Theory of Operation: VOCON Board | 2-25 |
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Audio Circuitry
A 13-bit CODEC, internal to the GCAP II IC and programmable by the dual-core processor through the SPI bus, converts microphone audio into a digital bit stream for processing by the DSP. The CODEC also converts receive audio data that was processed by the DSP into an analog audio signal for amplification to a speaker. The CODEC interfaces to the DSP through the 4-wire SAP bus. The CODEC clock, which is 256 kHz and is supplied to the DCLK pin. The CODEC 8 kHz CODEC frame synchronization signal is supplied to the FSYNC pin. The CODEC transmit data signal is on the TX pin, while the CODEC receive data signal is on the RX pin. For the CODEC to operate with those clock and frame sync signals, a 13 MHz clock (R302), generated by the digital-support IC, is supplied to the GCAP CLK_IN pin.
The GCAP II IC contains internal amplification, filtering, and multiplexing functionality for both receive and transmit audio. These functions are dual-core processor-programmable through the SPI bus. The input for the internal microphone audio (R540) is the MICIN_NEG pin, while the input for the external microphone audio (R566) is the AUX_MIC_NEG pin. The output for the speaker audio is the EXTOUT pin (C533).
2.5.2.2.2 5 V Regulator U505
The 5 V regulator uses UNSW_B+ as its input voltage. The digital-support IC WDI line controls the regulator’s SHUTDOWN pin. The 5 V supply (R503) is used by the digital-support IC U301, audio preamplifier U502, microphone bias circuitry (R531 and R563), digital-support IC protection diodes, bi-directional voltage translators, battery data-line isolation circuitry, and ESD protection circuitry.
2.5.2.2.3 1.55 V Regulator
The 1.55 V regulator is made up of the following components: R600, Q600, U600, C601, C600, R601, R602, R617, C605, C603, and C604. This circuit uses VSW1 to bias the regulator while VSW2 sources the current. This voltage is used by the dual-core processor U401 for its core voltage and clock amplifier.
2.5.2.2.4 Audio Pre-Amplifier U502
The audio pre-amplifier U502 is a single-package, 5-pin, op-amp supplied with 5 volts. This pre-amp is an active low-pass filter and provides a fixed gain, which is selected by the components R551 and R537. The input (U502 pin 4) of stage is the EXTOUT pin from the GCAP II IC, while the output (U502 pin 1) of this stage goes to the audio PA.
2.5.2.2.5 Audio Power Amplifier U503
The audio PA U503 consists of two BTL amplifiers, complementary outputs, and control logic. Each of the amplifiers has a fixed gain—the external audio PA gain is set by components R553 and R554, while the internal audio PA gain is set by components R549 and R550.
The MODE pin (U503 pin 4) voltage determines the operation of the amplifier. That voltage is controlled by the dual-core processor GPIO lines AUDIO_PA_EN (to Q505) and AUDIO_MODE_SEL (to Q506). Table 2-9describes how the dual-core processor GPIO lines configure the audio PA.
The SELECT pin (U503 pin 6) is used to switch the audio path between internal and external speaker. The voltage on that pin is determined by the EXT_SPKR_SEL line from the dual-core processor and the Q505 transistor. When the voltage at the SELECT pin is high (B+), the audio is routed to the internal speaker lines. When the voltage at the SELECT pin is low (V_select < 0.5V), the audio is routed to the external speaker lines.
