uart_regs.v

vlsi UART referene, use UART0_3

end

wire 	lsr_mask_condition;
wire 	iir_read;
wire  msr_read;
wire	fifo_read;
wire	fifo_write;

assign lsr_mask_condition = (re_i && addr_i == `UART_REG_LS && !dlab);
assign iir_read = (re_i && addr_i == `UART_REG_II && !dlab);
assign msr_read = (re_i && addr_i == `UART_REG_MS && !dlab);
assign fifo_read = (re_i && addr_i == `UART_REG_RB && !dlab);
assign fifo_write = (we_i && addr_i == `UART_REG_TR && !dlab);

// lsr_mask_d delayed signal handling
always @(posedge clk or posedge rst_i)
begin
	if (rst_i)
		lsr_mask_d <= #1 0;
	else // reset bits in the Line Status Register
		lsr_mask_d <= #1 lsr_mask_condition;
end

// lsr_mask is rise detected
assign lsr_mask = lsr_mask_condition && ~lsr_mask_d;

// msi_reset signal handling
always @(posedge clk or posedge rst_i)
begin
	if (rst_i)
		msi_reset <= #1 1;
	else
	if (msi_reset)
		msi_reset <= #1 0;
	else
	if (msr_read)
		msi_reset <= #1 1; // reset bits in Modem Status Register
end


//
//   WRITES AND RESETS   //
//
// Line Control Register
always @(posedge clk or posedge rst_i)
	if (rst_i)
		lcr <= #1 8'b00000011; // 8n1 setting
	else
	if (we_i && addr_i==`UART_REG_LC)
		lcr <= #1 dat_i;

// Interrupt Enable Register or UART_DL2
always @(posedge clk or posedge rst_i)
	if (rst_i)
	begin
		ier <= #1 4'b0000; // no interrupts after reset
		dl[`UART_DL2] <= #1 8'b0;
	end
	else
	if (we_i && addr_i==`UART_REG_IE)
		if (dlab)
		begin
			dl[`UART_DL2] <= #1 dat_i;
		end
		else
			ier <= #1 dat_i[3:0]; // ier uses only 4 lsb


// FIFO Control Register and rx_reset, tx_reset signals
always @(posedge clk or posedge rst_i)
	if (rst_i) begin
		fcr <= #1 2'b11; 
		rx_reset <= #1 0;
		tx_reset <= #1 0;
	end else
	if (we_i && addr_i==`UART_REG_FC) begin
		fcr <= #1 dat_i[7:6];
		rx_reset <= #1 dat_i[1];
		tx_reset <= #1 dat_i[2];
	end else begin
		rx_reset <= #1 0;
		tx_reset <= #1 0;
	end

// Modem Control Register
always @(posedge clk or posedge rst_i)
	if (rst_i)
		mcr <= #1 5'b0; 
	else
	if (we_i && addr_i==`UART_REG_MC)
			mcr <= #1 dat_i[4:0];

// Scratch register
// Line Control Register
always @(posedge clk or posedge rst_i)
	if (rst_i)
		scratch <= #1 0; // 8n1 setting
	else
	if (we_i && addr_i==`UART_REG_SR)
		scratch <= #1 dat_i;

// TX_FIFO or UART_DL1
always @(posedge clk or posedge rst_i)
	if (rst_i)
	begin
		dl[`UART_DL1]  <= #1 8'b0;
		tf_push   <= #1 1'b0;
		start_dlc <= #1 1'b0;
	end
	else
	if (we_i && addr_i==`UART_REG_TR)
		if (dlab)
		begin
			dl[`UART_DL1] <= #1 dat_i;
			start_dlc <= #1 1'b1; // enable DL counter
			tf_push <= #1 1'b0;
		end
		else
		begin
			tf_push   <= #1 1'b1;
			start_dlc <= #1 1'b0;
		end // else: !if(dlab)
	else
	begin
		start_dlc <= #1 1'b0;
		tf_push   <= #1 1'b0;
	end // else: !if(dlab)

// Receiver FIFO trigger level selection logic (asynchronous mux)
always @(fcr)
	case (fcr[`UART_FC_TL])
		2'b00 : trigger_level = 1;
		2'b01 : trigger_level = 4;
		2'b10 : trigger_level = 8;
		2'b11 : trigger_level = 14;
	endcase // case(fcr[`UART_FC_TL])
	
