[hdl] [bugfix] fix (again) offset calculation

Previous fix did not correctly account for when the most significant bit of a.frac was '1', as it would incorrectly consider it as a negative number. Also updated the header notes.

[hdl] [bugfix] fix (again) offset calculation
Previous fix did not correctly account for when the most significant bit of a.frac was '1', as it would incorrectly consider it as a negative number. Also updated the header notes.
24274099 · Dimitris Lampridis · 31e4f6f5 · 24274099
Commit 24274099 authored Sep 29, 2019 by Dimitris Lampridis
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tdc_ts_addsub.vhd hdl/rtl/tdc_ts_addsub.vhd +8 -7

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--- a/hdl/rtl/tdc_ts_addsub.vhd
+++ b/hdl/rtl/tdc_ts_addsub.vhd
@@ -6,17 +6,19 @@
 -- Author     : Tomasz Wlostowski
 -- Company    : CERN
 -- Created    : 2011-08-29
-- Last update: 2019-09-26
+-- Last update: 2019-09-29
 -- Platform   : FPGA-generic
 -- Standard   : VHDL'93
 -------------------------------------------------------------------------------
 -- Description: Pipelined timestamp adder with re-normalization of the result.
 -- Adds a to b, producing normalized timestamp q. A timestmap is normalized when
-- the 0 <= frac < 2**g_frac_bits, 0 <= coarse <= g_coarse_range-1 and utc >= 0.
+-- the 0 <= frac < g_frac_range, 0 <= coarse <= g_coarse_range-1 and utc >= 0.
 -- For correct operation of renormalizer, input timestamps must meet the
 -- following constraints:
-- 1. 0 <= (a/b)_frac_i <= 2**g_frac_bits-1
-- 2. -g_coarse_range+1 <= (a_coarse_i + b_coarse_i) <= 3*g_coarse_range-1
+-- 1. 0 <= a_tai + b_tai < 2**32 - 1
+-- 2. 0 <= a_frac < g_frac_range
+-- 3. -g_frac_range / 2 <= b_frac < g_frac_range / 2
+-- 4. -g_coarse_range+1 <= (a_coarse + b_coarse) <= 3*g_coarse_range-1
 -------------------------------------------------------------------------------
 --
 -- Copyright (c) 2011 CERN / BE-CO-HT
@@ -101,10 +103,9 @@ begin  -- rtl
        sums(0).slope <= a_i.slope;
        sums(0).meta  <= a_i.meta;

-        sums(0).tai    <= resize(signed(a_i.tai), 33) + resize(signed(b_i.tai), 33);
-        sums(0).frac   <= resize(signed(a_i.frac), 16) + resize(signed(b_i.frac), 16);
+        sums(0).tai    <= signed(resize(unsigned(a_i.tai) + unsigned(b_i.tai), 33));
        sums(0).coarse <= signed(a_i.coarse) + signed(b_i.coarse);
-
+        sums(0).frac   <= signed(resize(signed('0' & a_i.frac), 16) + resize(signed(b_i.frac), 16));
      else
        pipe(0) <= '0';
      end if;