Is it possible to have a Chisel "is" condition which behaves similar to a Verilog "default" case in a Chisel "switch" statement? This is required to prevent latches being inferred for combinatorial outputs from an FSM.
Verilog Example:
module detector (clk, rstn, in1, in2, out1, out2);
input clk, rstn, in1, in2;
output reg out1, out2;
localparam [1:0] IDLE = 2'b01;
localparam [1:0] GOT_IN1 = 2'b10;
reg [1:0] state, nxtState;
always @ (posedge clk or negedge rstn) begin
if (!rstn) begin
state <= IDLE;
end else begin
state <= nxtState;
end
end
always @ (*) begin
case (state)
IDLE: begin
if (in1) begin
out1 = 1'b1;
out2 = 1'b0;
nxtState = GOT_IN1;
end else begin
out1 = 1'b0;
out2 = 1'b0;
nxtState = IDLE;
end
end
GOT_IN1: begin
if (in2) begin
out1 = 1'b0;
out2 = 1'b1;
nxtState = IDLE;
end else begin
out1 = 1'b0;
out2 = 1'b0;
nxtState = GOT_IN1;
end
end
default: begin
out1 = 1'b0;
out2 = 1'b0;
nxtState = IDLE;
end
endcase
end
endmodule
Chisel2 allowed this type of behavior as a default value could be assigned to out1 and out2 in the switch statement outside of an "is" condition.
switch (state) {
io.out1 := UInt(0)
io.out2 := UInt(0)
is (IDLE) {
when (io.in1 === UInt(1)) {
io.out1 := UInt(1)
io.out2 := UInt(0)
state := GOT_IN1
} .otherwise {
io.out1 := UInt(0)
io.out2 := UInt(0)
state := IDLE
}
}
is (GOT_IN1) {
when (io.in2 === UInt(1)) {
io.out1 := UInt(0)
io.out2 := UInt(1)
state := IDLE
} .otherwise {
io.out1 := UInt(0)
io.out2 := UInt(0)
state := GOT_IN1
}
}
}
Chisel3 doesn't support this default assignment syntax like Chisel2. A build error gets flagged:
◾exception during macro expansion: java.lang.Exception: Cannot include blocks that do not begin with is() in switch. at chisel3.util.switch
Chisel3 doesn't appear to have any method to prevent a latch from being inferred on the out1 and out2 outputs. I understand that out1 and out2 assignments can be moved outside of the switch statement and handled using a conditional assignment. However, from a code visibility standpoint it's clearer to handle the assignments within the case statement for large FSMs with dozens of states and combinatorial outputs.
1 Answer
You can add bracket deliberately to identify some code should be read together. Something like:
{ // state machine block
io.out1 := UInt(0)
io.out2 := UInt(0)
when (state === IDLE) {
when (io.in1 === UInt(1)) {
io.out1 := UInt(1)
io.out2 := UInt(0)
state := GOT_IN1
} .otherwise {
io.out1 := UInt(0)
io.out2 := UInt(0)
state := IDLE
}
}
when (state === GOT_IN1) {
when (io.in2 === UInt(1)) {
io.out1 := UInt(0)
io.out2 := UInt(1)
state := IDLE
} .otherwise {
io.out1 := UInt(0)
io.out2 := UInt(0)
state := GOT_IN1
}
}
}
I believe Chisel will never generate latches. The first assignment takes effect if no later assignment change is enabled.
