CMX865AE4 Dual-Tone Detection Solution Enhances Communication Reliability

^{November 7, 2025 — With the continuous growth in demand for multifunctional communication in industrial IoT and intelligent control systems, single-chip solutions integrating multiple modem protocols are becoming the core of modern communication systems. The widely adopted CMX865AE4 multi-mode modem chip, with its exceptional integration and flexible communication capabilities, is providing innovative solutions for smart metering, remote control, and industrial automation.}

 

 

I.Core Technical Features of the Chip

 

 

^{The CMX865AE4 utilizes advanced mixed-signal processing technology to implement complete modem functionality within a single chip. Its core features include:}

 

^{Multi-Mode Communication Architecture}

^{Supports multiple modulation and demodulation schemes including FSK, DTMF, and CPT}

^{Integrated programmable tone generation and detection functions}

^{Compatible with standard communication protocols such as V.23 and Bell 202}

^{Flexible baud rate configuration supporting up to 1200 bps}

 

^{High-Integration Design}

^{Built-in precision bandpass filters and equalizers}

^{Integrated line drivers and receive amplifiers}

^{Complete 2/4-wire hybrid circuit functionality}

^{Programmable gain control and level detection capabilities}

 

^{Industrial-Grade Reliability}

^{Operating voltage range: 3.0V to 5.5V}

^{Industrial temperature range: -40℃ to +85℃}

^{Low-power design with standby current below 1μA}

^{Excellent anti-interference and EMC performance}

 

 

 

II. Functional Block Diagram

 

 

^{This diagram is a functional block diagram of the CMX865AE4, a highly integrated telecommunications signaling and communication chip primarily used for processing various types of audio signals, data modulation/demodulation, and signaling interactions in telephone networks. Based on its designation as a "Telecom Signalling Device (with DTMF Codec and Multi-Standard FSK Modem)", we will analyze the various modules in the diagram as follows:}

 

 

 

^{1.Core Function Positioning}

^{The CMX865A is a monolithic device integrating the following key functions:}

^{DTMF (Dual-Tone Multi-Frequency) signal generation and detection}

^{FSK (Frequency Shift Keying) modulation and demodulation}

^{Call progress tone detection}

^{Serial communication interface control}

^{Analog audio input/output processing}

 

^{Target Applications:}

^{Telephone answering machines}

^Modems

^{Telephone dial-up alarms in security systems}

^{Remote data transmission equipment}

 

2.Module Analysis

 

^{1. Serial Control Interface (C-BUS)
CSN, COMMAND DATA, REPLY DATA, SERIAL CLOCK:}

^{Used for communication with an external microcontroller to receive commands and return status.}

^{Utilizes an SPI-like or custom serial protocol to configure chip operating modes (e.g., DTMF transmission, FSK transceiver, tone detection, etc.).}

 

^{2. Data Registers and USART
Tx/Rx DATA REGISTERS & USART:}

^{Provides asynchronous serial communication capability for handling data streams from the host.}

^{Used for transmitting and receiving serial data in FSK mode.}

 

^{3. Modem Section}

^{FSK Modulator: Supports multiple standards including Bell 202 and V.23.
Transmit Path: Integrates filtering and equalization to ensure compliant output signals.
Receive Path: Provides filtering and demodulation for accurate data recovery.}

 

^{4. DTMF/Audio Processing Section}

^{DTMF/TONE GENERATOR:}

^{Generates DTMF signals (e.g., telephone keypad tones) or other single/composite tones.}

^{DTMF/TONE/CALL PROG/ANSWER TONE DETECTOR:}

^{Detects DTMF signals, call progress tones (e.g., dial tone, busy tone), or answering machine identification tones from the line.}

 

^{5. Analog Front-End}

^{Transmit Driver: Provides differential drive capability to the telephone line (TXA/TXAN).
Programmable Gain Reception: Features automatic or configurable gain to ensure input signal quality.
Analog Loopback: Incorporates a built-in local loopback path for system diagnostics and performance testing.}

 

^{6. Clock and Power Supply
XTAL / CLOCK:}

^{External crystal or clock input providing the chip's operating clock.}

^{VDD, VDEC, VSSD, VSSA:}

^{Power management pins, including digital power supply, analog power supply, digital ground, and analog ground, ensuring signal integ}

 

^{3.Typical Workflow}

 

