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CMSIS arm_rms_q15 defect

Aaron Scott
Note: This was originally posted on 13th July 2012 at http://forums.arm.com

There is a bug in arm_rms_q15 that saturates the sum before dividing for the mean.

00107 /* Truncating and saturating the accumulator to 1.15 format */
00108 sum = __SSAT((q31_t) (sum >> 15), 16);
00109
00110 in1 = (q15_t) (sum / blockSize);
00111
00112 /* Store the result in the destination */
00113 arm_sqrt_q15(in1, pResult);
and
00136 /* Truncating and saturating the accumulator to 1.15 format */
00137 sum = __SSAT((q31_t) (sum >> 15), 16);
00138
00139 in = (q15_t) (sum / blockSize);
00140
00141 /* Store the result in the destination */
00142 arm_sqrt_q15(in, pResult);
should be changed to something like this:
    /* Truncating and saturating the accumulator to 1.15 format */
    in = (q31_t) (sum >> 15);

    in1 = __SSAT((in / blockSize), 16);

    /* Store the result in the destination */
    arm_sqrt_q15(in1, pResult);
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