apb: Add APB RAM module and testbench

Signed-off-by: Alex Forencich <alex@alexforencich.com>

README.md

@@ -26,6 +26,7 @@ To facilitate the dual-license model, contributions to the project can only be a
 
 *  APB
     *  SV interface for APB
+    *  Single-port RAM
 *  AXI
     *  SV interface for AXI
     *  AXI to AXI lite adapter

src/apb/rtl/taxi_apb_ram.sv

@@ -0,0 +1,122 @@
+// SPDX-License-Identifier: CERN-OHL-S-2.0
+/*
+
+Copyright (c) 2025 FPGA Ninja, LLC
+
+Authors:
+- Alex Forencich
+
+*/
+
+`resetall
+`timescale 1ns / 1ps
+`default_nettype none
+
+/*
+ * APM RAM
+ */
+module taxi_apb_ram #
+(
+    // Width of address bus in bits
+    parameter ADDR_W = 16,
+    // Extra pipeline register on output
+    parameter logic PIPELINE_OUTPUT = 1'b0
+)
+(
+    input  wire logic  clk,
+    input  wire logic  rst,
+
+    /*
+     * APB slave interface
+     */
+    taxi_apb_if.slv    s_apb
+);
+
+// extract parameters
+localparam DATA_W = s_apb.DATA_W;
+localparam STRB_W = s_apb.STRB_W;
+
+localparam VALID_ADDR_W = ADDR_W - $clog2(STRB_W);
+localparam BYTE_LANES = STRB_W;
+localparam BYTE_W = DATA_W/BYTE_LANES;
+
+// check configuration
+if (BYTE_W * STRB_W != DATA_W)
+    $fatal(0, "Error: APB data width not evenly divisible (instance %m)");
+
+if (2**$clog2(BYTE_LANES) != BYTE_LANES)
+    $fatal(0, "Error: APB byte lane count must be even power of two (instance %m)");
+
+if (s_apb.ADDR_W < ADDR_W)
+    $fatal(0, "Error: APB address width is insufficient (instance %m)");
+
+logic mem_wr_en;
+logic mem_rd_en;
+
+logic s_apb_pready_reg = 1'b0, s_apb_pready_next;
+logic s_apb_pready_pipe_reg = 1'b0;
+logic [DATA_W-1:0] s_apb_prdata_reg = '0, s_apb_prdata_next;
+logic [DATA_W-1:0] s_apb_prdata_pipe_reg = '0;
+
+// (* RAM_STYLE="BLOCK" *)
+logic [DATA_W-1:0] mem[2**VALID_ADDR_W];
+
+wire [VALID_ADDR_W-1:0] s_apb_paddr_valid = VALID_ADDR_W'(s_apb.paddr >> (ADDR_W - VALID_ADDR_W));
+
+assign s_apb.prdata = PIPELINE_OUTPUT ? s_apb_prdata_pipe_reg : s_apb_prdata_reg;
+assign s_apb.pready = PIPELINE_OUTPUT ? s_apb_pready_pipe_reg : s_apb_pready_reg;
+assign s_apb.pslverr = 1'b0;
+assign s_apb.pruser = '0;
+assign s_apb.pbuser = '0;
+
+initial begin
+    // two nested loops for smaller number of iterations per loop
+    // workaround for synthesizer complaints about large loop counts
+    for (integer i = 0; i < 2**VALID_ADDR_W; i = i + 2**(VALID_ADDR_W/2)) begin
+        for (integer j = i; j < i + 2**(VALID_ADDR_W/2); j = j + 1) begin
+            mem[j] = '0;
+        end
+    end
+end
+
+always_comb begin
+    mem_wr_en = 1'b0;
+    mem_rd_en = 1'b0;
+
+    s_apb_pready_next = 1'b0;
+
+    if (s_apb.psel && s_apb.penable && (!s_apb_pready_reg && (PIPELINE_OUTPUT || !s_apb_pready_pipe_reg))) begin
+        s_apb_pready_next = 1'b1;
+
+        if (s_apb.pwrite) begin
+            mem_wr_en = 1'b1;
+        end else begin
+            mem_rd_en = 1'b1;
+        end
+    end
+end
+
+always_ff @(posedge clk) begin
+    s_apb_pready_reg <= s_apb_pready_next;
+
+    for (integer i = 0; i < BYTE_LANES; i = i + 1) begin
+        if (mem_wr_en && s_apb.pstrb[i]) begin
+            mem[s_apb_paddr_valid][BYTE_W*i +: BYTE_W] <= s_apb.pwdata[BYTE_W*i +: BYTE_W];
+        end
+    end
+
+    if (mem_rd_en) begin
+        s_apb_prdata_reg <= mem[s_apb_paddr_valid];
+    end
+
+    s_apb_prdata_pipe_reg <= s_apb_prdata_reg;
+    s_apb_pready_pipe_reg <= s_apb_pready_reg;
+
+    if (rst) begin
+        s_apb_pready_reg <= 1'b0;
+    end
+end
+
+endmodule
+
+`resetall

