mcpwm.rst

Motor Control Pulse Width Modulator (MCPWM)

{IDF_TARGET_NAME} has two MCPWM units which can be used to control different types of motors. Each unit has three pairs of PWM outputs.

MCPWM Overview

Further in documentation the outputs of a single unit are labeled PWMxA / PWMxB.

More detailed block diagram of the MCPWM unit is shown below. Each A/B pair may be clocked by any one of the three timers Timer 0, 1 and 2. The same timer may be used to clock more than one pair of PWM outputs. Each unit is also able to collect inputs such as SYNC SIGNALS, detect FAULT SIGNALS like motor overcurrent or overvoltage, as well as obtain feedback with CAPTURE SIGNALS on e.g. a rotor position.

MCPWM Block Diagram

Description of this API starts with configuration of MCPWM's Timer and Generator submodules to provide the basic motor control functionality. Then it discusses more advanced submodules and functionalities of a Fault Handler, signal Capture and Carrier.

Contents

• Configure a basic functionality of the outputs
• Operate the outputs to drive a motor
• Adjust how the motor is driven
• Synchronize sync timers to work together
• Capture external signals to provide additional control over the outputs
• Use Fault Handler to detect and manage faults
• Add a higher frequency Carrier, if output signals are passed through an isolation transformer
• Extra configuration of Resolution.

Configure

The scope of configuration depends on the motor type, in particular how many outputs and inputs are required, and what will be the sequence of signals to drive the motor.

In this case we will describe a simple configuration to control a brushed DC motor that is using only some of the available MCPWM's resources. An example circuit is shown below. It includes a H-Bridge to switch polarization of a voltage applied to the motor (M) and to provide sufficient current to drive it.

Example of Brushed DC Motor Control with MCPWM

Configuration covers the following steps:

1. Selection of a MCPWM unit that will be used to drive the motor. There are two units available on-board of {IDF_TARGET_NAME} and enumerated in :cppmcpwm_unit_t.
2. Initialization of two GPIOs as output signals within selected unit by calling :cppmcpwm_gpio_init. The two output signals are typically used to command the motor to rotate right or left. All available signal options are listed in :cppmcpwm_io_signals_t. To set more than a single pin at a time, use function :cppmcpwm_set_pin together with :cppmcpwm_pin_config_t.
3. Selection of a timer. There are three timers available within the unit. The timers are listed in :cppmcpwm_timer_t.
4. Setting of the timer frequency and initial duty within :cppmcpwm_config_t structure.
5. Setting timer resolution if necessary, by calling :cppmcpwm_group_set_resolution and :cppmcpwm_timer_set_resolution
6. Calling of :cppmcpwm_init with the above parameters to make the configuration effective.

Operate

To operate a motor connected to the MCPWM unit, e.g. turn it left or right, or vary the speed, we should apply some control signals to the unit's outputs. The outputs are organized into three pairs. Within a pair they are labeled "A" and "B" and each driven by a submodule called an "Generator". To provide a PWM signal, the Operator itself, which contains two Generator, should be clocked by one of three available Timers. To make the API simpler, each Timer is automatically associated by the API to drive an Operator of the same index, e.g. Timer 0 is associated with Operator 0.

There are the following basic ways to control the outputs:

• We can drive particular signal steady high or steady low with function :cppmcpwm_set_signal_high or :cppmcpwm_set_signal_low. This will make the motor to turn with a maximum speed or stop. Depending on selected output A or B the motor will rotate either right or left.
• Another option is to drive the outputs with the PWM signal by calling :cppmcpwm_start or :cppmcpwm_stop. The motor speed will be proportional to the PWM duty.
• To vary PWM's duty call :cppmcpwm_set_duty and provide the duty value in %. Optionally, you may call :cppmcpwm_set_duty_in_us, if you prefer to set the duty in microseconds. Checking of currently set value is possible by calling :cppmcpwm_get_duty. Phase of the PWM signal may be altered by calling :cppmcpwm_set_duty_type. The duty is set individually for each A and B output using :cppmcpwm_generator_t in specific function calls. The duty value refers either to high or low output signal duration. This is configured when calling :cppmcpwm_init, as discussed in section Configure, and selecting one of options from :cppmcpwm_duty_type_t.

