// i2c.vhd
//
//這是是能從一個外部NUM（AT24C02A）讀數據到一個256x8的外部SRAM塊的I2C主接口程序，
//當用于寫NUM的外部邏輯寄存器存取時，SRAM就從外部源讀數據并且把數據寫到特定的I2C地址。
//----------------------------------------------------------------------
//
// ????????????? Copyright 2004 Actel corporation
//
//----------------------------------------------------------------------
//
// Version 1.2? 06/04/04 J.Vorgert - working file
//
//----------------------------------------------------------------------

`timescale 1ns / 1ps

module i2c (Reset_n, CLK, INIT, IENB, IADDR, IDATA, ICLK, UPDT,
??????????? UENB, UADDR, UDATA, SDI, SDO, SCK);

input Reset_n;? /* active low reset */
input CLK??? ;? /* processor clock? */
output INIT?? ;? /* high during init */
output IENB?? ;? /* low to enable write */
output [7:0] IADDR? ;? /* init address */
output [7:0] IDATA? ;? /* init data */
output ICLK?? ;? /* init clock */

input UPDT?? ;? /* high to trigger mirror image update */
output UENB?? ;? /* low to enable fifo */
input [7:0] UADDR; /* write address */
input [7:0] UDATA;? /* write data */

input SDI??? ;? /* serial input */
output SDO??? ; /* active low open-drain drive enable - data */
output SCK??? ; /* active low open-drain drive enable - clock */

reg IENB;
reg INIT;
reg UENB;
reg BTCK;
wire STEN;????
reg? [3:0] CSTATE;??
reg? [3:0] BCNT? ;?
reg? [7:0] CCNT? ;?
reg DLY???? ;
reg D2 ;
wire D2I;
wire NKI???? ;
reg NACK??? ;
wire WRI???? ;
wire RDI???? ;
reg [8:0] BYTE?? ;
reg [8:0] SDATA? ;
wire LD_BYTE? ;
reg STSP???? ;
wire CTL_VAL? ;

always @ (posedge CLK or negedge Reset_n)
begin
? if(Reset_n == 1'b0)
???? BTCK <= 1'b0;
? else
???? BTCK <= #1 !BTCK;

end

// INIT is set at power-up and cleared when the state machine
// reaches state 0101.

always @ (negedge Reset_n or posedge CLK)
begin
? if(Reset_n == 1'b0)
???? INIT <= 1'b1;
? else if(CSTATE == 4'b0101)
??????? INIT <= #1 1'b0 ;
end

// This state machine is set-up to read/write data to an AT24C02A
// serial Flash memory

//這個狀態機是建立AT24C02A串行閃存" title="串行閃存">串行閃存的數據讀寫

// At power-up, the INIT bit is set, and the state machine executes
// a 'sequencial read' operation starting at address 0x000 and
// procedding until all 256 bytes have been read and forwarded into
// the internal memory block. The state machine then sends a
// stop bit to the Flash and clears the INIT control bit.
//
//在上電" title="上電">上電時，INIT被設置為高，狀態機從地址0x000開始執行“連續讀”操作，
//一直進行下去直到所有的256字節都被讀，然后向前到內部存儲區，
//狀態機然后給FLASH發送一個停止位并且清除INIT控制位。

// The state machine then waits for updt to be set.
// When the updt bit is set, the interface asserts u_enb low on a
// falling-edge of clk and addr/data is latched? on the next falling edge
// (rd_clk should be on the rising-edge).? The state machine writes
// data to the external FLASH memory one byte at a time whenever
// updt is asserted high.? If the FIFO remains 'not empty' then this
// block will poll the NVM until it is ready, and then proceed with
// a write cycle for the next byte.

