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==== Processor-External Memory Interface (WISHBONE) (AXI4-Lite)
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[cols="<3,<3,<4"]
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[frame="topbot",grid="none"]
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|=======================
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| Hardware source file(s): | neorv32_wishbone.vhd |
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| Software driver file(s): | none             | _implicitly used_
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| Top entity port:         | `wb_tag_o`  | request tag output (3-bit)
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|                          | `wb_adr_o`  | address output (32-bit)
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|                          | `wb_dat_i`  | data input (32-bit)
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|                          | `wb_dat_o`  | data output (32-bit)
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|                          | `wb_we_o`   | write enable (1-bit)
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|                          | `wb_sel_o`  | byte enable (4-bit)
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|                          | `wb_stb_o`  | strobe (1-bit)
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|                          | `wb_cyc_o`  | valid cycle (1-bit)
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|                          | `wb_lock_o` | exclusive access request (1-bit)
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|                          | `wb_ack_i`  | acknowledge (1-bit)
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|                          | `wb_err_i`  | bus error (1-bit)
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|                          | `fence_o`   | an executed `fence` instruction
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|                          | `fencei_o`  | an executed `fence.i` instruction
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| Configuration generics:  | _MEM_EXT_EN_ | enable external memory interface when _true_
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|                          | _MEM_EXT_TIMEOUT_ | number of clock cycles after which an unacknowledged external bus access will auto-terminate (0 = disabled)
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| Configuration constants in VHDL package file `neorv32_package.vhd`: | `wb_pipe_mode_c` | when _false_ (default): classic/standard Wishbone protocol; when _true_: pipelined Wishbone protocol
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|                                                                     | `xbus_big_endian_c` | byte-order (Endianness) of external memory interface; true=BIG, false=little (default)
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| CPU interrupts:          | none             |
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|=======================
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The external memory interface uses the Wishbone interface protocol. The external interface port is available
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when the _MEM_EXT_EN_ generic is _true_. This interface can be used to attach external memories, custom
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hardware accelerators additional IO devices or all other kinds of IP blocks. All memory accesses from the
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CPU, that do not target the internal bootloader ROM, the internal IO region or the internal data/instruction
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memories (if implemented at all) are forwarded to the Wishbone gateway and thus to the external memory
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interface.
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[TIP]
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When using the default processor setup, all access addresses between 0x00000000 and
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0xffff0000 (= beginning of processor-internal BOOT ROM) are delegated to the external memory
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/ bus interface if they are not targeting the (actually enabled/implemented) processor-internal
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instruction memory (IMEM) or the (actually enabled/implemented) processor-internal data memory
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(DMEM). See section <<_address_space>> for more information.
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**Wishbone Bus Protocol**
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The external memory interface either uses **standard** ("classic") Wishbone transactions (default) or
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**pipelined** Wishbone transactions. The transaction protocol is configured via the wb_pipe_mode_c constant
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in the in the main VHDL package file (`rtl/neorv32_package.vhd`):
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[source,vhdl]
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----
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-- (external) bus interface --
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constant wb_pipe_mode_c : boolean := false;
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----
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When `wb_pipe_mode_c` is disabled, all bus control signals including _STB_ are active (and stable) until the
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transfer is acknowledged/terminated. If `wb_pipe_mode_c` is enabled, all bus control except _STB_ are active
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(and stable) until the transfer is acknowledged/terminated. In this case, _STB_ is active only during the very
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first bus clock cycle.
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.Exemplary Wishbone bus accesses using "classic" and "pipelined" protocol
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[cols="^2,^2"]
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[grid="none"]
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|=======================
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a| image::wishbone_classic_read.png[700,300]
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a| image::wishbone_pipelined_write.png[700,300]
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| **Classic** Wishbone read access | **Pipelined** Wishbone write access
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|=======================
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[TOP]
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A detailed description of the implemented Wishbone bus protocol and the according interface signals
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can be found in the data sheet "Wishbone B4 – WISHBONE System-on-Chip (SoC) Interconnection
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Architecture for Portable IP Cores". A copy of this document can be found in the docs folder of this
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project.
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**Interface Latency**
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The Wishbone gateway introduces two additional latency cycles: Processor-outgoing and -incoming signals
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are fully registered. Thus, any access from the CPU to a processor-external devices requires +2 clock cycles.
