14.4.3.2 LFSR – Linear Feedback Shift Register

The Linear Feedback Shift register (LFSR) is a pseudorandom number generator that provides 15-bit values that can be used in applications to modify either the duty cycle and/or period by a small amount to dither the corresponding switching edges of the application circuit’s power transistors. This dithering can be useful in reducing peak EMI (Electromagnetic Interference) emissions.

Each read of the LFSR register will result in a new update of the LFSR value. The LFSR initializes at a Power-on Reset to 0x0000, and for successive reads it follows the deterministic sequence shown in Table 14-40. It has the equivalent circuit, as shown in Figure 14-34, which implements a Fibonacci form LFSR based on the primitive polynomial: x¹⁵ + x¹⁴ + 1 over GF(2). The circuit is modified by a Zero-Detect circuit that causes the 0x0000 value to be followed by 0x0001. Subsequent reads of the LFSR will cycle through all 15-bit values, other than 0x0000, before repeating. The high bit of the LFSR output is always ‘0’.

If the same LFSR value is to be used for multiple calculations, then the value read from the LFSR register should be saved in a temporary location for this purpose.

Successive readings of one particular bit of the LFSR forms a Pseudonoise (PN) sequence with an impulse auto-correlation function (shifted versions of the PN sequence are uncorrelated) and may be useful for dithering applications. The entire 15-bit LFSR value has auto-correlation properties that may be undesirable in some applications as a source of pseudorandom noise; its use should be validated in the end application.