rtl: Document calculation of LFSRs

Document how the LFSR initial values are generated. Also fix several
off-by-one errors (where the documented cycles was not quite right).

Signed-off-by: Sean Anderson <seanga2@gmail.com>

rtl/descramble.v

@@ -38,21 +38,19 @@ module descramble (
 	 * 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:
+	 * generated with:
 	 *
-	 * lfsr = 0x1ffff
+	 * $ scripts/lfsr.py 0x12000 90460 45125 625
-	 * 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 receiving consecutive idles before we
 	 * unlock. This must be greater than 361us (7.2.3.3(f)), which is
 	 * 45125 cycles at 125MHz.
 	 */
-	localparam UNLOCK_VALUE = 17'h29fc;
+	localparam UNLOCK_VALUE = 17'h05b08;
 	/* One 9000-byte jumbo frame plus an extra preamble */
-	localparam JUMBO_UNLOCK_VALUE = 17'h12d84;
+	localparam JUMBO_UNLOCK_VALUE = 17'h053f9;
 	/* One minimum-length packet plus some extra (5us or 625 cycles) */
-	localparam TEST_UNLOCK_VALUE = 17'h11077;
+	localparam TEST_UNLOCK_VALUE = 17'h020ef;
 	reg [16:0] unlock_counter, unlock_counter_next;
 
 	always @(*) begin

rtl/led_blinker.v

@@ -21,9 +21,14 @@ module led_blinker (
 
 	parameter LEDS		= 2;
 
+	/*
+	 * $ scripts/lfsr.py 0x140000 0x1fffff 16
+	 *
+	 * 16.78 ms
+	 */
 	localparam TIMER_RESET		= 21'h1ffffe;
-	/* 16 cycles before the end */
+	/* 16 cycles */
-	localparam TEST_TIMER_RESET	= 21'h0ccccf;
+	localparam TEST_TIMER_RESET	= 21'h13333f;
 
 	reg active, active_next;
 	reg [LEDS - 1:0] out_next, triggered, triggered_next;
@@ -50,7 +55,7 @@ module led_blinker (
 				active_next = 1;
 				out_next = triggered;
 			end
-			lfsr_next = test_mode ? 21'hCCCCF : 21'h1FFFFE;
+			lfsr_next = test_mode ? TEST_TIMER_RESET : TIMER_RESET;
 		end
 	end
 

rtl/phy_core.v

@@ -88,14 +88,13 @@ module phy_core (
 	);
 
 	/*
-	 * LFSR counter; see descramble.v for details on how these values were
+	 * $ scripts/lfsr.py 0x12000 41250 16
-	 * generated.
          *
-	 * 50000 cycles or 400 us at 125MHz
+	 * 41250 cycles or 330 us at 125MHz
 	 */
-	localparam STABILIZE_VALUE = 17'h1ac86;
+	localparam STABILIZE_VALUE = 17'h1590c;
 	/* 16 cycles; there's no instability while testing */
-	localparam TEST_STABILIZE_VALUE = 17'h11c71;
+	localparam TEST_STABILIZE_VALUE = 17'h038e3;
 
 	reg link_status_next;
 	initial link_status = 0;

rtl/uart_rx.v

@@ -25,10 +25,14 @@ module uart_rx (
 	output reg frame_error
 );
 
-	/* 1085 cycles, for 115200 baud with a 125 MHz clock */
+	/*
+	 * $ scripts/lfsr.py 0x500 1085 543 31 16
+	 *
+	 * 115200 baud with a 125 MHz clock
+	 */
 	parameter SLOW_FULL	= 11'h78c;
 	parameter SLOW_HALF	= 11'h202;
-	/* 31 cycles, for 4M baud with a 125 MHz clock */
+	/* 4M baud with a 125 MHz clock */
 	parameter FAST_FULL	= 11'h68e;
 	parameter FAST_HALF	= 11'h34c;
 

rtl/uart_tx.v

@@ -20,9 +20,13 @@ module uart_tx (
 	input high_speed
 );
 
-	/* 1085 cycles, for 115200 baud with a 125 MHz clock */
+	/*
+	 * $ scripts/lfsr.py 0x500 1085 31
+	 *
+	 * 115200 baud with a 125 MHz clock
+	 */
 	parameter SLOW_VALUE	= 11'h78c;
-	/* 31 cycles, for 4M baud with a 125 MHz clock */
+	/* 4M baud with a 125 MHz clock */
 	parameter FAST_VALUE	= 11'h68e;
 
 	reg ready_next;