^{Initialization:}

^{The host controller configures the chip's operating mode (e.g., FSK modulation/demodulation or DTMF transmission) via the C-BUS.}

 

^{Data Transmission:
Data is fed through the USART → processed by the FSK modulator → passed through the transmit filter → output via the buffer → transmitted over the telephone line.}

 

^{Data Reception:
Signals from the telephone line → enter the input amplifier → pass through the receive filter → undergo FSK demodulation → output to the host controller via the USART.}

 

^{Tone Processing:}

^{The DTMF detector continuously monitors input signals and reports valid tone detection via the C-BUS.}

^{The DTMF generator produces corresponding dual-frequency signals based on commands.}

 

^{4. Summary}
 

^{The CMX865A is a fully-featured telecommunications signaling chip that highly integrates analog front-end, modem, tone generation, and detection modules. It is suitable for various embedded devices requiring telephone network interfaces. Its design balances flexibility (configurable via serial interface) with compatibility (supporting multi-standard FSK and DTMF), making it an ideal choice for data communication and signaling interactions in traditional telephone systems.}

^{If you need further details about its register configuration or specific application circuits, I am available to provide additional assistance.}

 

 

 

 

III. Typical Application Circuit External Component Configuration Diagram

 

 

^{This diagram illustrates the typical application circuit external component configuration for the CMX865AE4, demonstrating the most fundamental peripheral circuits required to implement this chip in real-world projects. Let's analyze each section and its function in detail:}

 

 

 

^{Diagram Overview}

^{The core concept of this diagram is: a microcontroller communicates with the telephone line (PSTN) through the CMX865A. The upper part of the diagram shows the digital control and clock section, while the lower part illustrates the analog telephone line interface.}

 

^{Core Components Analysis}

^{1. Microcontroller Interface}

^{C-BUS Connection: Directly connect CSN, COMMAND DATA, SCLK, and REPLY DATA to the microcontroller GPIO pins.}

^{Interrupt Configuration: The IRQN pin requires a 68kΩ pull-up resistor (R1) to VDD to ensure reliable interrupt requests.}

^{Power Supply: Pay attention to the power connections in the pin 9-16 region.}

 

^{2. Clock Circuit
Pins 5 & 6: The chip requires an external clock to operate.}

^{X1: A high-precision 6.144MHz crystal (±300ppm). This frequency is related to telecommunications standards and can be divided to generate all necessary audio and modulation frequencies.}

^{C1, C2 (22pF): These capacitors are the crystal load capacitors, essential for stable crystal oscillation. Their capacitance values are typically specified by the crystal manufacturer.}

 

^{3. Power Supply and Decoupling
This is a critical section to ensure stable chip operation and prevent noise interference.}

^{VDD: Digital power supply positive terminal}

^{VSSD: Digital power supply ground}

^{VSSA: Analog power supply ground}

^{VBIAS: Internally generated analog bias voltage, requiring an external capacitor for filtering and stabilization}

 

^{Key External Components:}

^{C3, C4, C7 (100nF): These are decoupling/filter capacitors. Positioned near power pins to filter high-frequency noise and provide clean local power supply. C7 specifically stabilizes the VBIAS voltage.}

^{C5, C6 (10μF): These are energy storage/bypass capacitors. Used to handle instantaneous current fluctuations and ensure more stable power delivery.}

 

^{4. Telephone Line Interface}

^{Differential Drive: The TXA/TXAN (Pins 1 & 2) differential pair is used to drive the telephone line, enhancing noise immunity.}

^{Receive Input: RXAFB (Pin 3) serves as the receive input, requiring an external RC network to couple signals from the telephone line.}

^{Interface Protection: The receive path components R1 (matching/current limiting) and C8 (DC blocking) ensure secure and reliable signal transmission.}

 

^{Design Key Points and Rationale}

^{Separated Grounding: The diagram clearly distinguishes between VSSD (Digital Ground) and VSSA (Analog Ground). During PCB layout, analog and digital grounds are typically separated and connected at a single point (e.g., beneath the chip) to prevent noise from the digital section from interfering with sensitive analog signals. The "Ground plane connection" annotation in the diagram implies this practice.}

 

^{Signal Flow Path:}

^{Transmit Path: Microcontroller → C-BUS → CMX865A (internal DAC, filters, modulator) → TXA/TXAN → External line driver circuit (not fully shown in the diagram, such as DAA module) → Telephone line.}