Note

Call function :cppmcpwm_set_duty_type every time after :cppmcpwm_set_signal_high or :cppmcpwm_set_signal_low to resume with previously set duty cycle.

Adjust

There are couple of ways to adjust a signal on the outputs and changing how the motor operates.

• Set specific PWM frequency by calling :cppmcpwm_set_frequency. This may be required to adjust to electrical or mechanical characteristics of particular motor and driver. To check what frequency is set, use function :cppmcpwm_get_frequency.
• Introduce a dead time between outputs A and B when they are changing the state to reverse direction of the motor rotation. This is to make up for on/off switching delay of the motor driver FETs. The dead time options are defined in :cppmcpwm_deadtime_type_t and enabled by calling :cppmcpwm_deadtime_enable. To disable this functionality call :cppmcpwm_deadtime_disable.
• Synchronize outputs of operator submodules, e.g. to get raising edge of PWM0A/B and PWM1A/B to start exactly at the same time, or shift them between each other by a given phase. Synchronization is triggered by SYNC SIGNALS shown on the block diagram <mcpwm_block_diagram> of the MCPWM above, and defined in :cppmcpwm_sync_signal_t. To attach the signal to a GPIO call :cppmcpwm_gpio_init. You can then enable synchronization with function :cppmcpwm_sync_configure. As input parameters provide MCPWM unit, timer to synchronize, the synchronization signal and a phase to delay the timer.

Note

Synchronization signals are referred to using two different enumerations. First one :cppmcpwm_io_signals_t is used together with function :cppmcpwm_gpio_init when selecting a GPIO as the signal input source. The second one :cppmcpwm_sync_signal_t is used when enabling or disabling synchronization with :cppmcpwm_sync_configure or :cppmcpwm_sync_disable.

• Vary the pattern of the A/B output signals by getting MCPWM counters to count up, down and up/down (automatically changing the count direction). Respective configuration is done when calling :cppmcpwm_init, as discussed in section Configure, and selecting one of counter types from :cppmcpwm_counter_type_t. For explanation of how A/B PWM output signals are generated, see {IDF_TARGET_NAME} Technical Reference Manual > Motor Control PWM (MCPWM) [PDF].

Synchronize

Each PWM timer has a synchronization input and a synchronization output. The synchronization input can be selected from other timers' synchronization outputs or GPIO signals via the GPIO matrix. Timer's synchronization signal can be generated from either the input sync signal or when the count value reaches peak/zero. Thus, the PWM timers can be chained together with their phase-locked. During synchronization, the PWM timer clock prescaler will reset its counter in order to synchronize the PWM timer clock.

The functionality is enabled in following steps:

1. Make sure the PWM timer and operator are already configured so that sync will inherit its config (count mode, freq and duty).
2. Enabling sync input of the timer by invoking :cppmcpwm_sync_configure, selecting desired signal input from :cppmcpwm_sync_signal_t, and setting the desired phase range from 0 to 999 which is mapped to 0%~99.9%. 0 means zero phase is applied and output is fired at the same time. And selecting desired counting direction.
3. Enabling one of sync event source from another timer or from external GPIO input.

To sync with another timer:

Enabling sync output of another timer by invoking :cppmcpwm_set_timer_sync_output and selecting desired event to generate sync output from :cppmcpwm_timer_sync_trigger_t.

To sync with GPIO positive edge input (negative edge requires :cppmcpwm_sync_invert_gpio_synchro):

Configuring GPIOs to act as the sync signal inputs by calling functions :cppmcpwm_gpio_init or :cppmcpwm_set_pin, which were described in section Configure.

