23.3.4.7 Amplitude, Offset and Fractional Compare Value

Instead of writing an absolute value to the compare registers (TCEn.CMPn, TCEn.CMPnBUF and TCEn.AMP), it is possible to write these as a fractional value between 0 and 2. The fractional value is in UQ1.15 format, ranging from 0 to 2 - 2^-15. The actual compare value is calculated based on the Period (TCEn.PER), Amplitude (TCEn.AMP), and Offset (TCEn.OFFSET) registers.

The UQ1.15 format will cause up to one-bit loss in precision for high period or amplitude values (> 0x8000) but allows multiplication >1, needed for Single slope where 100% duty cycle is achieved when CMPn > PER.

The Amplitude and Offset registers are used to set the range for the compare registers. The amplitude is calculated automatically from the TCEn.PER register if scaling (SCALE in TCEn.CTRLD) is enabled. The calculation occurs when the AMP register is written.

When the AMP register is written, the value is multiplied by the existing PER value and then stored in the AMP register. The multiplication is triggered when the high byte is written (TCEn.AMPH).

The data read from the AMP register are the value after the multiplication. The offset is automatically calculated based on the selected SCALEMODE for the following clock cycle.

Rewrite the TCEn.AMP register to ensure the identical relationship between the two registers if changing the period.

Figure 23-15. Write to Amplitude and Offset Registers

Table 23-5. Effective Amplitude and Offset in Scaled Mode
SCALEMODE	Amplitude^(1,2)	Offset⁽³⁾
CENTER	AMP_frac * PER	(PER - AMP)/2
BOTTOM	AMP_frac * PER	0
TOP	AMP_frac * PER	(PER - AMP)
TOPBOTTOM	AMP_frac * PER	(PER - AMP)
Note: If multiplication gives result > `0xFFFF`, AMP is set to `0xFFFF`. AMP_frac is the value written to the AMP register when scaling is enabled. Offset is never negative, even if AMP > PER.

When writing a fractional value to the TCEn.CMPn or TCEn.CMPnBUF registers, they are multiplied first with the amplitude (TCEn.AMP) register, and then the offset (TCEn.OFFSET) is added. The multiplication is triggered when the high byte is written (TCEn.CMPnH or TCEn.CMPnBUFH) and completes as the data are written. The figure below shows the functionality for the CMPn registers, which works identically for the CMPnBUF registers.

The data read from the compare and compare buffer register are the value after the multiplication.

When changing the Period or Amplitude registers, the TCEn.CMPn or TCEn.CMPnBUF registers must be rewritten to ensure the same relationship to the period and amplitude.

Figure 23-16. Writing Fractional Value to Compare Register

The effective stored CMPn values are shown in the table below.

Table 23-6. Effective Compare Value in Scaled Mode⁽¹⁾⁽²⁾
SCALEMODE	Scaled CMPn Value (AMPEN = `0`)	Scaled CMPn Value (AMPEN = `1`)
CENTER	CMP_frac * PER	CMP_frac * AMP + OFFSET
BOTTOM	CMP_frac * PER	CMP_frac * AMP
TOP	CMP_frac * PER	CMP_frac * AMP + OFFSET
TOPBOTTOM	CMP_frac * PER	CMP_{f rac} = `0x0000` : BOTTOM CMP_frac ≥ `0x8000` : PER CMP_frac ≥ `0x4000` : CMP_frac * AMP CMP_frac < `0x4000` : CMP_frac * AMP + OFFSET
Note: If multiplication gives result > `0xFFFF`, CMPn is set to `0xFFFF`. CMP_frac is the value written to the CMPn register when scaling is enabled.

Waveform Range with SCALE Enabled

When the scaled write is enabled, the values written to the TCEn.CMPn and TCEn.CMPnBUF are between 0 and 1, giving a duty cycle range between 0% and 100% of the PWM period. When using amplitude reduction, the compare registers are multiplied with the amplitude (TCEn.AMP) register as the range for the fractional value, and the generated compare values are adjusted with the offset (TCEn.OFFSET) register.

Examples below show the effective compare value in percentage of the period for the different scale modes where the amplitude is set to 90% of the period. The offsets are calculated according to Table 23-6. The black lines show the written compare value, identical for all cases, and the red line shows the effective compare value as a percentage of the period (PER).

Figure 23-17. Scaled Writes (Amplitude = 90%)