Table 5-2: std_logic Allowed Values Value Meaning What Vivado synthesis does U initialized Not accepted by Vivado synthesis X unknown Treated as don’t care 0 low Treated as logic zero 1 high Treated as logic one Z high impedance Treated as high impedance W weak unknown Not accepted by Vivado synthesis L weak low Treated identically to 0 H weak high Treated identically to 1 - don’t care Treated as don’t care