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descramble: Use lsfr counter for unlock_timer 

The critical path often includes the unlock timer. Switch to an lfsr
implementation. This saves around 20 LUTs and reduces the critical path
from the carry chain (and the or reduction) to just the and reduction.

Signed-off-by: Sean Anderson <seanga2@gmail.com>
This commit is contained in:
Sean Anderson 2022-10-30 20:50:18 -04:00
parent 3ec1f4d77d
commit 1b47635644
1 changed files with 22 additions and 10 deletions
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32
rtl/descramble.v
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@ -34,14 +34,25 @@ module descramble (
initial idle_counter = CONSECUTIVE_IDLES; initial idle_counter = CONSECUTIVE_IDLES;
/* /*
* We use a LFSR for the unlock counter in order to relax the timing
* requirements. Although we could use a 16-bit register, we use
* a 17-bit one to reduce the number of taps we need. Values were
* generated with the following python script:
*
* lfsr = 0x1ffff
* for _ in range(2**17 - cycles - 1):
* lfsr = ((lfsr << 1) & 0x1ffff) | (((lfsr >> 16) & 1) ^ ((lfsr >> 13) & 1))
*
* The amount of time without recieving consecutive idles before we * The amount of time without recieving consecutive idles before we
* unlock. This must be greater than 361us (7.2.3.3(f)). 2^16-1 works * unlock. This must be greater than 361us (7.2.3.3(f)), which is
* out to around 524us at 125MHz. * 45125 cycles at 125MHz.
*/ */
localparam UNLOCK_TIME = 16'hffff; localparam UNLOCK_VALUE = 17'h29fc;
/* 5us, or around one minimum-length packet plus some extra */ /* One 9000-byte jumbo frame plus an extra preamble */
localparam TEST_UNLOCK_TIME = 16'd625; localparam JUMBO_UNLOCK_VALUE = 17'h12d84;
reg [15:0] unlock_counter, unlock_counter_next; /* One minimum-length packet plus some extra (5us or 625 cycles) */
localparam TEST_UNLOCK_VALUE = 17'h11077;
reg [16:0] unlock_counter, unlock_counter_next;
always @(*) begin always @(*) begin
ldd = { lfsr[8] ^ lfsr[10], lfsr[7] ^ lfsr[9] }; ldd = { lfsr[8] ^ lfsr[10], lfsr[7] ^ lfsr[9] };
@ -88,9 +99,10 @@ module descramble (
locked_next = 1; locked_next = 1;
unlock_counter_next = unlock_counter; unlock_counter_next = unlock_counter;
if (relock) begin if (relock) begin
unlock_counter_next = test_mode ? TEST_UNLOCK_TIME : UNLOCK_TIME; unlock_counter_next = test_mode ? TEST_UNLOCK_VALUE : UNLOCK_VALUE;
end else if (|unlock_counter) begin end else if (!(&unlock_counter)) begin
unlock_counter_next = unlock_counter - 1; unlock_counter_next[0] = unlock_counter[16] ^ unlock_counter[13];
unlock_counter_next[16:1] = unlock_counter[15:0];
end else begin end else begin
locked_next = 0; locked_next = 0;
end end
@ -109,7 +121,7 @@ module descramble (
lfsr <= 0; lfsr <= 0;
idle_counter <= CONSECUTIVE_IDLES; idle_counter <= CONSECUTIVE_IDLES;
relock <= 0; relock <= 0;
unlock_counter <= 0; unlock_counter <= 17'h1ffff;
locked <= 0; locked <= 0;
descrambled_valid <= 0; descrambled_valid <= 0;
end end