AHB-APB Bridge

dv/ahb/SimpleDecoder/SimpleDecoder_tb.cpp

@@ -1,4 +1,36 @@
 #include <catch2/catch_test_macros.hpp>
 #include <VSimpleDecoder_tl.h>
 
-TEST_CASE("SimpleDecoder, Dummy Test") {}
+
+TEST_CASE("SimpleDecoder, Basic Functionality") {
+    VSimpleDecoder_tl* decoder = new VSimpleDecoder_tl;
+    Verilated::traceEverOn(true);
+    decoder->clk = 0;
+    decoder->nReset = 1;
+    decoder->eval();
+    decoder->clk = 1;
+    decoder->eval();
+    decoder->clk = 0;
+    decoder->eval();
+    decoder->nReset = 0;
+    decoder->eval();
+    decoder->clk = 1;
+    decoder->eval();
+
+    //test for addresses 1 to 3
+    for (int address = 1; address <= 3; ++address) {
+        int expected_sel = 1 << (address - 1);
+
+        decoder->addr = address;
+        decoder->eval();
+        decoder->clk = 0; //falling edge
+        decoder->eval();
+        decoder->clk = 1; //rising edge
+        decoder->eval();
+
+        CAPTURE(address, expected_sel); //capture the values
+        REQUIRE(decoder->sel == expected_sel);
+    }
+    decoder->final();
+    delete decoder;
+}

rtl/apb/devices/Bridge.sv

@@ -4,9 +4,85 @@
   @note See https://developer.arm.com/documentation/ihi0011/a/AMBA-APB/APB-bridge/Interface-diagram
 */
 module Bridge (
-  AHBCommon_if.subordinate sub,
-  APBCommon_if.bridge bridge
+    AHBCommon_if.subordinate sub,
+    APBCommon_if.bridge bridge
 );
-  // Interface the two modports here
+    //three states
+    typedef enum logic [1:0] {
+        IDLE,
+        SETUP,
+        ENABLE
+    } state_t;
 
+    //signals
+    state_t state = IDLE;
+    logic [31:0] write_data_buffer;
+    logic [31:0] read_data_buffer;
+    logic [31:0] address_buffer;
+    logic write_buffer;
+    logic setup_done;
+
+    //interface the two modports here
+    always_ff @(posedge sub.clk or negedge sub.nReset) begin
+        if (!sub.nReset) begin
+            state <= IDLE;
+            sub.readyOut <= 1'b0; //will be set when finished
+            sub.rData <= 32'd0; //Clear the read data buffer on the AHB side
+            sub.resp <= 2'b00; //set default response to ok
+            bridge.addr <= 32'd0;//this buffer holds the address for the next APB transaction.
+            bridge.write <= 1'b0; //this is used to indicate the next operation is not a write by default
+            bridge.wData <= 32'd0; // store data that is to be written to the APB bus during a write transaction
+            bridge.selectors <= 1'b0; //select which peripheral on the APB bus should be active for the next transaction
+            bridge.enable <= 1'b0;
+            write_data_buffer <= 32'd0;
+            read_data_buffer <= 32'd0;
+            address_buffer <= 32'd0;
+            write_buffer <= 1'b0; //if set, write operation; if not, read operation
+            setup_done <= 1'b0; //flag that indicates if the setup stage of the current APB transaction is complete
+        end else begin
+            case (state)
+                IDLE: begin //wait for AHB signal
+                    if (sub.sel && !setup_done) begin
+                        address_buffer <= sub.addr;
+                        write_buffer <= sub.write;
+                        write_data_buffer <= sub.wData;
+                        state <= SETUP;
+                    end
+                end
+                SETUP: begin
+                    //set up APB transfer
+                    bridge.selectors <= 1'b1; //select the peripheral
+                    bridge.addr <= address_buffer;
+                    bridge.write <= write_buffer;
+                    bridge.wData <= write_data_buffer;
+                    bridge.enable <= 1'b0;
+                    setup_done <= 1'b1;
+                    state <= ENABLE;
+                end
+                ENABLE: begin
+                    //enable APB transfer and wait for the ready signal
+                    bridge.enable <= 1'b1;
+                    if (bridge.ready) begin
+                        //complete the AHB transfer
+                        sub.readyOut <= 1'b1;
+                        if (!write_buffer) begin
+                            read_data_buffer <= bridge.rData;
+                        end
+                        sub.resp <= 2'b00; //okresponse
+                        state <= IDLE;
+                        setup_done <= 1'b0;
+                    end
+                end
+            endcase
+        end
+    end
+
+    //read data back to AHB
+    always_comb begin
+        if (!write_buffer && sub.sel) begin
+            sub.rData = read_data_buffer;
+        end else begin
+            sub.rData = 32'bz;
+        end
+    end
 endmodule