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