Nuts and Volts - September 2008

bits are written to the PmodCLP’s data bus. We have to raise the PmodCLP’s E control pin logically high for a minimum of 220 ns and then bring it logically low for another 280 ns to allow the PmodCLP’s LCD electronics to process the function_set command. So, we start the lcd_clk ticks within the else clause by incrementing the lcd_clk register.

We give the PmodCLP’s LCD controller plenty of time to digest the signals on the RS pin and databus as the PmodCLP’s E control line is raised to a logically high level 1 μS later when the lcd_clk value has been incremented to 10 ticks. The E control line will remain at a logically high level for 10 more ticks. When the lcd_clk value is equal to 20 ticks,

the E control line will be driven logically low. We must wait a minimum of 37 μs before issuing the next command. The lcd_state register is loaded with the next desired state and the function_set state is exited when the lcd_clk expends another 38 μs (lcd_clk == 400). In addition to setting up the next state to hop to, upon exiting the function_set state we also clear the lcd_clk tick count to zero. As you can see in the function_set source code, we worked our way through the case statement code from bottom to top performing tasks according to the ticks contained within the lcd_clk. Note also that we always made sure that the lcd_state register was loaded with the currently executing state as we incremented the lcd_clk register to its exit value. There’s only one state that doesn’t operate in the manner we’ve just examined. However, I don’t think that you will have any problems grasping the concepts used in the msg_table code:

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msg_table:
      begin
            if(char_ptr == 17)

begin
  lcd_clk <= 0;
  lcd_state <= display_line2;
end

else if(char_ptr == 34)

begin
  lcd_clk <= 0;
  lcd_state <= idle;
end

else

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begin
  case(char_ptr)
  //LINE 1
  1:char2display <= “ “;
  2:char2display <= “N”;
  3:char2display <= “U”;
  4:char2display <= “T”;
  5:char2display <= “S”;
  6:char2display <= “ “;
  7:char2display <= “A”;
  8:char2display <= “N”;

■ PHOTO 9. I still get excited when things like this go as planned. In the end when the firmware and hardware are proven to work, it just doesn’t get any better than that.