Clamp line_counter to an even number when in interlace sync+video mode. Fixes bug where vsync would disappear half the time when switching into this mode.

1 files changed, 3 insertions, 2 deletions

diff --git a/mc6845.vhd b/mc6845.vhd
index 90c1108..e38207a 100644
--- a/mc6845.vhd
+++ b/mc6845.vhd
@@ -223,7 +223,7 @@ begin
 				

 				-- In interlace sync + video mode mask off the LSb of the

 				-- max scan line address

-				if r08_interlace(1 downto 0) = "11" then

+				if r08_interlace = "11" then

 					max_scan_line := r09_max_scan_line_addr(4 downto 1) & "0";

 				else

 					max_scan_line := r09_max_scan_line_addr;

@@ -253,7 +253,7 @@ begin
 						-- Increment field counter

 						field_counter <= field_counter + 1;					

 					else

-						-- On all other chracter rows within the field the row start address is

+						-- On all other character rows within the field the row start address is

 						-- increased by h_displayed and the row counter is incremented

 						ma_row_start := ma_row_start + r01_h_displayed;

 						row_counter <= row_counter + 1;

@@ -262,6 +262,7 @@ begin
 					-- Next scan line.  Count in twos in interlaced sync+video mode

 					if r08_interlace = "11" then

 						line_counter <= line_counter + 2;

+						line_counter(0) <= '0'; -- Force to even

 					else

 						line_counter <= line_counter + 1;

 					end if;