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Add a few FFs on the RX input to avoid metastability issues

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Signed-off-by: Anton Blanchard <anton@linux.ibm.com>
This commit is contained in:
Anton Blanchard
2020-01-19 14:34:15 +11:00
committed by Anton Blanchard
parent 8569ae0ab1
commit 61d5e61f09
1 changed files with 12 additions and 3 deletions
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15
fpga/pp_soc_uart.vhd
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@@ -109,6 +109,8 @@ architecture behaviour of pp_soc_uart is
signal wb_ack : std_logic; --! Wishbone acknowledge signal
signal rxd2 : std_logic := '1';
signal rxd3 : std_logic := '1';
begin
irq <= (irq_recv_enable and (not recv_buffer_empty))
@@ -118,6 +120,13 @@ begin
recv_buffer_input <= rx_byte;
-- Add a few FFs on the RX input to avoid metastability issues
process (clk) is
begin
rxd3 <= rxd2;
rxd2 <= rxd;
end process;
uart_receive: process(clk)
begin
if rising_edge(clk) then
@@ -131,7 +140,7 @@ begin
recv_buffer_push <= '0';
end if;
if sample_clk = '1' and rxd = '0' then
if sample_clk = '1' and rxd3 = '0' then
rx_sample_value <= rx_sample_counter;
rx_sample_delay <= 0;
rx_current_bit <= 0;
@@ -150,10 +159,10 @@ begin
when RECEIVE =>
if sample_clk = '1' and rx_sample_counter = rx_sample_value then
if rx_current_bit /= 7 then
rx_byte(rx_current_bit) <= rxd;
rx_byte(rx_current_bit) <= rxd3;
rx_current_bit <= rx_current_bit + 1;
else
rx_byte(rx_current_bit) <= rxd;
rx_byte(rx_current_bit) <= rxd3;
rx_state <= STOPBIT;
end if;
end if;