23.3.3.6 Split Mode - Two 8-Bit Timer/Counters

Split Mode Overview

A Split mode is provided to double the number of timers and PWM channels in the TCA. In this Split mode, the 16-bit timer/counter acts as two separate 8-bit timers, which each have three compare channels for PWM generation. The Split mode will only work with single-slope down-count. Event-controlled operation is not supported in Split mode.

The figure below shows single-slope PWM generation in Split mode. The waveform generator output is cleared at BOTTOM and set on the compare match between the counter value (TCAn.CNT) and the Compare n (TCAn.CMPn) register.

CMPn = BOTTOM or CMPn > TOP will produce a static low signal on WOn.

Figure 23-15. Single-Slope Pulse-Width Modulation in Split mode
Note: The maximum duty-cycle of the waveform output is TOP/(TOP+1).

Activating Split mode changes the functionality of some registers and register bits. The modifications are described in a separate register map (see Register Summary - Split Mode).

Split Mode Differences Compared to Normal Mode

  • Count:
    • Down-count only
    • Low Byte Timer Counter (TCAn.LCNT) register and High Byte Timer Counter (TCAn.HCNT) register are independent
  • Waveform generation:
    • Single-slope PWM only (WGMODE = SINGLESLOPE in the TCAn.CTRLB register)
  • Interrupt:
    • No change for Low Byte Timer Counter (TCAn.LCNT) register
    • Underflow interrupt for High Byte Timer Counter (TCAn.HCNT) register
    • No compare interrupt or flag for High Byte Compare n (TCAn.HCMPn) register
  • Event Actions: Not compatible
  • Buffer registers and buffer valid flags: Unused
  • Register Access: Byte access to all registers

Block Diagram

Figure 23-16. Timer/Counter Block Diagram Split Mode

Split Mode Initialization

When shifting between Normal mode and Split mode, the functionality of some registers and bits changes, but their values do not. For this reason, disabling the peripheral (ENABLE = 0 in the TCAn.CTRLA register) and doing a hard Reset (CMD = RESET in the TCAn.CTRLESET register) is recommended when changing the mode to avoid unexpected behavior.

To start using the timer/counter in basic Split mode after a hard Reset, follow these steps:
  1. Enable Split mode by writing a ‘1’ to the Split mode enable (SPLITM) bit in the Control D (TCAn.CTRLD) register.
  2. Write a TOP value to the Period (TCAn.PER) registers.
  3. Enable the peripheral by writing a ‘1’ to the Enable (ENABLE) bit in the Control A (TCAn.CTRLA) register.

    The counter will start counting clock ticks according to the prescaler setting in the Clock Select (CLKSEL) bit field in the TCAn.CTRLA register.

  4. The counter values can be read from the Counter bit field in the Counter (TCAn.CNT) registers.