| RTL Coding Guidelines |
| ===================== |
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| The following is a set of RTL coding guidelines with the purpose of maximizing the readability and maintainability |
| of the codebase, without being too restrictive or inconvenient. The goal is to achieve a consistent 'look and feel' |
| across the several modules and subsystems, so long-term and new contributors alike can infer some of the |
| module's functionality and, more importantly, its interface, without having to reverse-engineer the code. This will |
| hopefully promote contributions, facilitate code re-use and mitigate fragmentation. |
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| These guidelines should be general enough such that they are not restricted to a specific Hardware Description Language, unless otherwise noted. |
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| Each guideline is individually labeled so it can be easily referred to in the context of code reviews or issue creation. |
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| A - Interfaces |
| -- |
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| #### A.1 - Interface signal naming |
| In the context of RTL modules, an interface is a collection of input/output signals that, when used together, implement |
| a protocol that allows for the transfer of data in one, or both, directions. These closely-related signals should, therefore, |
| share a common prefix that makes their relationship clear. This becomes even more important when the interface is associated |
| with a specific clock domain. E.g.: lb_clk, lb_valid, lb_rnw, lb_wdata, lb_rdata. |
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| #### A.2 - Keeping interfaces standard |
| In order to promote interface re-use and facilitate module interoperability, custom interfaces and protocols should be avoided |
| whenever possible. Instead, conventional interface design patterns should be preferred, along with the use of traditional naming |
| for both control and data signals. E.g. lb_valid, lb_ready/lb_enable, lb_wdata, lb_wstb/lb_wmask. |
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| Deviations from this recommendation should be accompanied by detailed documentation that goes beyond what is usually expected. |
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| B - Signal naming |
| -- |
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| #### B.1 - Registered and Delayed signals |
| Registered or delayed versions of a signal should use the suffix '_r' or '_d'. Where multiple delays are required, |
| a number should be appended to the suffix. E.g.: valid_r for single delay; valid_r1, valid_r2 etc. for multiple delays. |
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| #### B.2 - Internal or Local signals |
| Internal or local signals that have no other purpose other than to create an internal version of a module's port should |
| use the port name followed by the suffix '_l' or '_i'. |
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| #### B.3 - Signal case |
| All signal names should be lower-case and use snake-case, not camel-case. E.g.: valid_out, data_in. |
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| #### B.4 - Active-low signals |
| Active-low signals should make use of the suffix _n. E.g.: reset_n, ce_n. |
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| C - Module declaration |
| -- |
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| #### C.1 - Parameter naming |
| Parameters should be in all-caps, snake-case, and sufficiently descriptive so that no additional explanation is required. |
| Common abbreviations, such as 'DWI' and 'AWI' for 'data width' and 'address width', are acceptable, but anything |
| more obscure is best avoided. |
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| #### C.2 - Parameter defaults |
| All parameters shall have reasonable defaults, such that if the module were to be simulated without any of its parameters |
| being overridden, it would perform its typical functionality. |
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| D - Module instantiation |
| -- |
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| #### D.1 - Instance names |
| Module instance names should be prefixed by 'i_', include the module name, or an easily recognizable variant of it (in cases where the |
| full module name is too long), and an optional suffix that clarifies the purpose of the instance or disambiguates between |
| instances of the same module. E.g.: i_sqrt, i_mixer_field, i_shortfifo_lb. |
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| #### D.2 - Unconnected ports |
| Unconnected output ports should never be omitted, but rather included in the module instantiation so it is obvious that they |
| have been intentionally left open. It is recommended that an inline comment is added to justify why the port is unused. |
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| #### D.3 - Port and parameter assignment |
| All ports and parameters should always be connected by name. Unlike unconnected ports, however, it is reasonable to omit unused parameters. |
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| E - Misc |
| -- |
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| #### E.1 - TODOs and unfinished code |
| TODOs and unfinished code are best avoided in checked-in code, but if they must be used they should be clearly marked with a 'TODO' tag. Alternative |
| tags should be avoided as they may more easily go unnoticed. |
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| #### E.2 - Comments |
| In addition to being useful, clear and concise, comments should not include question marks, rhetorical questions or hypotheticals. |
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| #### E.3 - Precedence rules |
| Non-obvious operator precedence rules should be avoided. Whenever precedences are not completely clear, parentheses should be used. |
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