^{Receive Path: Telephone line → External protection/step-down circuit → R1/C8 network → RXAFB → CMX865A (internal amplifiers, filters, demodulator/detector) → Status/data via C-BUS or IRQN → Microcontroller.}

 

^{Typical Applications:
This configuration enables the CMX865A to function as a modem + signaling detector. For example, in an automated alarm system, it can detect answering machine signals from incoming calls and then transmit data via FSK; or it can detect remote DTMF commands to control equipment.}

 

^{Summary
This diagram provides the minimum list of external components and connection methods required to put the CMX865A chip}

^{into practical use. It clearly demonstrates:}

 

^{How to connect to the main MCU (C-BUS + IRQN).}

^{How to provide a precise clock source (crystal + load capacitors).}

^{How to ensure clean power supply (multiple decoupling/filtering capacitors).}

^{How to couple analog signals with the telephone line (simple RC receiving network).}

 

^{Following this recommended configuration is the first step to ensure stable operation of the CMX865A. In a complete product design, more complex Data Access Arrangement (DAA) circuits are typically added after the TXA/TXAN output and before the RXAFB input. These circuits provide functions such as overvoltage protection, ring signal detection, line on/off-hook control, and 2-to-4-wire hybrid conversion.}

 

 

 

IV. Schematic Diagram of Typical 2-Wire Line Interface Circuit

 

 

^{This diagram illustrates a simplified analog interface circuit connecting the CMX865AE4 to a standard 600Ω 2-wire telephone line (i.e., the common telephone line we typically use). This is a critical part of the entire system, responsible for transmitting the signals generated by the chip to the line and introducing the signals from the line into the chip.}

 

 

 

 

^{1. Core Circuit Functions
Implements 2-wire (telephone line) to 4-wire (chip) interface conversion, primarily achieving:}

^{Impedance Matching: Ensures impedance matching between the chip and the 600Ω telephone line}

^{Signal Coupling: Accomplishes injection and extraction of transmit/receive signals}

^{Noise Suppression: Filters out out-of-band high-frequency interference}

^{Electrical Isolation: Blocks -48V DC high voltage to protect the chip}

 

^{Analysis of Component Functions
We will analyze the signal path by dividing it into the transmit path and the receive path:}

 

^{1. Transmit Path
The signal originates from the internal driver amplifier of the CMX865A.
The key component R13 (600Ω) serves as a terminal matching resistor, providing the standard 600Ω impedance for the telephone line to ensure signal quality and prevent reflections.
In practical designs, this resistance value can be fine-tuned according to specifications such as FCC and ITU-T.}

 

^{2. Receive Path
This circuit employs a resistive voltage divider network to achieve isolation between transmit and receive signals:}

^{R11 & R12: Form a voltage division and attenuation network, converting line differential signals into single-ended signals for the RXAFB pin.}

^{R11: Serves as the key adjustment resistor, matching line signal strength through resistance value tuning.}

^{C11 (100pF): Combines with R12 to form a high-frequency filter, effectively suppressing RF interference.}

 

^{3. Common/Filtering & Protection Unit}

^{C10 (33nF): Provides DC blocking coupling and low-pass filtering, blocking DC while passing AC audio signals, and works with R13 to suppress high-frequency noise}

^{3.3V Zener Diode: Delivers basic overvoltage protection, ensuring chip safety through voltage clamping}

^{Note: Practical applications should replace this simplified design with professional protection solutions like TVS tubes or gas discharge tubes}

 

^{VBIAS:
This is the bias voltage generated internally by the chip. The received signal is coupled to the RXAFB pin through C11, while the RXAFB pin is typically biased at the VBIAS voltage internally via a high-value resistor. VBIAS provides a stable DC operating point for the AC-coupled received signal.}

 

^{Design Summary}

^{1.Passive Hybrid Circuit}

^{Resistor network (R11/R12/R13) enables signal routing}

^{TX path to line via R13}

^{RX signal to RXAFB via R11/R12 divider}

^{Prevents TX-RX crosstalk (anti-sidetone)}

 

^{2.Simplified Architecture
Core signal conditioning only. Requires:}

^{Hook/ring control}

^{Ring detection}

^{Enhanced surge protection}

 

^{3.Applications
For PSTN/POTS systems:
Fax/Modem/Answering Machine/Auto-dial Alarms}

