The clock device design utilizes an I²C-programmable fractional phase-locked loop (PLL) to meet high-performance timing requirements, ensuring a frequency with zero parts per million (PPM) of synthesis error. High-performance clock ICs are essential in complex systems like printers, scanners, and routers, where they drive critical components such as processors, FPGAs, and data converters. These systems often require dynamic updates to the reference clock frequency to support protocols like PCIe and Ethernet.
The clock IC operates as an I²C slave, requiring the host controller to configure its internal PLL logic. This control logic can be written to the microcontroller, which acts as an I²C master. The microcontroller configures the internal volatile memory of the clock IC and controls the PLL, enabling dynamic updates to the system clock frequency through the onboard MCU-IC combination. Using programmable microcontrollers reduces the number of on-board ICs and traces, resulting in a more compact design and lower overall costs.
**Theory of Operation**
Figure 1 illustrates the basic PLL architecture for a high-performance clock device. The output frequency is determined by the following formula:
$$ f_{\text{OUT}} = \frac{f_{\text{REF}}}{\text{DIV}_R} \times \text{DIV}_N \div \text{DIV}_O $$
Where:
- $ f_{\text{REF}} $ is the input reference crystal frequency (typically 8 MHz to 48 MHz).
- $ \text{DIV}_R $ is the prescaler division factor.
- $ \text{DIV}_N $ is the fractional-N factor.
- $ \text{DIV}_O $ is the post-divider factor before output.
The orange block diagram in Figure 1 represents programmable parameters that can be set during manufacturing and stored in non-volatile memory. When powered up, these configurations are copied to volatile memory, allowing the PLL to produce the default clock output.
Runtime programming via the I²C interface enables users to modify volatile memory contents for immediate changes. The device also supports multiple predefined user configurations stored in non-volatile memory, accessible via the Frequency Select (FS) pin. This allows quick switching between different settings without reprogramming the entire system.
**High-Performance Clock Application Requirements**
These clock ICs are used in consumer, industrial, and networking applications, featuring multiple differential and single-ended outputs from different PLLs. They support critical standards like PCIe, USB, and 10 GbE, and include features such as VCXO and FS for enhanced flexibility.
**Role of Microcontrollers in Clock IC PLL Control**
Connecting the clock IC to a microcontroller, as shown in Figure 2, allows the microcontroller to send I²C commands to program the divider and adjust the PLL output. The PLL generates a tuning voltage based on the local oscillator frequency, which is adjusted dynamically to maintain synchronization.
**In-System Programming via I²C Interface**
This feature allows fast iterations in system design. Data is transferred via SCL and SDA pins, and the microcontroller executes the sequence to interact with the clock IC at runtime. For example, in a system requiring a clock signal that matches a sample rate, the microcontroller can adjust the PLL configuration to switch between two frequencies—155.52 MHz and 156.25 MHz—seamlessly.
**Update Configuration via Frequency Selection (FS) Pin**
High-performance clock devices support fast and slow switching modes. Fast switching handles output ON/OFF and MUX changes, while slow switching adjusts PLL parameters. Both modes ensure reliable operation without errors.
**External Reset Operation**
When an external reset is triggered, the clock IC enters low power mode, and the I²C bus and outputs go into a high-impedance state. After reset, the non-volatile memory is copied to volatile memory, restoring the default configuration.
**Voltage Controlled Crystal Oscillator (VCXO) Operation**
For applications requiring precise frequency tracking, the VCXO function allows analog feedback to adjust the PLL frequency independently of crystal characteristics or environmental factors. With built-in microcontroller modules, the control logic achieves high precision, down to six decimal places.
By using a microcontroller to manage the clock IC, developers can achieve dynamic frequency adjustments, reduce board complexity, and streamline design processes. IDE tools and PSoC devices further enhance development efficiency and cost savings. For more details, refer to the "4-PLL Spread Spectrum Clock Generator Getting Started" guide.
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