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**Bus Access Timeout**
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The Wishbone bus interface provides an option to configure a bus access timeout counter. The _MEM_EXT_TIMEOUT_
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top generic is used to specify the _maximum_ time (in clock cycles) a bus access can be pending before it is automatically
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terminated. If _MEM_EXT_TIMEOUT_ is set to zero, the timeout disabled an a bus access can take an arbitrary number of cycles to complete.
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When _MEM_EXT_TIMEOUT_ is greater than zero, the WIshbone adapter starts an internal countdown whenever the CPU
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accesses a memory address via the external memory interface. If the accessed memory / device does not acknowledge (via `wb_ack_i`)
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or terminate (via `wb_err_i`) the transfer within _MEM_EXT_TIMEOUT_ clock cycles, the bus access is automatically canceled
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(setting `wb_cyc_o` low again) and a load/store/instruction fetch bus access fault exception is raised.
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[TIP]
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This feature can be used as **safety guard** if the external memory system does not check for "address space holes". That means that addresses, which
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do not belong to a certain memory or device, do not permanently stall the processor due to an unacknowledged/unterminated bus access. If the external
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memory system can guarantee to access **any** bus access (even it targets an unimplemented address) the timeout feature should be disabled
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(_MEM_EXT_TIMEOUT_ = 0).
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**Wishbone Tag**
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The 3-bit wishbone `wb_tag_o` signal provides additional information regarding the access type. This signal
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is compatible to the AXI4 _AxPROT_ signal.
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* `wb_tag_o(0)` 1: privileged access (CPU is in machine mode); 0: unprivileged access
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* `wb_tag_o(1)` always zero (indicating "secure access")
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* `wb_tag_o(2)` 1: instruction fetch access, 0: data access
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**Exclusive / Atomic Bus Access**
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If the atomic memory access CPU extension (via _CPU_EXTENSION_RISCV_A_) is enabled, the CPU can
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request an atomic/exclusive bus access via the external memory interface.
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The load-reservate instruction (`lr.w`) will set the `wb_lock_o` signal telling the bus interconnect to establish a
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reservation for the current accessed address (start of an exclusive access). This signal will stay asserted until
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another memory access instruction is executed (for example a `sc.w`).
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The memory system has to make sure that no other entity can access the reservated address until `wb_lock_o`
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is released again. If this attempt fails, the memory system has to assert `wb_err_i` in order to indicate that the
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reservation was broken.
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[TIP]
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See section <<_bus_interface>> for the CPU bus interface protocol.
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**Endianness**
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The NEORV32 CPU and the Processor setup are *little-endian* architectures. To allow direct connection
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to a big-endian memory system the external bus interface provides an _Endianness configuration_. The
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Endianness (of the external memory interface) can be configured via the global `xbus_big_endian_c`
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constant in the main VHDL package file (`rtl/neorv32_package.vhd`). By default, the external memory
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interface uses little-endian byte-order.
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[source,vhdl]
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----
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-- (external) bus interface --
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constant xbus_big_endian_c : boolean := true;
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----
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Application software can check the Endianness configuration of the external bus interface via the
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_SYSINFO_FEATURES_MEM_EXT_ENDIAN_ flag in the processor's SYSINFO module (see section
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<<_system_configuration_information_memory_sysinfo>> for more information).
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**AXI4-Lite Connectivity**
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The AXI4-Lite wrapper (`rtl/top_templates/neorv32_top_axi4lite.vhd`) provides a Wishbone-to-
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AXI4-Lite bridge, compatible with Xilinx Vivado (IP packager and block design editor). All entity signals of
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this wrapper are of type _std_logic_ or _std_logic_vector_, respectively.
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The AXI Interface has been verified using Xilinx Vivado IP Packager and Block Designer. The AXI
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interface port signals are automatically detected when packaging the core.
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.Example AXI SoC using Xilinx Vivado
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image::neorv32_axi_soc.png[]
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[WARNING]
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Using the auto-termination timeout feature (_MEM_EXT_TIMEOUT_ greater than zero) is **not AXI4 compliant** as the AXI protocol does not support canceling of
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bus transactions. Therefore, the NEORV32 top wrapper with AXI4-Lite interface (`rtl/top_templates/neorv32_top_axi4lite`) configures _MEM_EXT_TIMEOUT_ = 0 by default.
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