//
//  STATUS REGISTERS  //
//

// Modem Status Register
reg [3:0] delayed_modem_signals;
always @(posedge clk or posedge rst_i)
begin
	if (rst_i)
	  begin
  		msr <= #1 0;
	  	delayed_modem_signals[3:0] <= #1 0;
	  end
	else begin
		msr[`UART_MS_DDCD:`UART_MS_DCTS] <= #1 msi_reset ? 4'b0 :
			msr[`UART_MS_DDCD:`UART_MS_DCTS] | ({dcd, ri, dsr, cts} ^ delayed_modem_signals[3:0]);
		msr[`UART_MS_CDCD:`UART_MS_CCTS] <= #1 {dcd_c, ri_c, dsr_c, cts_c};
		delayed_modem_signals[3:0] <= #1 {dcd, ri, dsr, cts};
	end
end


// Line Status Register

// activation conditions
assign lsr0 = (rf_count==0 && rf_push_pulse);  // data in receiver fifo available set condition
assign lsr1 = rf_overrun;     // Receiver overrun error
assign lsr2 = rf_data_out[1]; // parity error bit
assign lsr3 = rf_data_out[0]; // framing error bit
assign lsr4 = rf_data_out[2]; // break error in the character
assign lsr5 = (tf_count==5'b0 && thre_set_en);  // transmitter fifo is empty
assign lsr6 = (tf_count==5'b0 && thre_set_en && (tstate == /*`S_IDLE */ 0)); // transmitter empty
assign lsr7 = rf_error_bit | rf_overrun;

// lsr bit0 (receiver data available)
reg 	 lsr0_d;

always @(posedge clk or posedge rst_i)
	if (rst_i) lsr0_d <= #1 0;
	else lsr0_d <= #1 lsr0;

always @(posedge clk or posedge rst_i)
	if (rst_i) lsr0r <= #1 0;
	else lsr0r <= #1 (rf_count==1 && rf_pop && !rf_push_pulse || rx_reset) ? 0 : // deassert condition
					  lsr0r || (lsr0 && ~lsr0_d); // set on rise of lsr0 and keep asserted until deasserted 

// lsr bit 1 (receiver overrun)
reg lsr1_d; // delayed

always @(posedge clk or posedge rst_i)
	if (rst_i) lsr1_d <= #1 0;
	else lsr1_d <= #1 lsr1;

always @(posedge clk or posedge rst_i)
	if (rst_i) lsr1r <= #1 0;
	else	lsr1r <= #1	lsr_mask ? 0 : lsr1r || (lsr1 && ~lsr1_d); // set on rise

// lsr bit 2 (parity error)
reg lsr2_d; // delayed

always @(posedge clk or posedge rst_i)
	if (rst_i) lsr2_d <= #1 0;
	else lsr2_d <= #1 lsr2;

always @(posedge clk or posedge rst_i)
	if (rst_i) lsr2r <= #1 0;
	else lsr2r <= #1 lsr_mask ? 0 : lsr2r || (lsr2 && ~lsr2_d); // set on rise

// lsr bit 3 (framing error)
reg lsr3_d; // delayed

always @(posedge clk or posedge rst_i)
	if (rst_i) lsr3_d <= #1 0;
	else lsr3_d <= #1 lsr3;

always @(posedge clk or posedge rst_i)
	if (rst_i) lsr3r <= #1 0;
	else lsr3r <= #1 lsr_mask ? 0 : lsr3r || (lsr3 && ~lsr3_d); // set on rise

// lsr bit 4 (break indicator)
reg lsr4_d; // delayed

always @(posedge clk or posedge rst_i)
	if (rst_i) lsr4_d <= #1 0;
	else lsr4_d <= #1 lsr4;

always @(posedge clk or posedge rst_i)
	if (rst_i) lsr4r <= #1 0;
	else lsr4r <= #1 lsr_mask ? 0 : lsr4r || (lsr4 && ~lsr4_d);

// lsr bit 5 (transmitter fifo is empty)
reg lsr5_d;

always @(posedge clk or posedge rst_i)
	if (rst_i) lsr5_d <= #1 1;
	else lsr5_d <= #1 lsr5;

always @(posedge clk or posedge rst_i)
	if (rst_i) lsr5r <= #1 1;
	else lsr5r <= #1 (fifo_write) ? 0 :  lsr5r || (lsr5 && ~lsr5_d);

