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| Configuration generics: | 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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| Package constants: | `max_proc_int_response_time_c` | Access time window (#cycles)
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| CPU interrupts: | none |
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| CPU interrupts: | none |
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|=======================
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|=======================
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**Theory of Operation**
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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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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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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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If an accessed device responds with an error condition or do not respond within a specific _access time window_,
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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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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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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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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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`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 is automatically cleared when reading the
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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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`NEORV32_BUSKEEPER.CTRL` register. The _BUSKEEPER_ERR_TYPE_ indicated the tape or bus fault:
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writing the `NEORV32_BUSKEEPER.CTRL` register. The _BUSKEEPER_ERR_TYPE_ indicated the tape of the bus fault:
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* _BUSKEEPER_ERR_TYPE_ = `0` - "Device Error": The bus access exception was cause by the memory-mapped device that
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* _BUSKEEPER_ERR_TYPE_ = `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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has been accessed (the device asserted it's `err_o`).
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* _BUSKEEPER_ERR_TYPE_ = `1` - "Timeout Error": The bus access exception was caused by the Bus Keeper because the
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* _BUSKEEPER_ERR_TYPE_ = `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.
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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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[NOTE]
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Bus access fault exceptions are also raised if a physical memory protection rule is violated. In this case
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Bus access fault exceptions are also raised if a physical memory protection rule is violated. In this case
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the _BUSKEEPER_ERR_FLAG_ bit remains zero.
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the _BUSKEEPER_ERR_FLAG_ bit remains zero.
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Furthermore, application software can determine the source of the bus access fault via the _BUSKEEPER_ERR_SRC_ bit:
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* _BUSKEEPER_ERR_SRC_ = `0`: The error was cause during access via the <<_processor_external_memory_interface_wishbone_axi4_lite>>).
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* _BUSKEEPER_ERR_SRC_ = `1`: The error was cause during access to an processor-internal module.
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[NOTE]
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The Bus Keeper does not track **timeout errors** of processor-external accesses via the external memory bus interface.
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However, the external memory bus interface also provides an _optional_ and independent bus timeout feature
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(see section <<_processor_external_memory_interface_wishbone_axi4_lite>>).
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.BUSKEEPER register map (`struct NEORV32_BUSKEEPER`)
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.BUSKEEPER register map (`struct NEORV32_BUSKEEPER`)
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[cols="<2,<2,<4,^1,<4"]
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[cols="<2,<2,<4,^1,<4"]
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[options="header",grid="all"]
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[options="header",grid="all"]
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|=======================
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|=======================
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| Address | Name [C] | Bit(s), Name [C] | R/W | Function
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| Address | Name [C] | Bit(s), Name [C] | R/W | Function
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.3+<| `0xffffff7C` .3+<| `NEORV32_BUSKEEPER.CTRL` <|`0` _BUSKEEPER_ERR_TYPE_ ^| r/- <| Bus error type, valid if _BUSKEEPER_ERR_FLAG_ is set: `0`=device error, `1`=access timeout
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.2+<| `0xffffff7C` .2+<| `NEORV32_BUSKEEPER.CTRL` <|`0` _BUSKEEPER_ERR_TYPE_ ^| r/- <| Bus error type, valid if _BUSKEEPER_ERR_FLAG_ is set: `0`=device error, `1`=access timeout
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<|`1` _BUSKEEPER_ERR_SRC_ ^| r/- <| Error source: `0`=processor-internal, `1`=processor-external (via Wishbone bus interface)
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<|`31` _BUSKEEPER_ERR_FLAG_ ^| r/- <| Sticky error flag, clears after read or write access
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<|`31` _BUSKEEPER_ERR_FLAG_ ^| r/- <| Sticky error flag, clears after read
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|=======================
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|=======================
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