TCD Counter Capture

Because the TCD counter is asynchronous to the system clock it is not possible to read out the counter value directly. It is possible to capture the TCD counter value, synchronized to the IO clock domain in two different ways.

The capture logic contains two separate capture blocks, CAPTUREA and CAPTUREB, that can capture and synchronize the TCD counter value to the IO clock domain. CAPTUREA/B can be triggered by input Event A/B or by software.

The capture values can be read by reading first TCD.CAPTURExL and then TCD.CAPTURExH registers.

Captures Triggered by Input Events

To enable capture on input Event, write a '1' to the ACTION bit in the respective Event Control x register (TCD.EVCTRL) when configuring an Event input.

When a capture has occurred, the TRIGA/B flag is raised in the Interrupt Flags register (TCD.INTFLAGS). The according TRIGA/B interrupt is executed if enabled by writing a '1' to the respective Trigger Interrupt x Enable bit (TRIGx) in the Interrupt Control register (TCD.INTCTRL). By polling TRIGx in TCD.INTFLAGS, the user knows that a CAPTUREx value is available, and can read out the value by reading first the TCD.CAPTURExL and then TCD.CAPTURxH registers.


In order to do PWM capture both event A and B should be connected to the same asynchronous event channel that contains the PWM signal. To get information on the PWM signal one event input should be configured to capture the rising edge of the signal. The other event input should be configured to capture the falling edge of the signal.

Capture Triggered by Software

Software can capture the TCD value by writing a '1' to respective Software Capture A/B Strobe bit (SCAPTUREx) in the Control E register (TCD.CTRLE). When this command is executed and the Command Ready bit (CMDRDY) in the Status register (TCD.STATUS) reads '1' again, the CAPTUREA/B value is available. It can now be read by reading first the TCD.CAPTURExL and then the TCD.CAPTURxH registers.

Using Capture together with input modes

The capture functionality can be used together with input modes. The same event will then both capture the counter value and trigger a change in the counter flow depending on the input mode selected.


If the user what to reset the counter when running in One Ramp mode whenever the input event capture the counter value this can be done by using input event B and setting INPUTCTRLB register to 0x08.