/*
 *       i2c.v
 *
 *   Copyright (C) 2018, 2019 Mind Chasers Inc.
 *   
 *   Licensed under the Apache License, Version 2.0 (the "License");
 *   you may not use this file except in compliance with the License.
 *   You may obtain a copy of the License at
 *
 *       http://www.apache.org/licenses/LICENSE-2.0
 *
 *   Unless required by applicable law or agreed to in writing, software
 *   distributed under the License is distributed on an "AS IS" BASIS,
 *   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 *   See the License for the specific language governing permissions and
 *   limitations under the License.
 *
 *	function: I2C slave controller for communicating with internal controller
 *		
 */


`timescale 1ns /10ps


module i2c(
	input 	rstn,
	input 	clk, 
	
	// external I2C I/O
	input	scl_i,		// serial clock
	output	scl_o,		// serial clock out
	output	scl_oe,		// serial clock output enable
	
	input 	sda_i,		// serial data in
	output  reg	sda_o,	// serial data out
	output	reg sda_oe,	// serial data output enable
	
	// shared memory and controller data output
	output [7:0] mem_do,
	
	// dedicated memory interface
	output [10:0] mem_ad,
	output mem_ce,
	output reg mem_we,
	input [7:0] mem_di,
	
	// dedicated controller write interface
	output reg cont_we,
	output cont_done,
	
	// Controller->FIFO input interface
	input tx_fifo_empty,		// use for control and pass as msbit of data
	input [6:0] fifo_di,		// data from FIFO to transmit on I2C
	output reg fifo_re

);

localparam  CONT_SEL = 7'h10,
			DPRAM_SEL = 7'h20,
			SCI_SEL = 7'h30;
	
// slave-related signals

reg	scl_high, scl_low;
reg [4:0] bit_cnt;	// cnt the bits received
reg start;		// falling SDA while SCL is high
reg stop;		// rising SDA while SCL is high
reg	run;		// assert while running and counting bits
reg rwn;		// follows address

reg [6:0] dev_ad;	// 7-bit device address
reg [7:0] addr;		// address
reg [7:0] d;		// data received from external i2c controller during an I2C write
wire target_sel, dpram_sel, cont_sel;
reg scl_i_m1, scl_i_m2, sda_i_m1;	// delayed versions of the inputs
reg ack;

wire [7:0] i_di;	// internal data in muxed from external sources (e.g., DPRAM and controller)

assign i_di = dpram_sel ? mem_di : { tx_fifo_empty, fifo_di };

// master-related signals
//reg halt;		// assert if we lose arbitration, detect a zero in place of a one

assign scl_oe = 1'b0;

// since it's open drain, never drive the outputs high
assign scl_o = 1'b0;

// slave interface

/*
	debounce and capture the asynch inputs
	delayed signals, by default, they're 1 since they're open drain
*/
always @(posedge clk or negedge rstn)
	begin
		if (!rstn)
			begin
				scl_i_m1 <= 1'b1;
				scl_i_m2 <= 1'b1;
				sda_i_m1 <= 1'b1;
			end
		else
			begin
				scl_i_m1 <= scl_i;
				scl_i_m2 <= scl_i_m1;
				sda_i_m1 <= sda_i;
			end
	end

/*
	create a one shot when scl is first stable high 
	use this to register inputs
*/
always@(posedge clk or negedge rstn)
	begin
		if (!rstn)
			scl_high <= 1'b0;
		else if (scl_i && scl_i_m1 && ~scl_i_m2)
			scl_high <= 1'b1;
		else
			scl_high <= 1'b0;
	end

/*
	create a one shot when scl is first stable low
	use this to register outputs
*/
always@(posedge clk or negedge rstn)
	begin
		if (!rstn)
			scl_low <= 1'b0;
		else if ( !scl_i && !scl_i_m1 && scl_i_m2)
			scl_low <= 1'b1;
		else
			scl_low <= 1'b0;
	end
	
