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==== Processor-External Memory Interface (WISHBONE) (AXI4-Lite)

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|=======================
| Hardware source file(s): | neorv32_wishbone.vhd |
| Software driver file(s): | none             | _implicitly used_
| Top entity port:         | `wb_tag_o`  | request tag output (3-bit)
|                          | `wb_adr_o`  | address output (32-bit)
|                          | `wb_dat_i`  | data input (32-bit)
|                          | `wb_dat_o`  | data output (32-bit)
|                          | `wb_we_o`   | write enable (1-bit)
|                          | `wb_sel_o`  | byte enable (4-bit)
|                          | `wb_stb_o`  | strobe (1-bit)
|                          | `wb_cyc_o`  | valid cycle (1-bit)
|                          | `wb_lock_o` | exclusive access request (1-bit)
|                          | `wb_ack_i`  | acknowledge (1-bit)
|                          | `wb_err_i`  | bus error (1-bit)
|                          | `fence_o`   | an executed `fence` instruction
|                          | `fencei_o`  | an executed `fence.i` instruction
| Configuration generics:  | _MEM_EXT_EN_ | enable external memory interface when _true_
|                          | _MEM_EXT_TIMEOUT_ | number of clock cycles after which an unacknowledged external bus access will auto-terminate (0 = disabled)
| 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
|                                                                     | `wb_big_endian_c` | byte-order (Endianness) of external memory interface; true=BIG, false=little (default)
|                                                                     | `wb_rx_buffer_c` | enable register buffer for RX path (default)
| CPU interrupts:          | none             |
|=======================

The external memory interface uses the Wishbone interface protocol. The external interface port is available
when the _MEM_EXT_EN_ generic is _true_. This interface can be used to attach external memories, custom
hardware accelerators additional IO devices or all other kinds of IP blocks. All memory accesses from the
CPU, that do not target the internal bootloader ROM, the internal IO region or the internal data/instruction
memories (if implemented at all) are forwarded to the Wishbone gateway and thus to the external memory
interface.

[TIP]
When using the default processor setup, all access addresses between 0x00000000 and
0xffff0000 (= beginning of processor-internal BOOT ROM) are delegated to the external memory
/ bus interface if they are not targeting the (actually enabled/implemented) processor-internal
instruction memory (IMEM) or the (actually enabled/implemented) processor-internal data memory
(DMEM). See section <<_address_space>> for more information.

**Wishbone Bus Protocol**

The external memory interface either uses **standard** ("classic") Wishbone transactions (default) or
**pipelined** Wishbone transactions. The transaction protocol is configured via the wb_pipe_mode_c constant
in the in the main VHDL package file (`rtl/neorv32_package.vhd`):

[source,vhdl]
----
-- external bus interface --
constant wb_pipe_mode_c : boolean := false;
----

When `wb_pipe_mode_c` is disabled, all bus control signals including _STB_ are active (and stable) until the
transfer is acknowledged/terminated. If `wb_pipe_mode_c` is enabled, all bus control except _STB_ are active
(and stable) until the transfer is acknowledged/terminated. In this case, _STB_ is active only during the very
first bus clock cycle.

.Exemplary Wishbone bus accesses using "classic" and "pipelined" protocol
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|=======================
a| image::wishbone_classic_read.png[700,300]
a| image::wishbone_pipelined_write.png[700,300]
| **Classic** Wishbone read access | **Pipelined** Wishbone write access
|=======================


[TOP]
A detailed description of the implemented Wishbone bus protocol and the according interface signals
can be found in the data sheet "Wishbone B4 – WISHBONE System-on-Chip (SoC) Interconnection
Architecture for Portable IP Cores". A copy of this document can be found in the docs folder of this
project.

**Interface Latency**

By default, the Wishbone gateway introduces two additional latency cycles: processor-outgoing ("TX") and
processor-incoming ("RX") signals are fully registered. Thus, any access from the CPU to a processor-external devices
via Wishbone requires 2 additional clock cycles (at least; depending on device's latency).