src/apb/tb/taxi_apb_ram/Makefile

@@ -0,0 +1,54 @@
+# SPDX-License-Identifier: CERN-OHL-S-2.0
+#
+# Copyright (c) 2020-2025 FPGA Ninja, LLC
+#
+# Authors:
+# - Alex Forencich
+
+TOPLEVEL_LANG = verilog
+
+SIM ?= verilator
+WAVES ?= 0
+
+COCOTB_HDL_TIMEUNIT = 1ns
+COCOTB_HDL_TIMEPRECISION = 1ps
+
+RTL_DIR = ../../rtl
+LIB_DIR = ../../lib
+TAXI_SRC_DIR = $(LIB_DIR)/taxi/src
+
+DUT      = taxi_apb_ram
+COCOTB_TEST_MODULES = test_$(DUT)
+COCOTB_TOPLEVEL     = test_$(DUT)
+MODULE   = $(COCOTB_TEST_MODULES)
+TOPLEVEL = $(COCOTB_TOPLEVEL)
+VERILOG_SOURCES += $(COCOTB_TOPLEVEL).sv
+VERILOG_SOURCES += $(RTL_DIR)/$(DUT).sv
+VERILOG_SOURCES += $(RTL_DIR)/taxi_apb_if.sv
+
+# handle file list files
+process_f_file = $(call process_f_files,$(addprefix $(dir $1),$(shell cat $1)))
+process_f_files = $(foreach f,$1,$(if $(filter %.f,$f),$(call process_f_file,$f),$f))
+uniq_base = $(if $1,$(call uniq_base,$(foreach f,$1,$(if $(filter-out $(notdir $(lastword $1)),$(notdir $f)),$f,))) $(lastword $1))
+VERILOG_SOURCES := $(call uniq_base,$(call process_f_files,$(VERILOG_SOURCES)))
+
+# module parameters
+export PARAM_DATA_W := 32
+export PARAM_ADDR_W := 16
+export PARAM_STRB_W := $(shell expr $(PARAM_DATA_W) / 8 )
+export PARAM_PIPELINE_OUTPUT := 0
+
+ifeq ($(SIM), icarus)
+	PLUSARGS += -fst
+
+	COMPILE_ARGS += $(foreach v,$(filter PARAM_%,$(.VARIABLES)),-P $(COCOTB_TOPLEVEL).$(subst PARAM_,,$(v))=$($(v)))
+else ifeq ($(SIM), verilator)
+	COMPILE_ARGS += $(foreach v,$(filter PARAM_%,$(.VARIABLES)),-G$(subst PARAM_,,$(v))=$($(v)))
+
+	ifeq ($(WAVES), 1)
+		COMPILE_ARGS += --trace-fst
+		VERILATOR_TRACE = 1
+	endif
+endif
+
+include $(shell cocotb-config --makefiles)/Makefile.sim