It's normal condition that chained sync signal may have tens or even hundreds of nanoseconds of delay between each timer output due to hardware limitation. To sync two timers accurately it is required to have the third timer occupied to produce sync event that can be consumed parallel by other two timer, so that those two timer will have no delay between each other but have the same delay between the timer which provides events. Another solution is introducing an external GPIO event source so that all three timers can be synced together with no delay.

SOC_MCPWM_SWSYNC_CAN_PROPAGATE

Software sync event which triggered on one timer can be propagated to other timers on {IDF_TARGET_NAME}, which can be used as a tricky way to get all three timers synced without any extra requirement.

// configure timer0 as trigger source
mcpwm_set_timer_sync_output(MCPWM_UNIT_0, MCPWM_TIMER_0, MCPWM_SWSYNC_SOURCE_SYNCIN);
mcpwm_sync_config_t sync_conf = {
    .sync_sig = MCPWM_SELECT_TIMER0_SYNC,
    .timer_val = 0,
    .count_direction = MCPWM_TIMER_DIRECTION_UP,
};
mcpwm_sync_configure(TARGET_MCPWM_UNIT, MCPWM_TIMER_0, &sync_conf);
mcpwm_sync_configure(TARGET_MCPWM_UNIT, MCPWM_TIMER_1, &sync_conf);
mcpwm_sync_configure(TARGET_MCPWM_UNIT, MCPWM_TIMER_2, &sync_conf);
// then send soft sync event to timer0
mcpwm_timer_trigger_soft_sync(MCPWM_UNIT_0, MCPWM_TIMER_0);

If not required anymore, the capture functionality may be disabled with :cppmcpwm_sync_disable.

Capture

One of requirements of BLDC (Brushless DC, see figure below) motor control is sensing of the rotor position. To facilitate this task each MCPWM unit provides three sensing inputs together with dedicated hardware. The hardware is able to detect the input signal's edge and measure time between signals. As result the control software is simpler and the CPU power may be used for other tasks.

Example of Brushless DC Motor Control with MCPWM

The capture functionality may be used for other types of motors or tasks. The functionality is enabled in two steps:

1. Configuration of GPIOs to act as the capture signal inputs by calling functions :cppmcpwm_gpio_init or :cppmcpwm_set_pin, that were described in section Configure.
2. Enabling of the functionality itself by invoking :cppmcpwm_capture_enable_channel, selecting desired signal input from :cppmcpwm_capture_channel_id_t, setting the signal edge, signal count prescaler and user callback within :cppmcpwm_capture_config_t

Within the second step above a 32-bit capture timer is enabled. The timer runs continuously driven by the APB clock. The clock frequency is typically 80 MHz. On each capture event the capture timer’s value is stored in time-stamp register that may be then checked by calling :cppmcpwm_capture_signal_get_value. The edge of the last signal may be checked with :cppmcpwm_capture_signal_get_edge. Those data are also provided inside callback function as event data :cppcap_event_data_t

If not required anymore, the capture functionality may be disabled with :cppmcpwm_capture_disable_channel.

Capture prescale is different from other modules as it is applied to the input signal, not the timer source. Prescaler has maintained its own level state with the initial value set to low and is detecting the positive edge of the input signal to change its internal state. That means if two pairs of positive and negative edges are passed to input, the prescaler's internal state will change twice. ISR will report on this internal state change, not the input signal. For example, setting prescale to 2 will generate ISR callback on each positive edge of input if both edge is selected via :cppmcpwm_capture_config_t. Or each 2 positive edges of input if only one edge is selected though :cppmcpwm_capture_config_t.

Fault Handler

Each unit of the MCPWM is able to sense external signals with information about failure of the motor, the motor driver or any other device connected to the MCPWM. There are three fault inputs per unit that may be routed to user selectable GPIOs. The MCPWM may be configured to perform one of four predefined actions on A/B outputs when a fault signal is received:

• lock current state of the output
• set the output low
• set the output high
• toggle the output

The user should determine possible failure modes of the motor and what action should be performed on detection of particular fault, e.g. drive all outputs low for a brushed motor, or lock current state for a stepper motor, etc. As result of this action the motor should be put into a safe state to reduce likelihood of a damage caused by the fault.

