The Cortex-M0 DesignStart microcontroller from ARM utilizes an ARM Cortex-M0 processor core which requires a clock source with specific duty cycle requirements. Meeting these requirements is critical for proper operation of the Cortex-M0 microcontroller.
Overview of Cortex-M0 DesignStart Clocking
The Cortex-M0 DesignStart microcontroller features the ARM Cortex-M0 processor core which is targeted for low-cost and low-power embedded applications. The Cortex-M0 core is an extremely energy efficient 32-bit RISC processor optimized for microcontroller applications.
The Cortex-M0 utilizes a single-cycle datapath and two-stage instruction pipeline to deliver impressive performance and efficiency. However, in order to achieve maximum performance, the Cortex-M0 requires a clock source with tight duty cycle requirements.
The clock for the Cortex-M0 is typically generated by an on-chip oscillator circuit. This oscillator generates a square wave clock signal which drives the core logic of the processor. The duty cycle of this clock signal must be within specification for proper functionality.
Duty Cycle Requirements for Cortex-M0 Clock
The duty cycle requirements for the Cortex-M0 clock are:
- Minimum high time: 25 ns
- Maximum high time: 35 ns
- Minimum low time: 25 ns
- Maximum low time: 35 ns
- Duty cycle range: 40% – 60%
This means the high pulse width and low pulse width of the clock signal must each be between 25 ns and 35 ns. The overall duty cycle, which is the ratio of high time to the total period, must be between 40% and 60%.
Minimum High and Low Times
The 25 ns minimum high and low times are dictated by internal timing requirements of the Cortex-M0 core logic. Violating these minimum timing parameters could result in setup and hold time failures within the processor leading to unpredictable behavior.
The Cortex-M0 datapath and pipeline stages have been carefully optimized to operate properly with a 25 ns clock period. Reducing the high or low pulse width below 25 ns does not allow sufficient time for internal logic and flops to transition between states reliably.
Maximum High and Low Times
The 35 ns maximum high and low times help ensure the core achieves maximum performance. The Cortex-M0 pipeline is two stages and designed to support single-cycle throughput for most instructions when clocked with a period around 30 ns.
Increasing the high and low times beyond 35 ns reduces the maximum achievable clock frequency, thereby reducing performance. Excessive high and low times can also increase power consumption without any benefit.
Duty Cycle Range
The overall clock duty cycle, which is the percentage of time spent in the high state, must be between 40% and 60%. A duty cycle in this range allows symmetrical timing margins between internal clock rising and falling edges.
A clock signal with a duty cycle less than 40% reduces the available timing margin for logic triggered on the rising edge. Conversely, a duty cycle above 60% reduces the margin for falling edge triggered logic.
Keeping the duty cycle centered between 40% and 60% ensures balanced timing and reliability across internal clock domains while enabling maximum performance.
Impact of Out-of-Spec Duty Cycle
Operating the Cortex-M0 core with a clock source that violates the 25 ns min/35 ns max high/low time or 40-60% duty cycle requirements can result in a variety of issues including:
- Unreliable operation
- Logic errors and functional failures
- Reduced maximum clock speed
- Increased power consumption
- Intermittent crashes or lockups
The internal circuits and logic of the processor rely on the clock waveform meeting specification for timing margins and proper sequencing. Out-of-spec duty cycle degrades reliability and robustness which quickly leads to erratic behavior or even catastrophic failure.
An out-of-spec clock duty cycle can manifest as unreliable operation where the processor seems to work correctly sometimes but then begins exhibiting strange bugs or crashes at other times.
Intermittent failures are often the result of internal hold or setup time violations caused by asymmetric clock pulse widths. Conditions may appear stable but some logic paths are being stressed beyond safe limits.
Logic Errors and Functional Failures
More serious functionality issues like internal register corruption, execution errors, and pipeline stalls can occur if the clock duty cycle is sufficiently out of range.
Violating timing margins results in outright logic failures and circuits not functioning correctly. This leads to all manner of bizarre errors and malfunctions.
Reduced Maximum Clock Speed
An unbalanced clock duty cycle forces the maximum usable clock speed to be reduced. Logic paths constrained by the narrower portion of waveform limit the speed the entire system can run at.
For example, a 20% duty cycle clock significantly cuts maximum frequency even though the core power supply and temperature constraints might otherwise allow much higher speeds.
Increased Power Consumption
An out-of-spec duty cycle can also result in wasted power consumption. Excessive shoot-through current can occur when outputs switch under asymmetric timing conditions.
Power usage escalates but provides no performance benefit. This is especially detrimental in battery-powered applications.
Intermittent Crashes or Lockups
As conditions deteriorate far enough, the processor will begin exhibiting crashes, lockups, or resets. The system will run seemingly normally for a period of time before something triggers and causes a failure.
Intermittent faults like this are difficult to debug and fix. They get worse as temperature changes or voltage drops until the system is completely unusable.
Generating a Compliant Clock Source
Generating a clock source that meets the Cortex-M0 duty cycle requirements demands care in circuit design and component selection. Here are some tips for creating a robust clock for the Cortex-M0:
Use a high-precision oscillator circuit. RC oscillators and ceramic resonators often have uncontrolled duty cycles. A quartz crystal oscillator provides precision frequency control and decent duty cycle, but may still require calibration.
Add buffer stages to sharpen rise and fall times. Slow rise and fall times lead to variable pulse widths. Fast logic gates restore rapid transitions.
Duty Cycle Calibration
Implement adjustable duty cycle calibration. A trimmer capacitor on the oscillator provides a means to fine-tune the high/low pulse widths.
Duty Cycle Measurement
Measure duty cycle with a precision test setup. Adjust the trimmer capacitor while observing the duty cycle to dial in optimal values. This maximizes yield and ensures reliable operation.
Duty Cycle Monitoring
Implement real-time duty cycle monitoring using a simple analog comparator circuit. This allows detecting out-of-range conditions and taking action, such as trimming the oscillator on-the-fly or resetting the system.
The Cortex-M0 DesignStart microcontroller requires the clock source follow specific duty cycle requirements for proper operation. The core relies on minimum 25 ns and maximum 35 ns high/low times along with an overall duty cycle between 40% to 60%.
Care must be taken in the clock generation and conditioning circuitry to meet these parameters. Failing to comply with the duty cycle requirements can result in unreliable operation, logic errors, reduced performance, increased power consumption, and crashes.
With robust oscillator design, buffering, calibration, measurement, and monitoring the Cortex-M0 clock duty cycle can be optimized to deliver solid reliable performance from the Cortex-M0 DesignStart microcontroller.