//狀態機一直等待UPDT被置為高。當updt比特被置成高，當clk下降沿時，接口把u_enb設成低
//（rd_clk應該在上升沿）。這個狀態機每次updt為高時就寫一個字節數據到外部閃存。
//如果FIFO保持“非空”，則這個塊會一直等到它準備好，然后在接下來的比特進行一個寫周期。

// State Machine:
//
// 0000 - reset state:?? generate a start bit and load 0xA0 command
// 0001 - send byte:???? then load 0x00 address
// 0010 - send byte:???? generate a start bit and load 0xA1 command
// 0011 - send byte:???? clear byte count
// 0100 - receive byte:? if cnt /= FF: ack, cnt++, goto 0004 else: nack
// 0101 - stop:????????? assert stop bit and loop until updt = 1 - then
//?????????????????????? generate a start bit and load A0
// 0110 - send byte:???? send byte - if nack - goto 0101, else load Address
// 0111 - send byte:???? send data byte, load data
// 1000 - send byte:???? goto 0101
//
// In practice, the state machine is just a counter that starts at zero
// and counts up, then jumps back to 101 and counts up again,
// returning to zero only when reset_n is asserted low.
//在練習中，狀態機只是一個從0開始計數的計數器，加起來，然后跳到101，再重新計數，
//只有到reset_n被置為低時才返回到0。

assign STEN = ( BCNT[3] == 1'b1 &&
????????????????? (CSTATE[2] != 1'b1 || CSTATE[2:1] == 2'b11 ||
?????????????????? CSTATE[3]? == 1'b1 || (CSTATE == 4'b0100 && CCNT == 8'b11111111) ||
????????????????? (CSTATE == 4'b0101 && UPDT == 1'b1)))?1'b1:1'b0;

always @(negedge Reset_n or negedge CLK)
begin
? if(Reset_n == 1'b0)?
???? CSTATE <= 4'b0000;
? else
? begin
? if(STEN == 1'b1 && BTCK == 1'b0)
??????? begin
???????? if(CSTATE < 4'b0101 && NACK == 1'b1)
?????????? CSTATE <= #1 4'b0000 ;
??????? end
?? else
??????? begin
??????? if (CSTATE[3] == 1'b1 || NACK == 1'b1)
????????? CSTATE <= #1 4'b0101 ;
??????? else
????????? CSTATE <= #1 CSTATE + 1'b1 ;
??????? end
? end
end

// The bit counter (BCNT) is cleared at the state transition
// and during the first cycle of state '0011' (for start bit).
// incremented on the falling-edge of clk when BTCK is low.

//比特計數器在過渡狀態時和在狀態“0011”（起始比特）第一個" title="第一個">第一個循環期間被清空。
//在clk下降沿并且BTCK為低電平時，比特計數器增加。

always @ (negedge Reset_n or negedge CLK)
begin
? if(Reset_n == 1'b0)
???? begin
???? BCNT <= 4'b0000;
???? DLY? <= 1'b0;
???? end
? else
??? begin
???? if(BTCK == 1'b0)
?????? begin
?????? if(BCNT[3] == 1'b1 && CSTATE == 4'b0010)
????????? DLY <= #1 1'b1;
?????? else
????????? DLY <= 1'b0;
??????
?????? if(BCNT[3] == 1'b1 || (CSTATE == 4'b0011 && DLY == 1'b1))
????????? BCNT <= #1 4'b0000;
?????? else
????????? BCNT <= #1 BCNT + 1'b1;
?????? end
???
??? end
end

// The byte counter (CCNT) is cleared in state 0011.
//字節計數器在狀態0011時被清零。

// It is incremented during the ACK bit after each
// byte transfer in state 0100 to count 0x00-0xFF bytes
// as they are read from the NVM.? ccnt is used both as
// a control signal and as the iaddr output.

assign D2I = (BTCK == 1'b1 && BCNT[3] == 1'b1 && CSTATE == 4'b0100)?1'b1:1'b0;
??????????

always @ (negedge Reset_n or negedge CLK)
begin
? if(Reset_n == 1'b0)
?? begin
???? CCNT <= 8'b0;
???? D2?? <= 1'b0;
?? end
? else
?? begin
???? D2 <= #1 D2I;
???? if(CSTATE == 4'b0011)
??????? CCNT <= #1 8'b0;
???? else if(D2 == 1'b1)
??????? CCNT <= #1 CCNT + 1'b1;
???? end
end

// the following logic checks the ACK bit for all states except
// states '0100' and '0101' and asserts NACK if the data pin is
// high during the 9th bit of any transfer.? This is registered
// so that the value is present during state changes.

????????????
endmodule