RTC: Retry I2C transaction in case of error

Occasionally the I2C transaction is seen to fail with a NACK. Th e fix
is to wait a short while (20 ms), and then repeat the transaction until
it works.

M2M/vhdl/i2c/rtc_master.vhd

@@ -49,7 +49,7 @@ end entity rtc_master;
 
 architecture synthesis of rtc_master is
 
-  type cmd_t is (NOP_CMD, WRITE_CMD, WAIT_CMD, SHIFT_IN_CMD, SHIFT_OUT_CMD, END_CMD);
+  type cmd_t is (NOP_CMD, WRITE_CMD, WAIT_CMD, SHIFT_IN_CMD, SHIFT_OUT_CMD, VERIFY_CMD, END_CMD);
   type action_t is record
     cmd  : cmd_t;
     addr : std_logic_vector( 7 downto 0);
@@ -83,7 +83,7 @@ architecture synthesis of rtc_master is
     6 => (WAIT_CMD,     X"F1", X"0000"),   -- Wait until I2C command is accepted
     7 => (WAIT_CMD,     X"F1", X"0001"),   -- Wait until I2C is idle
     8 => (SHIFT_IN_CMD, X"00", X"0004"),   -- Read seven bytes from buffer
-    9 => (END_CMD,      X"00", X"0000")
+    9 => (VERIFY_CMD,   X"00", X"0000")
    );
 
   constant C_ACTION_LIST_WRITE_R3 : action_list_t := (
@@ -128,7 +128,7 @@ architecture synthesis of rtc_master is
     6 => (WAIT_CMD,      X"F1", X"0000"),   -- Wait until I2C command is accepted
     7 => (WAIT_CMD,      X"F1", X"0001"),   -- Wait until I2C is idle
     8 => (SHIFT_IN_CMD,  X"00", X"0004"),   -- Read seven bytes from buffer
-    9 => (END_CMD,       X"00", X"0000")
+    9 => (VERIFY_CMD,    X"00", X"0000")
    );
 
   constant C_ACTION_LIST_WRITE_R456 : action_list_t := (
@@ -201,8 +201,8 @@ architecture synthesis of rtc_master is
   constant C_ACTION_LIST_READ  : action_list_t := get_action_list_read(G_BOARD);
   constant C_ACTION_LIST_WRITE : action_list_t := get_action_list_write(G_BOARD);
 
-  type state_t is (RESET_ST, IDLE_ST, BUSY_ST);
-  signal state : state_t := IDLE_ST;
+  type state_t is (RESET_ST, IDLE_ST, BUSY_ST, VERIFY_ST);
+  signal state : state_t := RESET_ST;
 
   signal action_idx  : natural range 0 to 15;
   signal action      : action_t;
@@ -211,6 +211,25 @@ architecture synthesis of rtc_master is
   signal rtc_write   : std_logic_vector(71 downto 0);
   signal write       : std_logic;
 
+  constant C_SIM : boolean :=
+    -- synthesis translate_off
+    not
+    -- synthesis translate_on
+    false;
+
+  -- This does the same as the ternary operator "cond ? t : f" in the C language
+  pure function cond_select(cond : boolean; t : natural; f : natural) return natural is
+  begin
+    if cond then
+      return t;
+    else
+      return f;
+    end if;
+  end function cond_select;
+
+  constant C_COUNT_DOWN_MAX : natural := cond_select(C_SIM, 1_000, 1_000_000);
+  signal count_down : natural range 0 to C_COUNT_DOWN_MAX;
+
 begin
 
   rtc_busy_o <= '0' when state = IDLE_ST else '1';
@@ -311,12 +330,32 @@ begin
                   end if;
                 end if;
 
+              when VERIFY_CMD =>
+                count_down <= C_COUNT_DOWN_MAX;
+                rtc        <= post_read(G_BOARD, rtc);
+                state      <= VERIFY_ST;
+
               when END_CMD =>
-                rtc   <= post_read(G_BOARD, rtc);
                 state <= IDLE_ST;
 
             end case; -- action.cmd
           end if;
+
+        when VERIFY_ST =>
+          if rtc(47 downto 32) = X"0000" then
+            if count_down = 0 then
+              -- Reading from RTC did not work. Try again.
+              action_idx  <= 0;
+              action      <= C_ACTION_LIST_READ(0);
+              next_action <= '0';
+              state       <= BUSY_ST;
+              write       <= '0';
+            else
+              count_down <= count_down - 1;
+            end if;
+          else
+            state <= IDLE_ST;
+          end if;
       end case; -- state
 
       if next_action = '1' then