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phy: Improve clock accuracy at 0.5 dividers at the cost of some jitter

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This commit is contained in:
Markus Koch 2024-07-15 13:34:54 +02:00
parent 21a0d0e69a
commit 7379df7b74
1 changed files with 8 additions and 2 deletions
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10
trashernet/trashernet_phy.vhd
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@ -297,8 +297,11 @@ begin
-- -------------------------------------------------------------------------
transmitter : block
constant TX_STB_CNT_MAX : integer := integer(round(real(F_CLK) / real((F_ETH * 2)))) - 1;
signal tx_stb_cnt : integer range 0 to TX_STB_CNT_MAX;
constant TX_STB_CNT_IDEAL : real := real(F_CLK) / real((F_ETH * 2));
constant TX_STB_SKIP_ERROR : real := abs (round(TX_STB_CNT_IDEAL) - TX_STB_CNT_IDEAL);
constant TX_STB_CNT_MAX : integer := integer(round(TX_STB_CNT_IDEAL + 0.25)) - 1; -- Round up starting for error > 0.25
constant TX_STB_SKIP_SECOND : boolean := TX_STB_SKIP_ERROR >= 0.25; -- Skip one clock cycle every other symbol to hit clock rate in between when the divider is close to x.5
signal tx_stb_cnt : integer range 0 to TX_STB_CNT_MAX;
type tx_state_t is (IDLE, NLP, TX, IPG);
signal tx_state : tx_state_t;
@ -368,6 +371,9 @@ begin
if tx_stb_cnt = 0 then
tx_stb_cnt <= TX_STB_CNT_MAX;
if TX_STB_SKIP_SECOND and bit_stage = '1' then
tx_stb_cnt <= TX_STB_CNT_MAX - 1;
end if;
else
tx_stb_cnt <= tx_stb_cnt - 1;
end if;