// lsr bit 6 (transmitter empty indicator)
reg lsr6_d;

always @(posedge clk or posedge rst_i)
	if (rst_i) lsr6_d <= #1 1;
	else lsr6_d <= #1 lsr6;

always @(posedge clk or posedge rst_i)
	if (rst_i) lsr6r <= #1 1;
	else lsr6r <= #1 (fifo_write) ? 0 : lsr6r || (lsr6 && ~lsr6_d);

// lsr bit 7 (error in fifo)
reg lsr7_d;

always @(posedge clk or posedge rst_i)
	if (rst_i) lsr7_d <= #1 0;
	else lsr7_d <= #1 lsr7;

always @(posedge clk or posedge rst_i)
	if (rst_i) lsr7r <= #1 0;
	else lsr7r <= #1 lsr_mask ? 0 : lsr7r || (lsr7 && ~lsr7_d);

// Frequency divider
always @(posedge clk or posedge rst_i) 
begin
	if (rst_i)
		dlc <= #1 0;
	else
		if (start_dlc | ~ (|dlc))
  			dlc <= #1 dl - 1;               // preset counter
		else
			dlc <= #1 dlc - 1;              // decrement counter
end

// Enable signal generation logic
always @(posedge clk or posedge rst_i)
begin
	if (rst_i)
		enable <= #1 1'b0;
	else
		if (|dl & ~(|dlc))     // dl>0 & dlc==0
			enable <= #1 1'b1;
		else
			enable <= #1 1'b0;
end

// Delaying THRE status for one character cycle after a character is written to an empty fifo.
always @(lcr)
  case (lcr[3:0])
    4'b0000                             : block_value =  95; // 6 bits
    4'b0100                             : block_value = 103; // 6.5 bits
    4'b0001, 4'b1000                    : block_value = 111; // 7 bits
    4'b1100                             : block_value = 119; // 7.5 bits
    4'b0010, 4'b0101, 4'b1001           : block_value = 127; // 8 bits
    4'b0011, 4'b0110, 4'b1010, 4'b1101  : block_value = 143; // 9 bits
    4'b0111, 4'b1011, 4'b1110           : block_value = 159; // 10 bits
    4'b1111                             : block_value = 175; // 11 bits
  endcase // case(lcr[3:0])

// Counting time of one character minus stop bit
always @(posedge clk or posedge rst_i)
begin
  if (rst_i)
    block_cnt <= #1 8'd0;
  else
  if(lsr5r & fifo_write)  // THRE bit set & write to fifo occured
    block_cnt <= #1 block_value;
  else
  if (enable & block_cnt != 8'b0)  // only work on enable times
    block_cnt <= #1 block_cnt - 1;  // decrement break counter
end // always of break condition detection

// Generating THRE status enable signal
assign thre_set_en = ~(|block_cnt);


//
//	INTERRUPT LOGIC
//

assign rls_int  = ier[`UART_IE_RLS] && (lsr[`UART_LS_OE] || lsr[`UART_LS_PE] || lsr[`UART_LS_FE] || lsr[`UART_LS_BI]);
assign rda_int  = ier[`UART_IE_RDA] && (rf_count >= {1'b0,trigger_level});
assign thre_int = ier[`UART_IE_THRE] && lsr[`UART_LS_TFE];
assign ms_int   = ier[`UART_IE_MS] && (| msr[3:0]);
assign ti_int   = ier[`UART_IE_RDA] && (counter_t == 10'b0) && (|rf_count);

reg 	 rls_int_d;
reg 	 thre_int_d;
reg 	 ms_int_d;
reg 	 ti_int_d;
reg 	 rda_int_d;

// delay lines
always  @(posedge clk or posedge rst_i)
	if (rst_i) rls_int_d <= #1 0;
	else rls_int_d <= #1 rls_int;

always  @(posedge clk or posedge rst_i)
	if (rst_i) rda_int_d <= #1 0;
	else rda_int_d <= #1 rda_int;

always  @(posedge clk or posedge rst_i)
	if (rst_i) thre_int_d <= #1 0;
	else thre_int_d <= #1 thre_int;

always  @(posedge clk or posedge rst_i)
	if (rst_i) ms_int_d <= #1 0;
	else ms_int_d <= #1 ms_int;

always  @(posedge clk or posedge rst_i)
	if (rst_i) ti_int_d <= #1 0;
	else ti_int_d <= #1 ti_int;

