Cirrus Logic CDB470xx - Manual (Page 54)

6-3 Copyright 2014 Cirrus Logic, Inc. DS886DB11
CDB47xxS Single-Ended Schematic Descriptions
CDB47xxx User’s Manual
populated. The current limiter is scaled to make the CDB47xxxS capable of accepting analog signals of
up to 2 V
RMS . The CS470xx analog inputs register full-scale for an input amplitude of 2V RMS with this
input filter.
6.2.8 Output Filters
Figure 6-9 shows the output filters for the CDB47xxxS.
Each output of the CS470xx has an output filter that consists of an AC-coupling cap (22
F), a pull-down
resistor to prevent the output from floating when not connected to a load, a series resistor (560
) to
provide a voltage drop when the muting transistor is enabled, a filter cap (2200 pF) and a mute transistor
that will pull the output low when the mute control signal is enabled. The series resistor is small enough
that it does not affect the signal in normal operation, assuming a load of at least 5 k
 is connected to the
analog output of the board.
6.2.9 MCU Block Diagram
Figure 6-10 shows the MCU block diagram for the CDB47xxxS.
6.2.10 MCU
Figure 6-10 shows the MCU for the CDB47xxxS.
The USB connector (J25) is connected directly to the MCU (U15). All PC control must pass through the
MCU, since it has an integrated USB interface.
Due to the way the MCU multiplexes its serial control pins, and the multiplexing of the pin functions on the
CS470xx, there are two I
2
C bus isolators (U12 and U13) to prevent slave Comm ports from receiving
commands not intended for them.
The MCU has its own 32 Mbit SPI Flash (U14) for storing MCU firmware.
The LCD control port is multiplexed with other functions on the board, so a buffer (U20) is used to isolate
those logic lines from the LCD data when the MCU is refreshing the LCD display.
The button S6 is the board reset. When the MCU is reset, it reboots and re-initializes the entire board
according to the firmware stored in its internal Flash.
J36 is used only for updating the firmware in the MCU, and has no function in relation to the CS470xx.
6.2.11 User Interface Devices
Figure 6-12 shows the buttons, LEDs, and InfraRed receiver which comprise the standalone user
interface for the CDB47xxxS.
The four momentary contact push-button switches (S1-S4) are routed to the ADC in the MCU (U15).
The rotary encoder with integrated push-button switch (S6) is also routed to the ADC in the MCU (U15).
The LCD (LCD1) is the primary feedback device for the MCU user interface.
The LEDs D1-D4 are also part of the MCU feedback interface showing the MCU status.
LED D11 is used to indicate that the USB interface is active.