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Add comment on why partial multipliers are 18x18

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This commit is contained in:
Eddie Hung 2019-10-04 22:30:14 -07:00
parent 792cd31052
commit 6c5e1234e1
1 changed files with 8 additions and 4 deletions
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12
techlibs/xilinx/synth_xilinx.cc
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@ -342,10 +342,14 @@ struct SynthXilinxPass : public ScriptPass
if (check_label("map_dsp", "(skip if '-nodsp')")) {
if (!nodsp || help_mode) {
// NB: Xilinx multipliers are signed only
run("techmap -map +/mul2dsp.v -map +/xilinx/dsp_map.v -D DSP_A_MAXWIDTH=25 -D DSP_A_MAXWIDTH_PARTIAL=18 -D DSP_B_MAXWIDTH=18 "
"-D DSP_A_MINWIDTH=2 -D DSP_B_MINWIDTH=2 " // Blocks Nx1 multipliers
"-D DSP_Y_MINWIDTH=9 " // UG901 suggests small multiplies are those 4x4 and smaller
"-D DSP_SIGNEDONLY=1 -D DSP_NAME=$__MUL25X18");
run("techmap -map +/mul2dsp.v -map +/xilinx/dsp_map.v -D DSP_A_MAXWIDTH=25 "
"-D DSP_A_MAXWIDTH_PARTIAL=18 -D DSP_B_MAXWIDTH=18 " // Partial multipliers are intentionally
// limited to 18x18 in order to take
// advantage of the (PCOUT << 17) -> PCIN
// dedicated cascade chain capability
"-D DSP_A_MINWIDTH=2 -D DSP_B_MINWIDTH=2 " // Blocks Nx1 multipliers
"-D DSP_Y_MINWIDTH=9 " // UG901 suggests small multiplies are those 4x4 and smaller
"-D DSP_SIGNEDONLY=1 -D DSP_NAME=$__MUL25X18");
run("select a:mul2dsp");
run("setattr -unset mul2dsp");
run("opt_expr -fine");