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/* (c) Peter McGoron 2022
* This Source Code Form is subject to the terms of the Mozilla Public
* License, v.2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at https://mozilla.org/MPL/2.0/.
*/
module spi_slave
#(
parameter WID = 24, // Width of bits per transaction.
parameter WID_LEN = 5, // Length in bits required to store WID
parameter POLARITY = 0,
parameter PHASE = 0 // 0 = rising-read falling-write, 1 = rising-write falling-read.
)
(
input clk,
input sck,
input ss_L,
`ifndef SPI_SLAVE_NO_READ
output reg [WID-1:0] from_master,
input mosi,
`endif
`ifndef SPI_SLAVE_NO_WRITE
input [WID-1:0] to_master,
output miso,
`endif
output finished,
input rdy,
output err
);
wire ss = !ss_L;
reg sck_delay = 0;
reg [WID_LEN-1:0] bit_counter = 0;
reg ss_delay = 0;
reg ready_at_start = 0;
`ifndef SPI_SLAVE_NO_WRITE
reg [WID-1:0] send_buf = 0;
`endif
task read_data();
`ifndef SPI_SLAVE_NO_READ
from_master <= from_master << 1;
from_master[0] <= mosi;
`endif
endtask
task write_data();
`ifndef SPI_SLAVE_NO_WRITE
send_buf <= send_buf << 1;
miso <= send_buf[WID-1];
`endif
endtask
task setup_bits();
/* at Mode 00, the transmission starts with
* a rising edge, and at mode 11, it starts with a falling
* edge. For both modes, these are READs.
*
* For mode 01 and mode 10, the first action is a WRITE.
*/
if (POLARITY == PHASE) begin
miso <= to_master[WID-1];
send_buf <= to_master << 1;
end else begin
send_buf <= to_master;
end
endtask
task check_counter();
if (bit_counter == WID) begin
err <= ready_at_start;
end else begin
bit_counter <= bit_counter + 1;
end
endtask
always @ (posedge clk) begin
sck_delay <= sck;
ss_delay <= ss;
case ({ss_delay, ss})
2'b01: begin // rising edge of SS
bit_counter <= 0;
finished <= 0;
err <= 0;
ready_at_start <= rdy;
setup_bits();
end
2'b10: begin // falling edge
finished <= ready_at_start;
end
2'b11: begin
case ({sck_delay, sck})
2'b01: begin // rising edge
if (PHASE == 1) begin
write_data();
end else begin
read_data();
end
if (POLARITY == 0) begin
check_counter();
end
end
2'b10: begin // falling edge
if (PHASE == 1) begin
read_data();
end else begin
write_data();
end
if (POLARITY == 1) begin
check_counter();
end
end
default: ;
endcase
end
2'b00: if (!rdy) begin
finished <= 0;
err <= 0;
end
endcase
end
endmodule
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