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//------------------------------
// Module name: spi2dac
// Function: SPI interface for MPC4911 DAC
// Creator: Peter Cheung
// Version: 1.3
// Date: 8 Nov 2016
//------------------------------
module spi2dac (clk, data_in, load, dac_sdi, dac_cs, dac_sck, dac_ld);
input clk; // 50MHz system clock of DE0
input [9:0] data_in; // input data to DAC
input load; // Pulse to load data to dac
output dac_sdi; // SPI serial data out
output dac_cs; // chip select - low when sending data to dac
output dac_sck; // SPI clock, 16 cycles at half clk freq
output dac_ld;
//-------------Input Ports-----------------------------
// All the input ports should be wires
wire clk, load;
wire [9:0] data_in;
//-------------Output Ports-----------------------------
// Output port can be a storage element (reg) or a wire
reg dac_cs, dac_ld;
wire dac_sck, dac_sdi;
parameter BUF=1'b1; // 0:no buffer, 1:Vref buffered
parameter GA_N=1'b1; // 0:gain = 2x, 1:gain = 1x
parameter SHDN_N=1'b1; // 0:power down, 1:dac active
wire [3:0] cmd = {1'b0,BUF,GA_N,SHDN_N}; // wire to VDD or GND
// --- Submodule: Generate internal clock at 1 MHz -----
reg clk_1MHz; // 1Mhz clock derived from 50MHz
reg [4:0] ctr; // internal counter
parameter TIME_CONSTANT = 5'd24; // change this for diff clk freq
initial begin
clk_1MHz = 0; // don't need to reset - don't care if it is 1 or 0 to start
ctr = 5'b0; // ... Initialise when FPGA is configured
end
always @ (posedge clk) //
if (ctr==0) begin
ctr <= TIME_CONSTANT;
clk_1MHz <= ~clk_1MHz; // toggle the output clock for squarewave
end
else
ctr <= ctr - 1'b1;
// ---- end internal clock generator ----------
// ---- Detect posedge of load with a small state machine
// .... FF set on posedge of load
// .... reset when dac_cs goes high at the end of DAC output cycle
reg [1:0] sr_state;
parameter IDLE = 2'b00,WAIT_CSB_FALL = 2'b01, WAIT_CSB_HIGH = 2'b10;
reg dac_start; // set if a DAC write is detected
initial begin
sr_state = IDLE;
dac_start = 1'b0; // set while sending data to DAC
end
always @ (posedge clk)
case (sr_state)
IDLE: if (load==1'b0) sr_state <= IDLE;
else begin
sr_state <= WAIT_CSB_FALL;
dac_start <= 1'b1;
end
WAIT_CSB_FALL: if (dac_cs==1'b1) sr_state <= WAIT_CSB_FALL;
else sr_state <= WAIT_CSB_HIGH;
WAIT_CSB_HIGH: if (dac_cs==1'b0) sr_state <= WAIT_CSB_HIGH;
else begin
sr_state <= IDLE;
dac_start <= 1'b0;
end
default: sr_state <= IDLE;
endcase
//------- End circuit to detect start and end of conversion
//------- spi controller designed as a state machine
// .... with 17 states (idle, and S1-S16
// .... for the 16 cycles each sending 1-bit to dac)
reg [4:0] state;
initial begin
state = 5'b0; dac_ld = 1'b0; dac_cs = 1'b1;
end
always @(posedge clk_1MHz) begin
// default outputs and state transition
dac_cs <= 1'b0; dac_ld <= 1'b1;
state <= state + 1'b1; // move to next state by default
case (state)
5'd0: if (dac_start == 1'b0) begin
state <= 5'd0; // still waiting
dac_cs <= 1'b1;
end
5'd16: begin
dac_cs <= 1'b1; dac_ld <= 1'b0;
state <= 5'd0; // go back to idle state
end
default: begin // all other states
dac_cs <= 1'b0; dac_ld <= 1'b1;
state <= state + 1'b1; // default state transition
end
endcase
end // ... always
// shift register for output data
reg [15:0] shift_reg;
initial begin
shift_reg = 16'b0;
end
always @(posedge clk_1MHz)
if((dac_start==1'b1)&&(dac_cs==1'b1)) // parallel load data to shift reg
shift_reg <= {cmd,data_in,2'b00};
else // .. else start shifting
shift_reg <= {shift_reg[14:0],1'b0};
// Assign outputs to drive SPI interface to DAC
assign dac_sck = !clk_1MHz&!dac_cs;
assign dac_sdi = shift_reg[15];
endmodule
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