 

 

 

 

V. Application Circuit Schematic in Wireless Local Loop Scenarios

 

 

 

^{System Positioning: Wireless Local Loop}

^{Wireless Local Loop (also known as Fixed Wireless Access) is a solution that utilizes wireless technology (such as cellular networks, private radio networks, etc.) to replace traditional copper telephone lines, providing the last segment of telephone access for homes or offices.}

 

^{The core signal flow can be simplified as follows:
Traditional Telephone Network → Wireless Base Station → User-end Wireless Equipment → Standard Telephone Set}

^{The CMX865A is located inside the User-end Wireless Equipment (often referred to as the Fixed Station or Subscriber Unit).}

 

 

 

 

 

Core Role of CMX865A in This Architecture:

 

^{1.Protocol and Signal Converter:}

^{Downlink Direction (Network → Telephone Set):
The wireless module receives digital voice or data packets. The microcontroller controls the CMX865A via the C-BUS to convert these into standard FSK modulated signals (for caller ID, data communication) or DTMF tones, which are then transmitted to the telephone set through the SLIC.}

 

 

^{Uplink Direction (Telephone Set → Network):
Analog signals picked up from the telephone set (such as voice or DTMF dial tones) are sent by the SLIC to the CMX865A. The DTMF/Call Progress Tone Detector inside the CMX865A can recognize key presses, and its Receive Modem can demodulate FSK data. The results are reported to the microcontroller via the C-BUS and finally packaged and sent back to the network by the wireless module.}

 

 

^{2.Telecom Signaling Simulator:}

^{It is responsible for generating and detecting all standard PSTN (Public Switched Telephone Network) tones, such as dial tone, ring-back tone, busy tone, etc. This ensures that users of the wireless telephone receive an auditory experience and signaling interaction completely consistent with that of a wired telephone, achieving "wireless access, wired experience."}

 

 

^{3. Key Design Considerations}

^{1.Collaborative Design:}

^{The actual circuit must be designed in strict accordance with the datasheets of both the SLIC and the wireless module.}

^{Ensure level and impedance matching between the CMX865A and the SLIC, as well as protocol compatibility with the wireless module.}

 

^{2.Power Supply Decoupling:}

^{This is the top priority in the design. The wireless module is a major source of noise, and its burst currents can severely interfere with the sensitive CMX865A.}

^{Enhance power supply decoupling: Deploy capacitors of different values (e.g., 10μF, 100nF, 1nF) near the power pins of each chip to provide a low-impedance return path for noise. This prevents noise coupling between analog and digital circuits and ensures communication reliabili}

 

4.Summary

This application diagram clearly demonstrates that the CMX865A serves as a "network protocol translator" and "signaling processing hub" in wireless local loop systems. Its high level of integration significantly simplifies the design.

 

However, achieving a stable and reliable product depends not on the CMX865A itself, but on how well its interactions with the two "neighbors"—the SLIC and the wireless module—are managed. This is particularly critical for handling the severe power noise introduced by the wireless module. Meticulous power and grounding design is the key determinant of success for such products.

 

 

 

VI. Key Implementation Principles and Characteristics of Programmable Dual-Tone Detectors and Filters

 

 

^{Core Concept Analysis
These two diagrams collectively describe how the chip detects and identifies input audio signals (such as DTMF dual-tone multi-frequency signals or call progress tones). This represents a processing flow from analog signals to digital determination.}

 

^{Programmable Dual-Tone Detector
This block diagram depicts the overall architecture of the detector, and its workflow can be parsed as follows:}

 

 

 

^{1.Signal Separation:}

^{The input mixed audio signal (which may contain two different frequency tones) is first fed into two independent programmable bandpass filters.}

^{One filter is configured to only pass the first target frequency (e.g., the high-frequency group in DTMF).}

^{The other filter is set to only pass the second target frequency (e.g., the low-frequency group in DTMF).}

 

^{2.Frequency Detection:}

^{The initially separated single-tone signals output from each filter are fed into a frequency detector.}

 

^{Detection Principle:
The detector measures the time required for the input signal to complete a "programmable number" of full cycles.}

 

^{Example:
To detect a 697Hz signal, the detector might be set to count 10 cycles. For an exact 697Hz signal, the time required to complete 10 cycles is a fixed value.}

 

^{Judgment Logic:
The detector then compares this measured time with internally preset programmable upper and lower time limits.}

