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<html>
<head>
<link rel="shortcut icon" href="./favicon.ico">
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<meta name="description" content="Adds/subtracts the signed `increment_value` to the signed `accumulated_value` when `increment_valid` is pulsed high *for one cycle*. A new increment may be added when `increment_done` pulses high, in the same cycle if necessary.">
<title>Accumulator Binary Saturating</title>
</head>
<body>
<p class="inline bordered"><b><a href="./Accumulator_Binary_Saturating.v">Source</a></b></p>
<p class="inline bordered"><b><a href="./legal.html">License</a></b></p>
<p class="inline bordered"><b><a href="./index.html">Index</a></b></p>
<h1>Signed Binary Accumulator, with Saturation</h1>
<p>Adds/subtracts the signed <code>increment_value</code> to the signed
<code>accumulated_value</code> when <code>increment_valid</code> is pulsed high <em>for one cycle</em>.
A new increment may be added when <code>increment_done</code> pulses high, in the same
cycle if necessary. </p>
<p>Pulsing <code>load_valid</code> high for one cycle replaces the <code>accumulated_value</code>
with the <code>load_value</code>. A new load can be done when <code>load_done</code> pulses high,
in the same cycle if necessary.</p>
<p>Pulsing <code>clear</code> high for one cycle puts the accumulator back at
<code>INITIAL_VALUE</code>. The accumulator can be cleared again once <code>clear_done</code>
pulses high, in the same cycle if necessary.</p>
<p>Deasserting <code>clock_enable</code> freezes the accumulator: new increments, loads,
and clears are ignored, the internal pipeline (if any) holds steady, and
all outputs remain static.</p>
<p>When chaining accumulators, which may happen if you are incrementing in
unusual bases where each digit has its own accumulator, AND the
<code>accumulated_value_carry_out</code> of the previous accumulator with the signal
fed to the <code>increment_valid</code> input of the next accumulator. The
<code>increment_carry_in</code> is kept for generality.</p>
<h2>Saturation</h2>
<p>If the increment <strong>or the load</strong> would cause the accumulator to go past the
signed minimum or maximum limits, the accumulator will saturate at the
nearest limit value and also raise one or more of the min/max limit signals
until the next operation. <strong>The maximum limit must be greater or equal than
the minimum limit.</strong> If the limits are reversed, such that limit_max
< limit_min, the result will be meaningless.</p>
<h2>Pipelining and Concurrency</h2>
<p>This module is pipelined since we are chaining adder/subtractors together
inside the
<a href="./Adder_Subtractor_Binary_Saturating.html">Adder_Subtractor_Binary_Saturating</a>
module, so the total carry-chain is twice as long as expected, plus 2 more
bits to avoid overflow. Most of the time, this will take longer than your
clock cycle since the carry-chain of arithmetic logic is often a limiting
factor in timing closure.</p>
<p>We can't retime a pipeline from outside since there is a loop, so we
pipeline inside the loop here, and let that retime across the carry-chains.
The price to pay is a latency of EXTRA_PIPE_STAGES+1 cycles between an
increment, load, or clear, and the corresponding "done" signal. This
latency is why the input pulses must be asserted <em>for only one cycle</em> when
<code>EXTRA_PIPE_STAGES</code> is greater than zero, then wait until the command has
completed before pulsing again, else any latter increments or loads will
override the previous ones (since the accumulated_value will have not yet
updated).</p>
<pre>
`default_nettype none
module <a href="./Accumulator_Binary_Saturating.html">Accumulator_Binary_Saturating</a>
#(
parameter EXTRA_PIPE_STAGES = -1,
parameter WORD_WIDTH = 0,
parameter [WORD_WIDTH-1:0] INITIAL_VALUE = 0
)
(
input wire clock,
input wire clock_enable,
input wire clear,
output wire clear_done,
input wire increment_carry_in,
input wire increment_add_sub, // 0/1 --> +/-
input wire [WORD_WIDTH-1:0] increment_value,
input wire increment_valid,
output wire increment_done,
input wire [WORD_WIDTH-1:0] load_value,
input wire load_valid,
output wire load_done,
input wire [WORD_WIDTH-1:0] limit_max,
input wire [WORD_WIDTH-1:0] limit_min,
output wire [WORD_WIDTH-1:0] accumulated_value,
output wire accumulated_value_carry_out,
output wire [WORD_WIDTH-1:0] accumulated_value_carries,
output wire accumulated_value_at_limit_max,
output wire accumulated_value_over_limit_max,
output wire accumulated_value_at_limit_min,
output wire accumulated_value_under_limit_min
);
localparam WORD_ZERO = {WORD_WIDTH{1'b0}};
</pre>
<p>Here, we pipeline the inputs so that all signals further down are in sync,
and to place the register pipeline inside the loop formed by the
Adder_Subtractor_Binary_Saturating and the output register, so we can have
forward retiming move it into the Adder_Subtractor_Binary_Saturating logic.