The fault handler functionality is enabled in two steps:

1. Configuration of GPIOs to act as fault signal inputs. This is done in analogous way as described for capture signals in section above. It includes setting the signal level to trigger the fault as defined in :cppmcpwm_fault_input_level_t.
2. Initialization of the fault handler by calling either :cppmcpwm_fault_set_oneshot_mode or :cppmcpwm_fault_set_cyc_mode. These functions set the mode that MCPWM should operate once fault signal becomes inactive. There are two modes possible:

• State of MCPWM unit will be locked until reset - :cppmcpwm_fault_set_oneshot_mode.
• The MCPWM will resume operation once fault signal becoming inactive - :cppmcpwm_fault_set_cyc_mode.

The function call parameters include selection of one of three fault inputs defined in :cppmcpwm_fault_signal_t and specific action on outputs A and B defined in :cppmcpwm_action_on_pwmxa_t and :cppmcpwm_action_on_pwmxb_t.

Particular fault signal may be disabled at the runtime by calling :cppmcpwm_fault_deinit.

Carrier

The MCPWM has a carrier submodule used if galvanic isolation from the motor driver is required by passing the A/B output signals through transformers. Any of A and B output signals may be at 100% duty and not changing whenever motor is required to run steady at the full load. Coupling of non alternating signals with a transformer is problematic, so the signals are modulated by the carrier submodule to create an AC waveform, to make the coupling possible.

To use the carrier submodule, it should be first initialized by calling :cppmcpwm_carrier_init. The carrier parameters are defined in :cppmcpwm_carrier_config_t structure invoked within the function call. Then the carrier functionality may be enabled by calling :cppmcpwm_carrier_enable.

The carrier parameters may be then altered at a runtime by calling dedicated functions to change individual fields of the :cppmcpwm_carrier_config_t structure, like :cppmcpwm_carrier_set_period, :cppmcpwm_carrier_set_duty_cycle, :cppmcpwm_carrier_output_invert, etc.

This includes enabling and setting duration of the first pulse of the career with :cppmcpwm_carrier_oneshot_mode_enable. For more details, see {IDF_TARGET_NAME} Technical Reference Manual > Motor Control PWM (MCPWM) > PWM Carrier Submodule [PDF].

To disable carrier functionality call :cppmcpwm_carrier_disable.

Interrupts

Registering of the MCPWM interrupt handler is possible by calling :cppmcpwm_isr_register. Note if :cppmcpwm_capture_enable_channel is used then a default ISR routine will be installed hence please do not call this function to register any more.

Resolution

The default resolution for MCPWM group and MCPWM timer are configured to 10MHz and 1MHz in :cppmcpwm_init, which might be not enough for some applications. The driver also provides two APIs that can be used to override the default resolution: :cppmcpwm_group_set_resolution and :cppmcpwm_timer_set_resolution.

Note that, these two APIs won't update the frequency and duty automatically, to achieve that, one has to call :cppmcpwm_set_frequency and :cppmcpwm_set_duty accordingly.

To get PWM pulse that is below 15Hz, please set the resolution to a lower value. For high frequency PWM with limited step range, please set them with higher value.

Application Example

MCPWM example are located under: peripherals/mcpwm:

• Control of BLDC (brushless DC) motor with hall sensor feedback - peripherals/mcpwm/mcpwm_bldc_hall_control
• Brushed DC motor control - peripherals/mcpwm/mcpwm_brushed_dc_control
• Servo motor control - peripherals/mcpwm/mcpwm_servo_control
• HC-SR04 sensor with capture - peripherals/mcpwm/mcpwm_capture_hc_sr04

API Reference

inc/mcpwm_types.inc

inc/mcpwm.inc