// rise detection signals

wire 	 rls_int_rise;
wire 	 thre_int_rise;
wire 	 ms_int_rise;
wire 	 ti_int_rise;
wire 	 rda_int_rise;

assign rda_int_rise    = rda_int & ~rda_int_d;
assign rls_int_rise 	  = rls_int & ~rls_int_d;
assign thre_int_rise   = thre_int & ~thre_int_d;
assign ms_int_rise 	  = ms_int & ~ms_int_d;
assign ti_int_rise 	  = ti_int & ~ti_int_d;

// interrupt pending flags
reg 	rls_int_pnd;
reg	rda_int_pnd;
reg 	thre_int_pnd;
reg 	ms_int_pnd;
reg 	ti_int_pnd;

// interrupt pending flags assignments
always  @(posedge clk or posedge rst_i)
	if (rst_i) rls_int_pnd <= #1 0; 
	else 
		rls_int_pnd <= #1 lsr_mask ? 0 :  						// reset condition
							rls_int_rise ? 1 :						// latch condition
							rls_int_pnd && ier[`UART_IE_RLS];	// default operation: remove if masked

always  @(posedge clk or posedge rst_i)
	if (rst_i) rda_int_pnd <= #1 0; 
	else 
		rda_int_pnd <= #1 ((rf_count == {1'b0,trigger_level}) && fifo_read) ? 0 :  	// reset condition
							rda_int_rise ? 1 :						// latch condition
							rda_int_pnd && ier[`UART_IE_RDA];	// default operation: remove if masked

always  @(posedge clk or posedge rst_i)
	if (rst_i) thre_int_pnd <= #1 0; 
	else 
		thre_int_pnd <= #1 fifo_write || (iir_read & ~iir[`UART_II_IP] & iir[`UART_II_II] == `UART_II_THRE)? 0 : 
							thre_int_rise ? 1 :
							thre_int_pnd && ier[`UART_IE_THRE];

always  @(posedge clk or posedge rst_i)
	if (rst_i) ms_int_pnd <= #1 0; 
	else 
		ms_int_pnd <= #1 msr_read ? 0 : 
							ms_int_rise ? 1 :
							ms_int_pnd && ier[`UART_IE_MS];

always  @(posedge clk or posedge rst_i)
	if (rst_i) ti_int_pnd <= #1 0; 
	else 
		ti_int_pnd <= #1 fifo_read ? 0 : 
							ti_int_rise ? 1 :
							ti_int_pnd && ier[`UART_IE_RDA];
// end of pending flags

// INT_O logic
always @(posedge clk or posedge rst_i)
begin
	if (rst_i)	
		int_o <= #1 1'b0;
	else
		int_o <= #1 
					rls_int_pnd		?	~lsr_mask					:
					rda_int_pnd		? 1								:
					ti_int_pnd		? ~fifo_read					:
					thre_int_pnd	? !(fifo_write & iir_read) :
					ms_int_pnd		? ~msr_read						:
					0;	// if no interrupt are pending
end


// Interrupt Identification register
always @(posedge clk or posedge rst_i)
begin
	if (rst_i)
		iir <= #1 1;
	else
	if (rls_int_pnd)  // interrupt is pending
	begin
		iir[`UART_II_II] <= #1 `UART_II_RLS;	// set identification register to correct value
		iir[`UART_II_IP] <= #1 1'b0;		// and clear the IIR bit 0 (interrupt pending)
	end else // the sequence of conditions determines priority of interrupt identification
	if (rda_int)
	begin
		iir[`UART_II_II] <= #1 `UART_II_RDA;
		iir[`UART_II_IP] <= #1 1'b0;
	end
	else if (ti_int_pnd)
	begin
		iir[`UART_II_II] <= #1 `UART_II_TI;
		iir[`UART_II_IP] <= #1 1'b0;
	end
	else if (thre_int_pnd)
	begin
		iir[`UART_II_II] <= #1 `UART_II_THRE;
		iir[`UART_II_IP] <= #1 1'b0;
	end
	else if (ms_int_pnd)
	begin
		iir[`UART_II_II] <= #1 `UART_II_MS;
		iir[`UART_II_IP] <= #1 1'b0;
	end else	// no interrupt is pending
	begin
		iir[`UART_II_II] <= #1 0;
		iir[`UART_II_IP] <= #1 1'b1;
	end
end

endmodule