From 61d5e61f09199221ab15a632b3fe593e2fa8a52d Mon Sep 17 00:00:00 2001 From: Anton Blanchard Date: Sun, 19 Jan 2020 14:34:15 +1100 Subject: [PATCH] Add a few FFs on the RX input to avoid metastability issues Signed-off-by: Anton Blanchard --- fpga/pp_soc_uart.vhd | 15 ++++++++++++--- 1 file changed, 12 insertions(+), 3 deletions(-) diff --git a/fpga/pp_soc_uart.vhd b/fpga/pp_soc_uart.vhd index 879ea26..429bec4 100644 --- a/fpga/pp_soc_uart.vhd +++ b/fpga/pp_soc_uart.vhd @@ -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; -- 2.30.2