/* 
	one shot start/restart bit, sda 1 to 0 anytime while scl is high
*/
always @(posedge clk or negedge rstn)
	begin
		if (!rstn)	
			start <= 1'b0;
		else if ( scl_i == 1'b1 &&  scl_i_m1 == 1'b1 && sda_i == 1'b0 &&  sda_i_m1 == 1'b1 ) 
			start <= 1'b1;
		else
			start <= 1'b0;
	end
	
/*
	one shot stop bit, sda 0 to 1 anytime while scl is high
*/
assign cont_done = stop & ~rwn;

always @(posedge clk or negedge rstn)
	begin
		if (!rstn )
			stop <= 1'b0;
		else if ( scl_i == 1'b1 &&  scl_i_m1 == 1'b1 && sda_i == 1'b1 &&  sda_i_m1 == 1'b0 ) 
			stop <= 1'b1;
		else
			stop <= 1'b0;
	end

/*
	This I2C block runs between start and stop while run == 1
*/
always @(posedge clk or negedge rstn)
	begin
		if (!rstn)
			run <= 1'b0;
		else if ( start )
			run <= 1'b1;
		else if ( stop ) 
			run <= 1'b0;
	end
	

/* 
	bit_cnt indicates the state of the i2c transfer:
	start/stop act as synchronous resets
	otherwise, bit_cnt can change when scl is low
	
	31 (1F): reserved for first cycle after start / idle state
	0:6: dev_ad
	7: rwn, ACK is clocked out
	8: 
	9:16: if rwn == 0 addr, else d, ACK is clocked out on 16
	17:  ( restart to 9 if read )
	18:25: write data, ack write data is clocked out on 25
	26:  ( restart to 18 for write data )
*/
always @(posedge clk or negedge rstn)
	begin
		if ( !rstn )
			bit_cnt <= 5'h1f;
		else if ( start || stop )
			bit_cnt <= 5'h1f;
		else if ( run && scl_low )
			begin
				if ( rwn && bit_cnt == 5'd17 )
					bit_cnt <= 5'd9;
				else if ( bit_cnt == 5'd26 )
					bit_cnt <= 5'd18;
				else
					bit_cnt <= bit_cnt + 1;
			end
	end
	
/*
	shift device address (dev_ad) into the module from the i2c bus
*/
always @(posedge clk or negedge rstn)	
	begin
		if ( !rstn )
			dev_ad <= 7'h0;
		else if ( stop ) 
			dev_ad <= 7'h0;
		else if ( run && scl_high && bit_cnt <= 5'h6 )
			dev_ad <= { dev_ad[5:0], sda_i };
	end
	

/*
	shift I2C memory addr into the module from the i2c bus during first cycle
	auto increment on subsequent byte reads during same I2C cycle
*/
always @(posedge clk or negedge rstn)	
	begin
		if (!rstn)
				addr <= 8'h00;
		else if ( run && scl_high)
			begin
				if ( !rwn && bit_cnt >= 5'd9 && bit_cnt <= 5'd16 )
					addr <= { addr[6:0], sda_i };
				else if ( (rwn && bit_cnt == 5'd17) || (!rwn && bit_cnt == 5'd26)  ) 
					addr <= addr + 1;	
			end
	end


/*
*	shift write data (d) into the module from the i2c bus.
*/
always @(posedge clk or negedge rstn)	
	begin
		if (!rstn)
				d <= 8'ha5;
		else if ( run && scl_high && !rwn && !start && bit_cnt >= 5'd18 && bit_cnt < 5'd26 )
				d <= { d[6:0], sda_i };
	end


	
assign dpram_sel =  ( dev_ad == DPRAM_SEL ) ? 1 : 0;
assign cont_sel = ( dev_ad == CONT_SEL ) ? 1 : 0;
assign target_sel = cont_sel | dpram_sel;	   

