verilog数码管计数器倒数编程问题

求大佬看一下为什么设定一个数值倒数不显示0，并且循环几次十位直接变成9，然后从99倒数。always @ ( posedge CLK or negedge RSTn ) if( !RSTn ) begin i = 4d0; rNum = 24d0; rNumber = 24d0; end else case( i ) 0: if(C1==0) begin rNum[7:4] = 4d2; rNum[3:0] = 4d7; i = i + 1b1; end 1: if( C1 == T100MS ) begin rNum[3:0] = rNum[3:0] - 1b1; i = i + 1b1; if(rNum[7:4]==4d0&&rNum[3:0]==4b0) begin rNum[7:4] = 4d2; rNum[3:0] = 4d6; end end 2: if( rNum[3:0] == 4d0 ) begin rNum[7:4] = rNum[7:4] - 1b1; rNum[3:0] = 4d9; i = i + 1b1; end else i = i + 1b1; 3: if( rNum[7:4] == 4d0) begin rNum[7:4] = rNum[7:4]; i = i + 1b1; end else i = i + 1b1;