Accumulation

By default, conversion results are stored in the Result register as right-adjusted 16-bit values. The eight Least Significant bits (LSbs) are then located in the low byte of the Result register. By writing a ‘1’ to the Left Adjust Result (LEFTADJ) bit in the Control A (ADCn.CTRLA) register, the values will be left-adjusted by placing the eight Most Significant bits (MSbs) in the high byte of the Result register.

The result from multiple consecutive conversions can be accumulated. The number of samples to be accumulated is specified by the Sample Accumulation Number Select (SAMPNUM) bit field in the Control B (ADCn.CTRLB) register. When accumulating more than 16 samples, the result might be too large to match the 16-bit Result register size. To avoid overflow, the LSbs of the result are truncated to fit within the available register size.

The two following tables show how the Result (ADCn.RES) register value is stored for single-ended and differential conversions.

Table 1. Result Format in Single-Ended Mode
Accumulations	LEFTADJ	RES[15:8]									RES[7:0]
Accumulations	LEFTADJ	Bit 15	Bit 14	Bit 13	Bit 12	Bit 11	Bit 10	Bit 9	Bit 8	Bit 7	Bit 6	Bit 5	Bit 4	Bit 3	Bit 2	Bit 1	Bit 0
1	`0`	`0`	`0`	`0`	`0`	Conversion [11:0]
1	`1`	Conversion [11:0]												`0`	`0`	`0`	`0`
2	`0`	`0`	`0`	`0`	Accumulation [12:0]
2	`1`	Accumulation [12:0]													`0`	`0`	`0`
4	`0`	`0`	`0`	Accumulation [13:0]
4	`1`	Accumulation [13:0]														`0`	`0`
8	`0`	`0`	Accumulation [14:0]
8	`1`	Accumulation [14:0]															`0`
16	`0`	Accumulation [15:0]
16	`1`	Accumulation [15:0]
32, 64, 128	`0`	Truncated Accumulation [15:0]
32, 64, 128	`1`	Truncated Accumulation [15:0]

Table 2. Result Format in Differential Mode
Accumulations	LEFTADJ	RES[15:8]									RES[7:0]
Accumulations	LEFTADJ	Bit 15	Bit 14	Bit 13	Bit 12	Bit 11	Bit 10	Bit 9	Bit 8	Bit 7	Bit 6	Bit 5	Bit 4	Bit 3	Bit 2	Bit 1	Bit 0
1	`0`	Sign extension				Signed conversion [11:0]
1	`1`	Signed conversion [11:0]												`0`	`0`	`0`	`0`
2	`0`	Sign extension			Signed accumulation [12:0]
2	`1`	Signed accumulation [12:0]													`0`	`0`	`0`
4	`0`	Sign extension		Signed accumulation [13:0]
4	`1`	Signed accumulation [13:0]														`0`	`0`
8	`0`	Sign extension	Signed accumulation [14:0]
8	`1`	Signed accumulation [14:0]															`0`
16	`0`	Signed accumulation [15:0]
16	`1`	Signed accumulation [15:0]
32, 64, 128	`0`	Signed truncated accumulation [15:0]
32, 64, 128	`1`	Signed truncated accumulation [15:0]