/*
	register ack during I2C reads when bit_cnt == 17
*/
always @(posedge clk or negedge rstn)
	begin
		if ( !rstn )
			ack <= 1'b0;
		else if ( run && scl_high && rwn && bit_cnt == 5'd17 )
			ack <= ~sda_i;
	end

/*
	register rwn bit
*/
always @(posedge clk or negedge rstn)
	begin
		if ( !rstn )
			rwn <= 1'b1;
		else if ( stop )
			rwn <= 1'b1;
		else if ( run && scl_high && bit_cnt == 5'd7 )
			rwn <= sda_i;
	end
	
/*
	sda_oe logic, note that the bit_cnt will be changing simultaneously, so it's one behind
*/
always @(posedge clk or negedge rstn)
	begin
		if ( !rstn )
			begin
				sda_oe <= 1'b0;
				sda_o <= 1'b1;
			end
		else if ( stop )
			begin
				sda_oe <= 1'b0;
				sda_o <= 1'b1;
			end			
		else if ( scl_low ) 
			begin
				// ack the first byte written by master if it's addressed to our I2C slave
				if ( target_sel && bit_cnt == 5'd7 )
					begin
						sda_oe <= 1'b1;
						sda_o <= 1'b0;
					end
				// ack write address
				else if ( target_sel && rwn == 1'b0 && bit_cnt == 5'd16 )
					begin
						sda_oe <= 1'b1;
						sda_o <= 1'b0;
					end
				// ack write data
				else if ( target_sel && rwn == 1'b0 && bit_cnt == 5'd25 )
					begin
						sda_oe <= 1'b1;
						sda_o <= 1'b0;
					end
				// drive read data
				else if ( target_sel && rwn == 1'b1 && bit_cnt >= 5'd8 && bit_cnt <=5'd15 )	// xmt data for read cycle
					begin
						sda_oe <= 1'b1;
						sda_o <= i_di[7-bit_cnt[2:0]];
					end
				// drive data for first bit of burst read cycle, multi-byte if we get an ack
				// if no ack, then the burst read or single read is done
				else if ( target_sel && rwn == 1'b1 && ack && bit_cnt == 5'd17 )	
					begin
						sda_oe <= 1'b1;
						sda_o <= i_di[7];
					end
				else
					begin
						sda_oe <= 1'b0;
						sda_o <= 1'b1;		// don't care
					end
			end
	end
	
/* DPRAM Control */
assign mem_ad = {3'b0, addr};
assign mem_ce = 1'b1;	

/*
		driver: mem_do 
		shared between both cont and dpram interfaces
		drive addr for first byte since this is the controller cmd 
*/
assign mem_do = ( bit_cnt <= 5'd17 ) ? addr : d;

// mem_we bit
always @(posedge clk or negedge rstn)
	begin
		if ( !rstn )
			mem_we <= 1'b0;
		else if ( run && scl_high && dpram_sel && !rwn && bit_cnt == 5'd26 )
			mem_we <= 1'b1;
		else
			mem_we <= 1'b0;
	end
	
// cont_we bit is asserted at the end of both command and data bytes
always @(posedge clk or negedge rstn)
	begin
		if ( !rstn )
			cont_we <= 1'b0;
		else if ( run && scl_high && cont_sel && !rwn && (bit_cnt == 5'd26 || bit_cnt == 5'd17) )
			cont_we <= 1'b1;
		else
			cont_we <= 1'b0;
	end
	
/*
*	drive: fifo_re
*	Stop asserting during a burst if we don't get an ACK (use sda_i for real-time ACK)
*/
always @(posedge clk or negedge rstn)
	begin
		if ( !rstn )
			fifo_re <= 1'b0;
		else if ( run && scl_high && cont_sel && rwn && !tx_fifo_empty && ( bit_cnt == 5'd8 || ( bit_cnt == 5'd17 && !sda_i ) ) )
			fifo_re <= 1'b1;
		else
			fifo_re <= 1'b0;
	end
	
	
endmodule