src/apb/tb/taxi_apb_ram/test_taxi_apb_ram.py

@@ -0,0 +1,213 @@
+#!/usr/bin/env python
+# SPDX-License-Identifier: CERN-OHL-S-2.0
+"""
+
+Copyright (c) 2025 FPGA Ninja, LLC
+
+Authors:
+- Alex Forencich
+
+"""
+
+import itertools
+import logging
+import os
+import random
+
+import cocotb_test.simulator
+import pytest
+
+import cocotb
+from cocotb.clock import Clock
+from cocotb.triggers import RisingEdge, Timer
+from cocotb.regression import TestFactory
+
+from cocotbext.axi import ApbBus, ApbMaster
+
+
+class TB(object):
+    def __init__(self, dut):
+        self.dut = dut
+
+        self.log = logging.getLogger("cocotb.tb")
+        self.log.setLevel(logging.DEBUG)
+
+        cocotb.start_soon(Clock(dut.clk, 10, units="ns").start())
+
+        self.apb_master = ApbMaster(ApbBus.from_entity(dut.s_apb), dut.clk, dut.rst)
+
+    def set_idle_generator(self, generator=None):
+        if generator:
+            self.apb_master.set_pause_generator(generator())
+
+    async def cycle_reset(self):
+        self.dut.rst.setimmediatevalue(0)
+        await RisingEdge(self.dut.clk)
+        await RisingEdge(self.dut.clk)
+        self.dut.rst.value = 1
+        await RisingEdge(self.dut.clk)
+        await RisingEdge(self.dut.clk)
+        self.dut.rst.value = 0
+        await RisingEdge(self.dut.clk)
+        await RisingEdge(self.dut.clk)
+
+
+async def run_test_write(dut, data_in=None, idle_inserter=None):
+
+    tb = TB(dut)
+
+    byte_lanes = tb.apb_master.byte_lanes
+
+    await tb.cycle_reset()
+
+    tb.set_idle_generator(idle_inserter)
+
+    for length in range(1, byte_lanes*2):
+        for offset in range(byte_lanes):
+            tb.log.info("length %d, offset %d", length, offset)
+            addr = offset+0x1000
+            test_data = bytearray([x % 256 for x in range(length)])
+
+            await tb.apb_master.write(addr-4, b'\xaa'*(length+8))
+
+            await tb.apb_master.write(addr, test_data)
+
+            data = await tb.apb_master.read(addr-1, length+2)
+
+            assert data.data == b'\xaa'+test_data+b'\xaa'
+
+    await RisingEdge(dut.clk)
+    await RisingEdge(dut.clk)
+
+
+async def run_test_read(dut, data_in=None, idle_inserter=None):
+
+    tb = TB(dut)
+
+    byte_lanes = tb.apb_master.byte_lanes
+
+    await tb.cycle_reset()
+
+    tb.set_idle_generator(idle_inserter)
+
+    for length in range(1, byte_lanes*2):
+        for offset in range(byte_lanes):
+            tb.log.info("length %d, offset %d", length, offset)
+            addr = offset+0x1000
+            test_data = bytearray([x % 256 for x in range(length)])
+
+            await tb.apb_master.write(addr, test_data)
+
+            data = await tb.apb_master.read(addr, length)
+
+            assert data.data == test_data
+
+    await RisingEdge(dut.clk)
+    await RisingEdge(dut.clk)
+
+
+async def run_stress_test(dut, idle_inserter=None):
+
+    tb = TB(dut)
+
+    await tb.cycle_reset()
+
+    tb.set_idle_generator(idle_inserter)
+
+    async def worker(master, offset, aperture, count=16):
+        for k in range(count):
+            length = random.randint(1, min(32, aperture))
+            addr = offset+random.randint(0, aperture-length)
+            test_data = bytearray([x % 256 for x in range(length)])
+
+            await Timer(random.randint(1, 100), 'ns')
+
+            await master.write(addr, test_data)
+
+            await Timer(random.randint(1, 100), 'ns')
+
+            data = await master.read(addr, length)
+            assert data.data == test_data
+
+    workers = []
+
+    for k in range(16):
+        workers.append(cocotb.start_soon(worker(tb.apb_master, k*0x1000, 0x1000, count=16)))
+
+    while workers:
+        await workers.pop(0).join()
+
+    await RisingEdge(dut.clk)
+    await RisingEdge(dut.clk)
+
+
+def cycle_pause():
+    return itertools.cycle([1, 1, 1, 0])
+
+
+if getattr(cocotb, 'top', None) is not None:
+
+    for test in [run_test_write, run_test_read, run_stress_test]:
+
+        factory = TestFactory(test)
+        factory.add_option("idle_inserter", [None, cycle_pause])
+        factory.generate_tests()
+
+
+# cocotb-test
+
+tests_dir = os.path.abspath(os.path.dirname(__file__))
+rtl_dir = os.path.abspath(os.path.join(tests_dir, '..', '..', 'rtl'))
+lib_dir = os.path.abspath(os.path.join(tests_dir, '..', '..', 'lib'))
+taxi_src_dir = os.path.abspath(os.path.join(lib_dir, 'taxi', 'src'))
+
+
+def process_f_files(files):
+    lst = {}
+    for f in files:
+        if f[-2:].lower() == '.f':
+            with open(f, 'r') as fp:
+                l = fp.read().split()
+            for f in process_f_files([os.path.join(os.path.dirname(f), x) for x in l]):
+                lst[os.path.basename(f)] = f
+        else:
+            lst[os.path.basename(f)] = f
+    return list(lst.values())
+
+
+@pytest.mark.parametrize("data_w", [8, 16, 32])
+def test_taxi_apb_ram(request, data_w):
+    dut = "taxi_apb_ram"
+    module = os.path.splitext(os.path.basename(__file__))[0]
+    toplevel = module
+
+    verilog_sources = [
+        os.path.join(tests_dir, f"{toplevel}.sv"),
+        os.path.join(rtl_dir, f"{dut}.sv"),
+        os.path.join(rtl_dir, "taxi_apb_if.sv"),
+    ]
+
+    verilog_sources = process_f_files(verilog_sources)
+
+    parameters = {}
+
+    parameters['DATA_W'] = data_w
+    parameters['ADDR_W'] = 16
+    parameters['STRB_W'] = parameters['DATA_W'] // 8
+    parameters['PIPELINE_OUTPUT'] = 0
+
+    extra_env = {f'PARAM_{k}': str(v) for k, v in parameters.items()}
+
+    sim_build = os.path.join(tests_dir, "sim_build",
+        request.node.name.replace('[', '-').replace(']', ''))
+
+    cocotb_test.simulator.run(
+        simulator="verilator",
+        python_search=[tests_dir],
+        verilog_sources=verilog_sources,
+        toplevel=toplevel,
+        module=module,
+        parameters=parameters,
+        sim_build=sim_build,
+        extra_env=extra_env,
+    )

