RS232 chargen reset problem

Altera_Forum · ‎04-14-2013

Hello!

I'm actually a software engineer, new to the 'dark arts'! Anyway I'm trying to make a 'chargen' over RS232, and I have succeeded in making it transmit ASCII characters 0x21 to 0x7e.

But I am trying to make it transmit this again each time a button is pressed, to no avail. I am targetting a Cyclone II EP2C8Q208C8N and using a USB-to-RS232 adapter.

When I program the FPGA, the characters are output, then nothing (as expected). But then pressing the reset button does nothing. I have tried running it through ModelSim and found nothing untoward.

Can anyone offer any suggestions? Also I can't post the code because apparently I can't post links :confused:

Thanks,

Altera_Forum · ‎04-14-2013

Apparently if I post an at-sign, even in a CODE block, the forum thinks i'm posting a link, even if I untick "Automatically parse links in text". Anyway I replaced them with *AT*


 
module rs232_send
  (
  input wire SYSCLK, // 50MHz
  input wire RST_B, // reset switch, active low
  output reg UART_TX
  );
 reg  COUNTER, COUNTER_N;
 wire SER_CLK;
 reg  TX_STATE, TX_STATE_N;
 reg  LETTER, LETTER_N;
 wire ENABLE;
 reg UART_TX_N;
 initial
 begin
  COUNTER <= 0;
  TX_STATE <= 0;
  TX_STATE_N <= 0;
  LETTER <= 7'h21; // start with '!' character
  LETTER_N <= 7'h21;
  UART_TX <= 1;
 end
 assign SER_CLK = (COUNTER == 9'd434); // 50MHz / 434 = 115207 (close enough)
 assign ENABLE = (LETTER != 7'h7e);
 always *AT* (posedge SYSCLK or negedge RST_B)
 begin
  if(!RST_B)
  begin
   COUNTER <= 0;
   TX_STATE <= 0;
   LETTER <= 7'h21;
   UART_TX <= 1;
  end
  else
  begin
   COUNTER <=# 1 COUNTER_N;
   TX_STATE <=# 1 TX_STATE_N;
   LETTER <=# 1 LETTER_N;
   UART_TX <=# 1 UART_TX_N;
  end
 end
 always *AT* (*)
 begin
  if(COUNTER == 9'd434)
   COUNTER_N = 0;
  else
   COUNTER_N = COUNTER + 9'h1;
 end
 always  *AT* (*)
 begin
  if(SER_CLK)
  begin
   case(TX_STATE)
     4'd0: UART_TX_N = 0; // start bit
     4'd1: UART_TX_N = LETTER; // first nibble
     4'd2: UART_TX_N = LETTER;
     4'd3: UART_TX_N = LETTER;
     4'd4: UART_TX_N = LETTER;
     4'd5: UART_TX_N = LETTER; // second nibble start
     4'd6: UART_TX_N = LETTER;
     4'd7: UART_TX_N = LETTER;
     4'd8: UART_TX_N = 0; // top bit is always zero, as ASCII is 7-bit
     4'd9: UART_TX_N = 1; // stop bit
     default: UART_TX_N = 1; // idle
    endcase
   end
  else
   UART_TX_N = UART_TX;
 end
 always *AT* (posedge SYSCLK)
 begin
 if(SER_CLK & ENABLE)
 begin
  if(TX_STATE == 4'd9)
  begin
   TX_STATE_N <= 4'h0;
   if(LETTER == 7'h7e)
    LETTER_N <= 7'h21;
   else
    LETTER_N <= LETTER + 7'h1;
  end
  else
  begin
   TX_STATE_N <= TX_STATE + 4'h1;
   LETTER_N <= LETTER;
  end
 end
 end
 endmodule

Altera_Forum · ‎04-15-2013

A simple reason could be your RST_B signal is not correctly assigned to the right fpga pin. Serial communications starts upon fpga programming because registers are initially in the correct state.

Anyway your code is pretty bad: you'd better synchronizing everything to SYSCLK, namely use a single process always @ (posedge SYSCLK)

There's no point in triggering on RST_B negedge, when you can simply sample it with the fast sysclk; and assigning registers inside a always @ (*) process is not a recommended practice: how is this supposed to be synthesized in real hardware? Latches? Flip-flops, but which clock?

Altera_Forum · ‎04-15-2013

Thank you for the tips! :) I reworked it a little and removed the always * block, and moved it into a single process, as you suggested. And now it works as expected :D


module rs232_send
	(
	input wire SYSCLK, // 50MHz
	input wire RST_B, // reset switch, active low
	output reg UART_TX
	);
reg  COUNTER;
wire SER_CLK;
reg  TX_STATE;
reg  LETTER;
wire ENABLE;
initial
begin
	COUNTER <= 0;
	TX_STATE <= 0;
	LETTER <= 7'h21; // start with '!' character
	UART_TX <= 1;
end
assign SER_CLK = (COUNTER == 16'd5208); // 50MHz / 5208 = 9600
assign ENABLE = (LETTER != 7'h7e);
always *AT* (posedge SYSCLK)
begin
	if(!RST_B)
	begin
		COUNTER <= 0;
		TX_STATE <= 4'hA;
		LETTER <= 7'h21;
		UART_TX <= 1;
	end
	else
	begin
		if(!ENABLE)
			UART_TX <= 1;
		else if(SER_CLK)
		begin
			COUNTER <= 0;
			case(TX_STATE)
				4'd0: UART_TX <= 0; // start bit
				4'd1: UART_TX <= LETTER; // first nibble
				4'd2: UART_TX <= LETTER;
				4'd3: UART_TX <= LETTER;
				4'd4: UART_TX <= LETTER;
				4'd5: UART_TX <= LETTER; // second nibble start
				4'd6: UART_TX <= LETTER;
				4'd7: UART_TX <= LETTER;
				4'd8: UART_TX <= 0;
				4'd9: UART_TX <= 1; // stop bit
				default: UART_TX <= 1; // idle
			endcase
			if(TX_STATE == 4'd9)
			begin
				TX_STATE <= 4'h0;
				if(LETTER == 7'h7e)
					LETTER <= 7'h21;
				else
					LETTER <= LETTER + 7'h1;
			end
			else
			begin
				TX_STATE <= TX_STATE + 4'h1;
			end
		end
		else
			COUNTER <= COUNTER + 16'h1;
	end
end
endmodule

If there's anything else that's bad about this code, I would be very grateful to hear it.

Thanks again,

Altera_Forum · ‎04-16-2013

Ok. Now it's fine.

You could possibly remove these lines

      if(LETTER == 7'h7e)
	    LETTER <= 7'h21;
     else

since they are useless: infact when LETTER reaches 7e , ENABLE becomes false and you wouldn't get here any more, until after the RST_B signal has restored LETTER to 7'h21 initial value.

I'd also replace the if(!ENABLE) statement with the actual test on COUNTER value, because imho it is clearer.

Please note that your code is already ok as it is. The suggested changes won't affect its functionality but only serves to improve readability and better understanding the way code operates. This may seem not worthy here but it becomes fondamental in a bigger design.

Altera_Forum · ‎04-20-2013

Thanks again for your help and advice. Despite being a simple project in the grand scheme of things, this has taught me a lot :)