Added pin definitions for SD card interface. Fixed bug in 6522 shift-register implementation (shift on CB1 rising edge should occur even in SR disabled mode). Works with MMBEEB SD card interface ROM.

1 files changed, 18 insertions, 22 deletions

diff --git a/m6522.vhd b/m6522.vhd
index fe82b3f..7cd026c 100644
--- a/m6522.vhd
+++ b/m6522.vhd
@@ -733,7 +733,7 @@ begin
         free_run := '0';

 

         case r_acr(4 downto 2) is

-          when "000" => ena := '0';

+          when "000" => ena := '0'; cb1_ip := '1';

           when "001" => ena := '1'; cb1_op := '1'; use_t2 := '1';

           when "010" => ena := '1'; cb1_op := '1';

           when "011" => ena := '1'; cb1_ip := '1';

@@ -745,35 +745,31 @@ begin
         end case;

 

         -- clock select

-        if (ena = '0') then

-          sr_strobe <= '1';

-        else

-          if (cb1_ip = '1') then

-            sr_strobe <= I_CB1;

-          else

-            if (sr_cnt(3) = '0') and (free_run = '0') then

-              sr_strobe <= '1';

-            else

-              if ((use_t2 = '1') and t2_sr_ena) or

-                 ((use_t2 = '0') and (phase = "00")) then

-                  sr_strobe <= not sr_strobe;

-              end if;

-            end if;

-          end if;

-        end if;

+        -- SR still runs even in disabled mode (on rising edge of CB1).  It

+        -- just doesn't generate any interrupts.

+        -- Ref BBC micro advanced user guide p409

+		if (cb1_ip = '1') then

+		  sr_strobe <= I_CB1;

+		else

+		  if (sr_cnt(3) = '0') and (free_run = '0') then

+		    sr_strobe <= '1';

+		  else

+			if ((use_t2 = '1') and t2_sr_ena) or

+			  ((use_t2 = '0') and (phase = "00")) then

+			    sr_strobe <= not sr_strobe;

+			end if;

+		  end if;

+		end if;

 

         -- latch on rising edge, shift on falling edge

         if sr_write_ena then

           r_sr <= load_data;

-        elsif (ena = '1') then -- use shift reg

-

+        else

           if (dir_out = '0') then

             -- input

             if (sr_cnt(3) = '1') or (cb1_ip = '1') then

               if sr_strobe_rising then

-                r_sr(0) <= I_CB2;

-              elsif sr_strobe_falling then

-                r_sr(7 downto 1) <= r_sr(6 downto 0);

+                r_sr <= r_sr(6 downto 0) & I_CB2;

               end if;

             end if;

             sr_out <= '1';