If the attached Wishbone network / peripheral already provides output registers or if the Wishbone network is not relevant
for timing closure, the default buffering of incoming ("RX") data within the gateway can be disabled.
The configuration is done via the `wb_rx_buffer_c` constant in the in the main VHDL package file (`rtl/neorv32_package.vhd`):

[source,vhdl]
----
-- external bus interface --
constant wb_rx_buffer_c : boolean := false; -- false to implement "async" RX (non-default)
----

**Bus Access Timeout**

The Wishbone bus interface provides an option to configure a bus access timeout counter. The _MEM_EXT_TIMEOUT_
top generic is used to specify the _maximum_ time (in clock cycles) a bus access can be pending before it is automatically
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.

When _MEM_EXT_TIMEOUT_ is greater than zero, the WIshbone adapter starts an internal countdown whenever the CPU
accesses a memory address via the external memory interface. If the accessed memory / device does not acknowledge (via `wb_ack_i`)
or terminate (via `wb_err_i`) the transfer within _MEM_EXT_TIMEOUT_ clock cycles, the bus access is automatically canceled
(setting `wb_cyc_o` low again) and a load/store/instruction fetch bus access fault exception is raised.

[TIP]
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
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
memory system can guarantee to access **any** bus access (even it targets an unimplemented address) the timeout feature should be disabled
(_MEM_EXT_TIMEOUT_ = 0).

**Wishbone Tag**

The 3-bit wishbone `wb_tag_o` signal provides additional information regarding the access type. This signal
is compatible to the AXI4 _AxPROT_ signal.

* `wb_tag_o(0)` 1: privileged access (CPU is in machine mode); 0: unprivileged access
* `wb_tag_o(1)` always zero (indicating "secure access")
* `wb_tag_o(2)` 1: instruction fetch access, 0: data access

**Exclusive / Atomic Bus Access**

If the atomic memory access CPU extension (via _CPU_EXTENSION_RISCV_A_) is enabled, the CPU can
request an atomic/exclusive bus access via the external memory interface.

The load-reservate instruction (`lr.w`) will set the `wb_lock_o` signal telling the bus interconnect to establish a
reservation for the current accessed address (start of an exclusive access). This signal will stay asserted until
another memory access instruction is executed (for example a `sc.w`).

The memory system has to make sure that no other entity can access the reservated address until `wb_lock_o`
is released again. If this attempt fails, the memory system has to assert `wb_err_i` in order to indicate that the
reservation was broken.

[TIP]
See section <<_bus_interface>> for the CPU bus interface protocol.

**Endianness**

The NEORV32 CPU and the Processor setup are *little-endian* architectures. To allow direct connection
to a big-endian memory system the external bus interface provides an _Endianness configuration_. The
Endianness (of the external memory interface) can be configured via the global `wb_big_endian_c`
constant in the main VHDL package file (`rtl/neorv32_package.vhd`). By default, the external memory
interface uses little-endian byte-order.

[source,vhdl]
----
-- external bus interface --
constant wb_big_endian_c : boolean := true;
----

Application software can check the Endianness configuration of the external bus interface via the
_SYSINFO_FEATURES_MEM_EXT_ENDIAN_ flag in the processor's SYSINFO module (see section
<<_system_configuration_information_memory_sysinfo>> for more information).

**AXI4-Lite Connectivity**

The AXI4-Lite wrapper (`rtl/templates/system/neorv32_SystemTop_axi4lite.vhd`) provides a Wishbone-to-
AXI4-Lite bridge, compatible with Xilinx Vivado (IP packager and block design editor). All entity signals of
this wrapper are of type _std_logic_ or _std_logic_vector_, respectively.

The AXI Interface has been verified using Xilinx Vivado IP Packager and Block Designer. The AXI
interface port signals are automatically detected when packaging the core.

.Example AXI SoC using Xilinx Vivado
image::neorv32_axi_soc.png[]

[WARNING]
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
bus transactions. Therefore, the NEORV32 top wrapper with AXI4-Lite interface (`rtl/templates/system/neorv32_SystemTop_axi4lite`) configures _MEM_EXT_TIMEOUT_ = 0 by default.