^{If the measured time falls within the allowed range, it indicates that the input signal frequency matches the target frequency.}

^{If the time is too short, it means the input frequency is higher than expected.}

^{If the time is too long, it means the input frequency is lower than expected.}

 

^{3.Result Output:}

^{Only when both frequency detectors simultaneously determine that their respective frequencies are present in the input signal, and other conditions such as amplitude are also met, will the chip finally confirm the detection of a valid tone pair and notify the main controller via an interrupt or status register.}

 

^{Design Advantage:
This "cycle timing" method typically demonstrates superior performance in noise immunity and precision compared to some other approaches, making it particularly suitable for the less-than-pristine signals common in telecommunications environments.}

 

 

^{Filter Implementation
This diagram illustrates the technology used to implement the aforementioned bandpass filters.}

 

 

^{Filter Type: 4th-order IIR (Infinite Impulse Response) filter.}

^{IIR Filter Characteristics:}

High Efficiency: Compared to FIR (Finite Impulse Response) filters with equivalent performance, IIR filters require fewer computational stages and can achieve steeper roll-off characteristics with lower computational load.

 

^{Feedback Structure: By utilizing output feedback, IIR filters can achieve sharp frequency selection with relatively fewer resources, making them highly suitable for implementing high-performance bandpass filtering in resource-constrained embedded environments like this chip.}

 

^{Function: These 4th-order IIR bandpass filters serve as the first critical gatekeepers in the signal path. Their task is to significantly attenuate any noise and interference signals outside the target frequency range, providing only "purified" single-tone signals to the subsequent frequency detectors, thereby ensuring detection accuracy.}

 

^{Summary
By combining these two diagrams, we can understand the tone detection mechanism of the CMX865AE4:}

 

^{1.Separation: First, a pair of 4th-order IIR bandpass filters is used to preliminarily separate and purify the input dual-tone signal.}

^{2.Measurement: Next, high-precision digital cycle timers measure and verify the frequency of each single tone.}

^{3.Decision: Finally, a programmable tolerance window is applied for judgment, ultimately confirming a valid tone pair.}

 

^{This hardware-implemented detection solution is reliable, precise, and does not consume main controller resources, perfectly meeting the high demands for real-time performance and reliability in telecommunications signaling processing.}

 

 

VII. Line Interface Signal Hybrid Configuration

 

 

^{Core Concept
This diagram illustrates an analog signal mixing circuit. Its primary purpose is to "insert" or "superimpose" an additional audio signal (such as voice prompts from a microcontroller, alarm tones, or other audio sources) onto the transmission path without demodulating or interfering with the normal communication between the CMX865A and the telephone line.}

 

 

 

<sup>Key Design Considerations Analysis
1. Impedance Matching and Signal Source Requirements
The text explicitly states the critical requirements for the signal source:</sup>

^{Chip Input Impedance: The static impedance of the CMX865A receive input (likely the RXAFB pin) is approximately 100kΩ.}

^{Signal Source Output Impedance: The output impedance of the external signal source should be around 10kΩ or lower.}

 

^{Reasoning: This follows the classic 10:1 impedance ratio rule. To ensure efficient signal voltage transfer from the source to the load without significant attenuation, the source impedance must be much lower than the load impedance. With a 10kΩ source impedance and a 100kΩ load impedance, the voltage division results in minimal signal attenuation, which is negligible.}

 

^{Tri-state Capability: The signal source must have a tri-state (high-impedance) output capability.}

^{Reason: This is to prevent the low output impedance of the external signal source from causing unnecessary voltage division and attenuation of the CMX865A's output signal when the chip itself is transmitting. When external signal insertion is not required, the signal source should enter a high-impedance state, effectively "disconnecting" from the line to avoid interfering with the normal operation of the CMX865A.}

 

^{2. AC Coupling
The diagram shows the use of capacitors for AC coupling, with the text providing important clarification on this:}

^{Purpose: The main function of the AC coupling capacitor is to block the DC component. It only allows AC signals to pass through, preventing the DC bias voltage of the external signal source from affecting the precise internal DC operating point of the CMX865A input, and vice versa.}

 

^{Non-essential: The text explicitly states that AC coupling can be omitted if the line interface itself does not require it. This means that if the DC levels of the external signal source and the CMX865A input are compatible, the design can be simplified.}

