Implementing TCD Clock Control in Embedded Systems: Examples and Tips

Troubleshooting Common TCD Clock Control Issues and FixesTroubleshooting TCD (Timer/Counter for Control Devices) clock control issues requires a methodical approach: confirm clock sources, verify configuration registers, trace clock distribution, check gating and power domains, and isolate software vs. hardware causes. Below is a comprehensive guide covering typical problems, diagnostic techniques, and practical fixes you can apply in embedded systems using TCD peripherals.

What is TCD clock control (brief)

TCD peripherals rely on properly configured clock sources and prescalers to produce accurate timing for PWM, input capture, and event scheduling. Clock control involves selecting the clock source (internal oscillator, main system clock, PLL, or external clock), setting prescalers/dividers, enabling peripheral clocks in power/clock management units, and managing clock gating during low-power modes.

Common symptoms and their likely causes

• Peripheral doesn’t start or shows no output
  • Cause: Peripheral clock disabled, clock gating active, or power domain off.
• Incorrect frequency or timing (PWM duty/frequency wrong)
  • Cause: Wrong clock source selected, incorrect prescaler/divider, or clock jitter from unstable oscillator.
• Intermittent operation or glitches
  • Cause: Clock switching issues, race conditions during clock source transitions, EMI, or misconfigured synchronization.
• Unexpected resets or lockups when enabling/disabling clock
  • Cause: Improper sequencing of power/clock control, enabling peripheral before power domain is ready, or clock source unstable (PLL not locked).
• High power consumption with TCD active
  • Cause: Clock left enabled when not required, running at high frequency, or failure to use low-power gating properly.
• Timer drift over long periods
  • Cause: Use of imprecise internal RC oscillator without calibration, temperature-related oscillator drift, or PLL drift.

Step-by-step diagnostic checklist

1. Confirm peripheral clock enable

   • Check the microcontroller’s Clock/Power Management Unit (PMU/CCU) registers to ensure the TCD peripheral clock bit is set.
   • Verify there’s no higher-level OS or driver disabling the clock.

2. Verify clock source and frequency

   • Inspect the clock source selection registers. Ensure the expected source (SYSCLK, PLL, OSC, or external) is chosen.
   • If available, read the system clock tree or use on-chip measurement units (if present) to measure the actual clock frequency.

3. Check prescalers/dividers and timer configuration

   • Confirm prescaler and divider values in TCD registers match calculated values for desired frequency.
   • Recalculate expected timer ticks: Timer_frequency = Clock_source / Prescaler / (TOP+1).

4. Validate PLL and oscillator health

   • Confirm PLL is locked before switching the peripheral to use it.
   • Check stabilization delays after enabling external oscillators.

5. Inspect power domains and gating

   • Ensure the peripheral’s power domain is powered before enabling clocks.
   • Check for automatic gating features that may disable clocks in low-power states.

6. Look for synchronization and register-writes issues

   • Some TCD registers require write synchronization or specific sequences; verify per datasheet.
   • Avoid changing clock source while the timer is running unless recommended.

7. Reproduce the issue under controlled conditions

   • Run simplified test code that only enables the clock and toggles an output at known intervals.
   • Use an oscilloscope or logic analyzer to observe clock/timer signals.

8. Review interrupt and DMA interactions

   • Ensure interrupts or DMA transfers related to TCD aren’t stalled or masking operations.
   • Verify NVIC priorities and that ISR handlers clear flags properly.

9. Examine silicon errata and software library bugs

   • Check manufacturer errata for known TCD clock issues and recommended workarounds.
   • Confirm you’re using correct and up-to-date HAL/driver versions.

Practical fixes and examples

• Fix: Peripheral clock bit not set

  • Action: Enable the clock in the PMU/CCU before configuring TCD registers. Example (pseudocode):

    
    CLOCK_ENABLE(TCD); while (!CLOCK_READY(TCD)) { } TCD->CTRL = desired_config; 

• Fix: Wrong prescaler value

  • Action: Recalculate and write correct prescaler. If using formula: TOP = Clock_source/Prescaler/Freq – 1. Example: For 1 kHz PWM from 48 MHz clock with prescaler 48 → Timer_freq = 1 MHz; TOP = 1e6/1e3 – 1 = 999.

• Fix: PLL not locked

  • Action: Wait for PLL lock flag before switching source or enabling dependent peripherals. Insert required delays after enabling oscillators.

• Fix: Synchronization issues writing control registers

  • Action: Use the required write sequence or read-back confirmation if datasheet specifies. Example:

    
    TCD->CTRL = new_val; tmp = TCD->CTRL; // read-back to ensure write complete 

• Fix: Clock gating during low-power

  • Action: Configure power/perf modes or use retention/gating exemptions for TCD when needed. Ensure wake-up sources are set.

• Fix: Timer drift due to RC oscillator

  • Action: Calibrate oscillator against known reference (RTC or external oscillator) or use a crystal/PLL for precision.

• Fix: Intermittent glitches when switching clock sources

  • Action: Stop the timer, switch clock, verify stability, then restart. Use recommended sequencing from the datasheet.

Tools and measurements to help debugging

• Oscilloscope: verify the actual output waveform, frequency, and jitter.
• Logic analyzer: capture enable/disable sequences and peripheral signals.
• On-chip clock monitor/perf counters: measure internal clock frequencies if available.
• Software: minimal reproducer firmware, register-dump utility, and driver-level logging.
• Thermal chamber or temperature tests: expose oscillator drift problems.

Example troubleshooting flow (concise)

1. Confirm TCD clock enable bit in PMU.
2. Verify clock source = expected; measure frequency.
3. Check prescaler and TOP values; recalc expected output.
4. Use oscilloscope to observe output; note jitter or absence.
5. If absent, check power domain and PLL lock.
6. Consult errata; update HAL/firmware; retest.

Preventive best practices

• Always enable and verify peripheral clocks before register configuration.
• Use crystal/PLL for precise timing; reserve internal RC for non-critical timing.
• Implement safe sequencing: stop timers before changing clock sources or prescalers.
• Add read-back or status checks after writes to critical clock-control registers.
• Use clear abstraction in firmware that centralizes clock control to avoid conflicting code paths.
• Add unit tests and hardware-in-the-loop tests for timing-critical functions.

When to escalate to hardware or vendor support

• If oscilloscope shows expected clock at pin but timer still misbehaves.
• If behavior matches documented silicon errata or cannot be resolved with firmware workarounds.
• If you suspect an electrical fault (damaged oscillator circuitry, power instability).

If you want, I can: provide a specific register-sequence example for a particular microcontroller (STM32, AVR, NXP, Microchip, etc.), write minimal reproducible test firmware, or walk through an oscilloscope capture you provide. Which MCU or board are you using?