Lesson 04: System Clock

Clock Sources and General Features

Figure 4.1: PSoC 5LP Clock Propagation Diagram

Internal Oscikkators:

• Internal Main Oscillator (IMO):
  • The primary clock source for most designs, providing stable output frequencies without external components.
  • Factory trimmed, the IMO's accuracy varies from ±1% at 3 MHz to ±7% at 74 MHz
  • No external components required
  • Often used with PLL for higher frequencies
  • It outputs at frequencies of 3 MHz ±1%, 6 MHz ±2%, 12 MHz ±3%, 24 MHz ±0.25%, 48 MHz ±5%, 62.6MHz  ±7% and 74.7 MHz ±7%
• Internal Low-Speed Oscillator (ILO):
  • Generates 1 kHz, 33 kHz, and 100 kHz frequencies
  • 1 kHz is used for the central timewheel, 100 kHz for the fast timewheel, and 33 kHz as a reduced accuracy version of the ECO clock.
  • Used for low-power operations (watchdog timer, sleep timer, timewheels)

External Oscillators:

• MHz External Crystal Oscillator (MHzECO, XTAL):
  • Utilizes an external crystal for high-precision
  • 4 ~ 25 MHz, high precision with external crystal
  • Used with PLL for high-frequency, high-accuracy systems
  • It can generate CPU and system clocks up to the device's maximum frequency when used with the PLL
• 32.768 kHz External Crystal Oscillator (XTAL 32kHz):
  • Used for RTC

Clock Generation and Distribution:

• Clock Doubler:
  • Doubles the frequency of an input clock
  • Typically used for the USB block, it operates at an input frequency of 24 MHz to provide 48 MHz
• Phase-Locked Loop (PLL):
  • Multiplies low-frequency clocks to higher frequencies
  • Phase lock achieved within 250 μs
  • 24 ~ 80 MHz output, 4032 discrete ratios
  • Used with IMO or MHzECO
  • Disable before entering low power modes
• Clock Dividers:
  • 8 general-purpose 16-bit clock dividers
  • Dedicated divider for CPU bus and CPU clock

Automatic Clock Configuration:

PSoC Creator IDE allows automatic configuration of the clocking system based on overall system requirements. Designers specify desired frequencies and accuracies, and the software configures the clocks accordingly.

Figure 4.2: Clock Configuration

Guidelines for Implementation:

• Select Clock Sources: Choose the appropriate internal and external clock sources based on your system's requirements.
• Configure Clocks in PSoC Creator: Use the PSoC Creator IDE to specify clock frequencies and accuracies.
• Consider Power and Accuracy: Balance the need for high clock frequency and accuracy with power consumption considerations.
• Use PLL for Higher Frequencies: Employ the PLL to multiply lower frequency clocks from the IMO or MHzECO to higher frequencies as needed, considering the trade-off between frequency, accuracy, and power consumption.
• Implement Clock Dividers: Utilize the 16-bit clock dividers to generate specific frequencies for different parts of your system, like a UART baud rate generator.
• Manage Power Modes: Remember to disable the PLL before entering low-power modes to conserve energy.

Steps for Clock System Setup:

• Determine System Requirements: Identify your application's clock frequency and accuracy requirements.
• Select Clock Sources: Choose from the internal oscillators (IMO, ILO) or external oscillators (MHzECO) based on your needs.
• Configure PLL (if needed): Use the PLL for generating higher frequencies from a stable, low-frequency source. Configure the PLL settings in the PSoC Creator IDE.
• Set Up Clock Dividers: Use the 16-bit clock dividers to create specific clock frequencies for various subsystems.
• Configure Clock Doubler (for USB): If your system includes a USB block, configure the clock doubler to provide the required frequency.
• Adjust for Low Power Consumption: If your application demands low power, consider using the ILO and managing clock frequencies and operational modes accordingly.
• Finalize Configuration in PSoC Creator: Once all clock sources and settings are selected, finalize your configuration in the PSoC Creator IDE. The software will automatically arrange the clocking system based on your inputs.

After configuring the clock system, thoroughly test your PSoC device to ensure all clocks function as expected under various operating conditions and modes.

The PSoC 5LP clocking system offers a versatile and programmable approach to managing system clocks. By understanding and utilizing its features, designers can effectively tailor the clocking system to meet the specific needs of their applications, balancing performance, power consumption, and cost.

Table 4.1: Oscillator Summary

Figure 4.3:  PSoC Creator "Configure System Clocks" Dialog

The PSoC 5LP clocking system is a critical part of your design, dictating the timing and operation of various components. The 'Configure System Clocks' dialog in PSoC Creator provides a graphical representation of the system clocks and their relationships, helping to simplify configuration.

Access the Configure System Clocks Dialog:

• Open your project in PSoC Creator.
• Navigate to the Design-Wide Resources (DWR) tab.
• Locate the "Clocks" section and click the "Configure System Clocks" button.

Understand the Graphical Diagram:

• Clock Enablement: A checkmark indicates an enabled clock. Grayed-out clocks are disabled, and some cannot be disabled.
• Clock Paths: Lines show potential clock signal routes. Gray lines represent inactive paths in the current configuration.
• Error Indicators: Red error icons  highlight incorrect configurations. Hover over the icon for a detailed message.

Master Clock and Bus Clock:

• Master Clock: The primary source for generating all other PSoC clocks (including bus clock). It has the highest or equal frequency to all other PSoC clocks.
• Bus Clock: Derived from the master clock and serves as the clock source for the CPU, DMA, DFB, and other major blocks. It usually operates at the same frequency as the Master Clock but serves different functions.

Configuration Steps:

• Select Clock Sources: Choose appropriate clock sources (IMO, PLL, ECO, etc.) based on your system's requirements for frequency and accuracy.
• Enable/Disable Clocks: Check/uncheck boxes to enable or disable specific clocks.
• Adjust Clock Settings: Modify clock frequencies, dividers, and other settings as needed.
• Resolve Errors: Address any errors indicated by red icons.
• Review Configuration: Carefully examine the diagram and settings to ensure they align with your design goals. After making necessary adjustments, review the entire clock configuration to ensure all components are correctly synchronized.
• Rebuild Project: Save your configuration and recompile the project to apply the changes.

Testing and Validation:

• Test your design to confirm that all components function as expected with the new clock configuration.
• Pay special attention to timing-sensitive components to ensure they operate correctly with the adjusted clock settings.

Device-Specific Variations:

• The available clocks and options within the dialog may differ depending on the specific PSoC 5LP device you're using.
• Some devices may organize the dialog into tabs for high-frequency and low-frequency clocks.

Additional Considerations:

• Power Consumption: Higher clock frequencies generally consume more power.
• Accuracy: Select clock sources and PLL settings to achieve the required accuracy for your application.
• USB: For USB functionality, use either the IMO in USB mode or the Clock Doubler with a 24 MHz input.
• Low-Power Modes: Disable the PLL and use the ILO for low-power operations.

Proper configuration of the PSoC 5LP clocking system is crucial for the optimal performance of your design. Taking the time to accurately set up and verify each clock ensures your system runs reliably and efficiently.