60 lines
3.1 KiB
Text
60 lines
3.1 KiB
Text
|
<<<
|
||
|
|
:sectnums:
|
||
|
|
==== Internal Bus Monitor (BUSKEEPER)
|
||
|
|
|
||
|
|
[cols="<3,<3,<4"]
|
||
|
|
[frame="topbot",grid="none"]
|
||
|
|
|=======================
|
||
|
|
| Hardware source file(s): | neorv32_buskeeper.vhd |
|
||
|
|
| Software driver file(s): | none |
|
||
|
|
| Top entity port: | none |
|
||
|
|
| Configuration generics: | none |
|
||
|
|
| Package constants: | `max_proc_int_response_time_c` | Access time window (#cycles)
|
||
|
|
| CPU interrupts: | none |
|
||
|
|
|=======================
|
||
|
|
|
||
|
|
|
||
|
|
**Theory of Operation**
|
||
|
|
|
||
|
|
The Bus Keeper is a fundamental component of the processor's internal bus system that ensures correct bus operations
|
||
|
|
to maintain execution safety. The Bus Keeper monitors every single bus transactions that is intimated by the CPU.
|
||
|
|
If an accessed device responds with an error condition or do not respond within a specific _access time window_,
|
||
|
|
the according bus access fault exception is raised. The following exceptions can be raised by the Bus Keeper
|
||
|
|
(see section <<_traps_exceptions_and_interrupts>> for all available CPU traps):
|
||
|
|
|
||
|
|
* `TRAP_CODE_I_ACCESS`: error during instruction fetch bus access
|
||
|
|
* `TRAP_CODE_S_ACCESS`: error during data store bus access
|
||
|
|
* `TRAP_CODE_L_ACCESS`: error during data load bus access
|
||
|
|
|
||
|
|
The **access time window**, in which an accessed device has to respond, is defined by the `max_proc_int_response_time_c`
|
||
|
|
constant from the processor's VHDL package file (`rtl/neorv32_package.vhd`). The default value is **15 clock cycles**.
|
||
|
|
|
||
|
|
In case of a bus access fault exception application software can evaluate the Bus Keeper's control register
|
||
|
|
`NEORV32_BUSKEEPER.CTRL` to retrieve further details of the bus exception. The _BUSKEEPER_ERR_FLAG_ bit indicates
|
||
|
|
that an actual bus access fault has occurred. The bit is sticky once set and is automatically cleared when reading or
|
||
|
|
writing the `NEORV32_BUSKEEPER.CTRL` register. The _BUSKEEPER_ERR_TYPE_ bit defines the type of the bus fault:
|
||
|
|
|
||
|
|
* `0` - "Device Error": The bus access exception was cause by the memory-mapped device that
|
||
|
|
has been accessed (the device asserted it's `err_o`).
|
||
|
|
* `1` - "Timeout Error": The bus access exception was caused by the Bus Keeper because the
|
||
|
|
accessed memory-mapped device did not respond within the access time window. Note that this error type can also be raised
|
||
|
|
by the optional timeout feature of the <<_processor_external_memory_interface_wishbone_axi4_lite>>).
|
||
|
|
|
||
|
|
[NOTE]
|
||
|
|
Bus access fault exceptions are also raised if a physical memory protection (PMP) rule is violated. In this case
|
||
|
|
the _BUSKEEPER_ERR_FLAG_ bit remains zero (since the error signal is not triggered by the BUSKEEPER but by
|
||
|
|
the CPU's PMP logic).
|
||
|
|
|
||
|
|
|
||
|
|
**Register Map**
|
||
|
|
|
||
|
|
.BUSKEEPER register map (`struct NEORV32_BUSKEEPER`)
|
||
|
|
[cols="<2,<2,<4,^1,<4"]
|
||
|
|
[options="header",grid="all"]
|
||
|
|
|=======================
|
||
|
|
| Address | Name [C] | Bit(s), Name [C] | R/W | Function
|
||
|
|
.2+<| `0xffffff78` .2+<| `NEORV32_BUSKEEPER.CTRL` <|`0` _BUSKEEPER_ERR_TYPE_ ^| r/- <| Bus error type, valid if _BUSKEEPER_ERR_FLAG_
|
||
|
|
<|`31` _BUSKEEPER_ERR_FLAG_ ^| r/c <| Sticky error flag, clears after read or write access
|
||
|
|
| `0xffffff7c` | - | _reserved_ | r/c | _reserved_ (mirrored access to `CTRL`)
|
||
|
|
|=======================
|