(Backwards retiming is more difficult, and not supported by Vivado
post-synth optimizations)</p>
<pre>
wire clear_pipelined;
wire increment_carry_in_pipelined;
wire increment_add_sub_pipelined;
wire [WORD_WIDTH-1:0] increment_value_pipelined;
wire increment_valid_pipelined;
wire [WORD_WIDTH-1:0] load_value_pipelined;
wire load_valid_pipelined;
wire [WORD_WIDTH-1:0] limit_max_pipelined;
wire [WORD_WIDTH-1:0] limit_min_pipelined;
wire [WORD_WIDTH-1:0] accumulated_value_pipelined;
generate
if (EXTRA_PIPE_STAGES == 0) begin : gen_no_pipe
assign clear_pipelined = clear;
assign increment_carry_in_pipelined = increment_carry_in;
assign increment_add_sub_pipelined = increment_add_sub;
assign increment_value_pipelined = increment_value;
assign increment_valid_pipelined = increment_valid;
assign load_value_pipelined = load_value;
assign load_valid_pipelined = load_valid;
assign limit_max_pipelined = limit_max;
assign limit_min_pipelined = limit_min;
assign increment_carry_in_pipelined = increment_carry_in;
assign accumulated_value_pipelined = accumulated_value;
end
else if (EXTRA_PIPE_STAGES > 0) begin: gen_extra_pipe
localparam PIPELINE_WIDTH = (WORD_WIDTH * 5) + 5;
localparam PIPELINE_WORD_ZERO = {PIPELINE_WIDTH{1'b0}};
localparam PIPELINE_ZERO = {EXTRA_PIPE_STAGES{PIPELINE_WORD_ZERO}};
<a href="./Register_Pipeline.html">Register_Pipeline</a>
#(
.WORD_WIDTH (PIPELINE_WIDTH),
.PIPE_DEPTH (EXTRA_PIPE_STAGES),
// concatenation of each stage initial/reset value
.RESET_VALUES (PIPELINE_ZERO)
)
accumulator_pipeline
(
.clock (clock),
.clock_enable (clock_enable),
.clear (1'b0),
.parallel_load (1'b0),
.parallel_in (PIPELINE_ZERO),
// verilator lint_off PINCONNECTEMPTY
.parallel_out (),
// verilator lint_on PINCONNECTEMPTY
.pipe_in ({limit_max, limit_min, increment_valid, increment_value, load_valid, load_value, increment_add_sub, increment_carry_in, accumulated_value, clear}),
.pipe_out ({limit_max_pipelined, limit_min_pipelined, increment_valid_pipelined, increment_value_pipelined, load_valid_pipelined, load_value_pipelined, increment_add_sub_pipelined, increment_carry_in_pipelined, accumulated_value_pipelined, clear_pipelined})
);
end
endgenerate
</pre>
<p><strong>After this point, only use the pipelined inputs.</strong></p>
<p>If we are <em>loading</em> then substitute the <code>accumulated_value</code> and <code>carry_in</code> with
zero, and the <code>increment</code> with the <code>load_value</code>.
If we are <em>clearing</em> then substitute the <code>accumulated_value</code> and <code>carry_in</code> with
zero, and the <code>increment</code> with the <code>INITIAL_VALUE</code>.
