60 lines
2.5 KiB
ReStructuredText
60 lines
2.5 KiB
ReStructuredText
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.. _backup_ram:
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Safety Backup RAM
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=================
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Overview
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--------
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The STM controller's backup RAM is used to store different kinds of information that shall be preserved if the controller resets.
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The hardware setup is missing a separate powersupply for the controller's backup domain. Therefore, the backup RAM is cleared, when the power is cut.
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The backup RAM is used to store permanent error flags (See :ref:`safety_flags`). This ensures the flags stay present, even if a system reset is performed. The only way to clear them is by cutting the power.
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Because cutting the power is a way to clear the backup RAM, no separate method for clearing the error entries in the backup RAM is defined.
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The backup RAM contents are protected by a `CRC Checksum`_.
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The backup RAM is initialized and checked after boot. If the controller starts from a powered down state,
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the backup RAM is empty. This is detected by an invalid `Header`_ at the beginning of the backup RAM. If this is the case, the safety ocntoller
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will create a valid backup RAM image with a `Header`_, empty `Status Flag Entries`_, an empty `Error Memory`_, and a valid `CRC Checksum`_.
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If the Header is valid during boot (verified by plausible values and correct magic numbers), the backup RAM is CRC checked.
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In case of a CRC error, the Backup RAM is wiped and reinitialized. On top of that, the error flag :ref:`safety_flags_safety_mem_corrupt` is set.
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.. note:: It may be possible that future versions of the hardware include a backup RAM battery / Goldcap. In this case, a way to clear the error memory will be implemented,
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because it will no longer be possible to clear the error memory by cutting the power.
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On top of that, the backup memory will also contain the calibration data.
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Partitioning and Entries
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------------------------
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The backup RAM consists of multiple sections. The memory section are listed below.
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Header
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~~~~~~
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The backup memory header is located at offset address:
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.. doxygendefine:: SAFETY_MEMORY_HEADER_ADDRESS
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The header is defined by the following structure:
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.. doxygenstruct:: safety_memory_header
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The validity of the header is checked, if the magic and inverse amgic fields contain the correct values, and if the offset address pointers
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have values that are located inside the error memory and are not ``0`` or the same value.
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The safety memory header magic is:
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.. doxygendefine:: SAFETY_MEMORY_MAGIC
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Status Flag Entries
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~~~~~~~~~~~~~~~~~~~
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Error Memory
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~~~~~~~~~~~~
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CRC Checksum
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~~~~~~~~~~~~
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