 

^{Capacitance Value Selection: If AC coupling is used, the choice of capacitance value is critical.}

^{Principle: The capacitive reactance (Xc) must not be too large at the system's lowest operating frequency to avoid excessive signal attenuation.}

 

^{Calculation Formula: Capacitive reactance Xc = 1 / (2πfC), where f is the frequency and C is the capacitance value.}

 

^{Design Basis: For the CMX865A, the lowest frequency component is approximately 300Hz (the starting point of the telephone voice frequency band). Therefore, the capacitance value must be sufficiently large to ensure that its reactance at 300Hz is much smaller than the circuit's input impedance (100kΩ).}

 

^{Example: A 100nF (0.1μF) capacitor has a reactance of approximately 5.3kΩ at 300Hz. Compared to the 100kΩ input impedance, this results in minimal attenuation, making it a reasonable choice.}

 

^{Summary and Applications
This configuration diagram reveals the flexibility of the CMX865A interface. Through this circuit, designers can achieve:}

 

^{Voice Prompts: In automated alarm systems, play "The system is dialing" voice prompts before transmitting FSK data.}

 

^{Background Music or Broadcasting: Mix music signals into the communication line.}

 

^{Multi-channel Signal Multiplexing: Sequentially or simultaneously transmit audio signals from different sources to the line.}

 

^{The key to successfully implementing this circuit lies in:}

^{1.Using a signal source with sufficiently low output impedance (≤10kΩ) and tri-state control capability.}

^{2.If AC coupling is required, selecting appropriate coupling capacitor values based on the minimum frequency of 300Hz to ensure low-frequency signals are not excessively attenuated.}

 

 

VIII. Caller ID Function Implementation

 

 

 

^{Core Concept Analysis
The core of this circuit is a hook-switch-controlled, switchable impedance network. Its purpose is to optimize signal reception by altering the line termination impedance under specific operating conditions.}

 

 

 

 

^{Circuit Working Principle
1.Purpose: In the on-hook state, the impedance at the telephone line end is typically high (e.g., through a ring detection circuit). This high impedance may attenuate the caller ID signal (an FSK data transmitted between the first and second rings) to an unrecognizable level. This circuit is designed to address this issue.}

 

^{2.Operating Modes:}

^{On-Hook State: When the phone is on-hook, the switch in the diagram closes. At this time, resistor R13 (e.g., 600Ω) is precisely connected in parallel to the line, providing a standard, matched termination impedance for the line. This ensures that the FSK signal of the caller ID is transmitted to the receiving end (RXAFB) of the CMX865A with minimal reflection and attenuation, significantly improving data reception reliability.}

^{Off-Hook State: When the phone is off-hook and a call begins, this switch should remain closed.}

 

^{Critical Design Warning and Risks
The text explicitly points out the serious issues of operating the switch in the off-hook state with this design:}

 

^{1.Impedance Mismatch and Signal Reflection:}

^{Problem: If the switch is opened during the off-hook state, the externally added 600Ω matching resistor will be abruptly removed. This will cause a sharp deterioration in the return loss of the line interface, rendering it "unacceptable."}

 

^{Consequence: Impedance mismatch will lead to significant reflection of the received voice/data signals. This generates echo and distorts the received signal, severely degrading call quality or data transmission reliability.}

 

^{2.Line Transient Interference:}

^{Problem: Opening or closing the switch during an active call (off-hook state) is equivalent to abruptly altering the electrical characteristics of the line.}

 

^{Consequence: This action injects unwanted transient pulses into the telephone line. Such pulses would be perceived by the other party as harsh "clicks" or pops, severely impacting user experience and potentially violating telecommunications regulations.}

 

^{Summary and Application Guidance
This diagram illustrates a conditional and limited-use enhancement technique:}

 

^{Its correct application scenario is:
The switch should only be closed during the on-hook state to reliably receive caller ID signals. Before or after entering the off-hook state, the switch status should remain fixed to avoid switching.}

 

^{Its main risk lies in:
Operating the switch during the off-hook state, which will disrupt call quality and generate noise.}

 

^{Therefore, when implementing this function, the system firmware must enforce strict state machine control:
Ensure that switch operations only occur during the on-hook period, and once a call (off-hook state) is established, any switching action must be prohibited. This is a design introduced to optimize a specific function (Caller ID) and requires meticulous management.}