Converting a load or clear to an addition to zero prevents us from loading
a value outside the given limits, which could really upset things in the
enclosing logic, and will set the output status bits correctly.</p>
<pre>
reg gate_accumulated_value = 1'b0;
always @(*) begin
gate_accumulated_value = (load_valid_pipelined == 1'b1) || (clear_pipelined == 1'b1);
end
wire [WORD_WIDTH-1:0] accumulated_value_gated;
wire increment_carry_in_gated;
<a href="./Annuller.html">Annuller</a>
#(
.WORD_WIDTH (WORD_WIDTH + 1),
.IMPLEMENTATION ("AND")
)
gate_accumulated
(
.annul (gate_accumulated_value == 1'b1),
.data_in ({accumulated_value_pipelined, increment_carry_in_pipelined}),
.data_out ({accumulated_value_gated, increment_carry_in_gated})
);
reg [WORD_WIDTH-1:0] increment_selected = WORD_ZERO;
always @(*) begin
increment_selected = (load_valid_pipelined == 1'b1) ? load_value_pipelined : increment_value_pipelined;
increment_selected = (clear_pipelined == 1'b1) ? INITIAL_VALUE : increment_selected;
end
</pre>
<p>Apply the increment to the current accumulator value, or the load value to
an accumulator value of zero, or the initial value to an accumulator value
of zero, all with saturation.</p>
<pre>
wire [WORD_WIDTH-1:0] incremented_value_internal;
wire accumulated_value_carry_out_internal;
wire [WORD_WIDTH-1:0] accumulated_value_carries_internal;
wire accumulated_value_at_limit_max_internal;
wire accumulated_value_over_limit_max_internal;
wire accumulated_value_at_limit_min_internal;
wire accumulated_value_under_limit_min_internal;
<a href="./Adder_Subtractor_Binary_Saturating.html">Adder_Subtractor_Binary_Saturating</a>
#(
.WORD_WIDTH (WORD_WIDTH)
)
add_increment
(
.limit_max (limit_max_pipelined),
.limit_min (limit_min_pipelined),
.add_sub (increment_add_sub_pipelined), // 0/1 -> A+B/A-B
.carry_in (increment_carry_in_gated),
.A (accumulated_value_gated),
.B (increment_selected),
.sum (incremented_value_internal),
.carry_out (accumulated_value_carry_out_internal),
.carries (accumulated_value_carries_internal),
.at_limit_max (accumulated_value_at_limit_max_internal),
.over_limit_max (accumulated_value_over_limit_max_internal),
.at_limit_min (accumulated_value_at_limit_min_internal),
.under_limit_min (accumulated_value_under_limit_min_internal)
);
</pre>
<p>Then, update the accumulator register and other outputs sychronized to
it. Update the registers if load or increment or the clear pulse is valid. </p>
<pre>
reg enable_output = 1'b0;
always @(*) begin
enable_output = (increment_valid_pipelined == 1'b1) || (load_valid_pipelined == 1'b1) || (clear_pipelined == 1'b1);
enable_output = (enable_output == 1'b1) && (clock_enable == 1'b1);
end
<a href="./Register.html">Register</a>
#(
.WORD_WIDTH (WORD_WIDTH),
.RESET_VALUE (INITIAL_VALUE)
)
accumulator
(
.clock (clock),
.clock_enable (enable_output),
.clear (1'b0),
.data_in (incremented_value_internal),
.data_out (accumulated_value)
);
<a href="./Register.html">Register</a>
#(
.WORD_WIDTH (WORD_WIDTH),
.RESET_VALUE (WORD_ZERO)
)
carries
(
.clock (clock),
.clock_enable (enable_output),
.clear (1'b0),
.data_in (accumulated_value_carries_internal),
.data_out (accumulated_value_carries)
);
localparam STATUS_BITS_COUNT = 5;
localparam STATUS_BITS_ZERO = {STATUS_BITS_COUNT{1'b0}};
<a href="./Register.html">Register</a>
#(
.WORD_WIDTH (STATUS_BITS_COUNT),
.RESET_VALUE (STATUS_BITS_ZERO)
)
status_bits
(
.clock (clock),
.clock_enable (enable_output),
.clear (1'b0),
.data_in ({accumulated_value_carry_out_internal, accumulated_value_at_limit_max_internal, accumulated_value_over_limit_max_internal, accumulated_value_at_limit_min_internal, accumulated_value_under_limit_min_internal}),
.data_out ({accumulated_value_carry_out, accumulated_value_at_limit_max, accumulated_value_over_limit_max, accumulated_value_at_limit_min, accumulated_value_under_limit_min})
);
</pre>
<p>Finally, output the "done" signals, which are the pipelined command pulses
plus one register delay to synchronize them to the updated
<code>accumulated_value</code> and related data.</p>
<pre>
<a href="./Register.html">Register</a>
#(
.WORD_WIDTH (1),
.RESET_VALUE (1'b0)
)
for_clear
(
.clock (clock),
.clock_enable (clock_enable),
.clear (1'b0),
.data_in (clear_pipelined),
.data_out (clear_done)
);
<a href="./Register.html">Register</a>
#(
.WORD_WIDTH (1),
.RESET_VALUE (1'b0)
)
for_increment
(
.clock (clock),
.clock_enable (clock_enable),
.clear (1'b0),
.data_in (increment_valid_pipelined),
.data_out (increment_done)
);
<a href="./Register.html">Register</a>
#(
.WORD_WIDTH (1),
.RESET_VALUE (1'b0)
)
for_load
(
.clock (clock),
.clock_enable (clock_enable),
.clear (1'b0),
.data_in (load_valid_pipelined),
.data_out (load_done)
);
endmodule
</pre>
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