #0 is the control interface. Alternative setting #0 is the only possible setting for interface #0. Alternative setting #0 describes the standard audio control interface. The audio control interface is constructed with a series of terminal connections. The PCM2912A has the following 10 terminals:

• Input terminal (Terminal ID#1) for audio analog input for sidetone
• Feature unit (Unit ID#2) for sidetone PGA
• Input terminal (Terminal ID#3) for isochronous out stream
• Mixer unit (Unit ID#4) for sidetone mixing
• Feature unit (Unit ID#5) for analog output PGA
• Output terminal (Terminal ID#6) for audio analog output
• Input terminal (Terminal ID#7) for audio analog input
• Feature unit (Unit ID#8) for analog input PGA
• Mixer unit (Unit ID#A) for analog input
• Output terminal (Terminal ID#B) for isochronous in stream

Input terminal #3 is defined as USB stream (terminal type 0x0101). Input terminal #3 can accept two-channel audio streams constructed by the left and right channels. Output terminal #6 is defined as a speaker (terminal type 0x0301). Input terminals #1 and #7 are defined as Microphone (terminal type 0x0201). Physically, these two input terminals are the same input, but logically duplicated. Output terminal B is defined as a USB stream (terminal type 0x0101). Output terminal B is a single-channel audio stream. Mixer unit #4 multiplexes the analog input (sidetone) and the audio data of the digital-to-analog converter (DAC). Mixer unit A is placed in front of output terminal B. Mixer unit A has no impact on recording data. Mixer units #4 and A do not have programming capability.

Feature unit #5 supports the following sound control features for analog outputs:

• Volume control
• Mute control

The built-in volume controller can be manipulated by an audio-class-specific request from 0 dB to –76 dB in steps of 1 dB. An individual (L and R) channel can be set for different values. The built-in mute controller can be manipulated by an audio-class-specific request. Only the master mute control request is acceptable.

Feature unit #2 supports the following sound control features for analog input (sidetone):

• Volume control
• Mute control

The built-in volume controller can be manipulated by an audio-class-specific request from 0 dB to –76 dB in 1-dB steps. Only the master volume control is acceptable. The built-in mute controller can be manipulated by audio-class-specific request. Only the master mute control request is acceptable.

Feature unit #8 supports the following sound control features for analog input (microphone record input):

• Volume control
• Mute control

The built-in analog volume controller can be manipulated by an audio-class-specific request from +30 dB to –12 dB in 1-dB steps. The built-in mute controller can be manipulated by an audio-class-specific request. Only the master mute control request is acceptable.

9.5.1.3 Interface #1

Interface #1 is the audio streaming interface for data output. Table 4 lists the three alternative settings for Interface #1. Alternative setting #0 is the zero bandwidth setting. All other alternative settings are operational settings.

9.5.1.4 Interface #2

Interface #2 is the audio streaming interface for data output. Table 5 shows the two alternative settings for Interface #2. Alternative setting #0 is the Zero Band Width setting. Alternative setting #1 is an operational setting.

9.5.1.5 Endpoints

The PCM2912A has the following three endpoints:

• Control endpoint (EP #0)
• Isochronous out audio data stream endpoint (EP #1)
• Isochronous in audio data stream endpoint (EP #2)

The control endpoint is the default endpoint. The control endpoint controls all functions of the PCM2912A by the standard USB request and USB audio class-specific request from the host. The isochronous out audio data stream endpoint is an audio sink endpoint, which receives the PCM audio data. The isochronous out audio data stream endpoint accepts the asynchronous transfer mode. The isochronous in audio data stream endpoint is an audio source endpoint, which transmits the PCM audio data. The isochronous in audio data stream endpoint uses synchronous transfer mode.

9.5.1.6 Internal Regulator

All required power sources are generated by five internal regulators.

Each regulator generates 3.3 V (typical, without load) from V_BUS (pin 2). Each regulator has an output pin and a ground return pin (as described in Table 6); this pair must be decoupled with an appropriate capacitor. Note that this capacitance affects inrush-current limitation. One band-gap reference circuit supplies reference voltage for all regulators. BGND (pin 1) is provided for reference ground of the band-gap reference.