src/apb/tb/taxi_apb_ram/test_taxi_apb_ram.sv

@@ -0,0 +1,54 @@
+// SPDX-License-Identifier: CERN-OHL-S-2.0
+/*
+
+Copyright (c) 2025 FPGA Ninja, LLC
+
+Authors:
+- Alex Forencich
+
+*/
+
+`resetall
+`timescale 1ns / 1ps
+`default_nettype none
+
+/*
+ * APB RAM testbench
+ */
+module test_taxi_apb_ram #
+(
+    /* verilator lint_off WIDTHTRUNC */
+    parameter DATA_W = 32,
+    parameter ADDR_W = 16,
+    parameter STRB_W = (DATA_W/8),
+    parameter PIPELINE_OUTPUT = 0
+    /* verilator lint_on WIDTHTRUNC */
+)
+();
+
+logic clk;
+logic rst;
+
+taxi_apb_if #(
+    .DATA_W(DATA_W),
+    .ADDR_W(ADDR_W+16),
+    .STRB_W(STRB_W)
+) s_apb();
+
+taxi_apb_ram #(
+    .ADDR_W(ADDR_W),
+    .PIPELINE_OUTPUT(PIPELINE_OUTPUT)
+)
+uut (
+    .clk(clk),
+    .rst(rst),
+
+    /*
+     * AXI4-Lite slave interface
+     */
+    .s_apb(s_apb)
+);
+
+endmodule
+
+`resetall