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Saturate EMA sum to PROD_W range to prevent arithmetic overflow on sustained inputs #2

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Saturate EMA sum to PROD_W range to prevent arithmetic overflow on sustained inputs #2

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21 changes: 19 additions & 2 deletions src/lowpass_ema.vhd
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Original file line number Diff line number Diff line change
Expand Up @@ -126,6 +126,13 @@ ARCHITECTURE rtl OF lowpass_ema IS
SIGNAL avg_rms : REAL := 0.0;
SIGNAL avg_real : REAL := 0.0;

CONSTANT EXTRA_W : NATURAL := 4; -- guard bits: max shift_left + 1 for the add
CONSTANT SAT_W : NATURAL := PROD_W + EXTRA_W;
CONSTANT SAT_MAX_V : signed(SAT_W -1 DOWNTO 0) := to_signed(2**(PROD_W-1) - 1, SAT_W);
CONSTANT SAT_MIN_V : signed(SAT_W -1 DOWNTO 0) := to_signed(-(2**(PROD_W-1)), SAT_W);

SIGNAL sum_wide : signed(SAT_W -1 DOWNTO 0);


BEGIN

Expand All @@ -144,8 +151,18 @@ ASSERT False REPORT "FULL_SCALE: " & real'image(FULL_SCALE) SEVERITY NOTE;
-- pragma translate_on


sum <= shift_left(resize(mult_data, PROD_W), MULT_DATA_SHIFT) +
shift_left(resize(mult_sum, PROD_W), MULT_SUM_SHIFT);
-- ============================================================================
-- BEFORE:
-- sum <= shift_left(resize(mult_data, PROD_W), MULT_DATA_SHIFT) +
-- shift_left(resize(mult_sum, PROD_W), MULT_SUM_SHIFT);
-- ============================================================================

sum_wide <= shift_left(resize(mult_data, SAT_W), MULT_DATA_SHIFT) +
shift_left(resize(mult_sum, SAT_W), MULT_SUM_SHIFT);

sum <= to_signed(2**(PROD_W-1) - 1, PROD_W) WHEN sum_wide > SAT_MAX_V ELSE
to_signed(-(2**(PROD_W-1)), PROD_W) WHEN sum_wide < SAT_MIN_V ELSE
resize(sum_wide, PROD_W);

sum_shift <= resize(shift_right(sum, SUM_SHIFT_W), PROD_W - SUM_SHIFT_W);

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