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[cols="<3,<3,<4"]
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[frame="topbot",grid="none"]
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[frame="topbot",grid="none"]
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|=======================
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|=======================
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| Hardware source file(s): | neorv32_buskeeper.vhd |
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| Hardware source file(s): | neorv32_buskeeper.vhd |
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| Software driver file(s): | none | explicitly used
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| Software driver file(s): | none |
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| Top entity port: | none |
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| Top entity port: | none |
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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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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 and is automatically cleared when reading or
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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_ indicated the tape of the bus fault:
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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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* _BUSKEEPER_ERR_TYPE_ = `0` - "Device Error": The bus access exception was cause by the memory-mapped device that
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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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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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* `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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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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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 (PMP) 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 (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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**NULL Address Check**
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The bus keeper can ensure that no accesses are permitted to NULL addresses (`addr = 0x00000000`). These kind of
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access often occur when using uninitialized pointers. If the _BUSKEEPER_NULL_CHECK_EN_ bit is set, any access to
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address zero (instruction fetch, load data, store data) will raise an according bus exception. This flag
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automatically clears on a hardware reset.
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If a NULL address access has been detected the _BUSKEEPER_ERR_FLAG_ flag is set and the _BUSKEEPER_ERR_TYPE_
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flag is cleared indicating a "Device Error".
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[NOTE]
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Address 0 is normally used by the IMEM and contains boot code instructions that are executed _once_ right after
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hardware reset. Hence, activating the bus keeper's NULL check in application code will not corrupt code execution
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at all.
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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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.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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.3+<| `0xffffff7C` .3+<| `NEORV32_BUSKEEPER.CTRL` <|`0` _BUSKEEPER_ERR_TYPE_ ^| r/- | Bus error type, valid if _BUSKEEPER_ERR_FLAG_
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<|`16` _BUSKEEPER_NULL_CHECK_EN_ ^| r/w <| Enable NULL address check when set
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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 or write access
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|=======================
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|=======================
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