- RISC-V implementation of rv32i for application intended use
- no
ecall,ebreak,fenceor counters
- no
- multi-cycle with ad-hoc pipeline
- cache to 2 MB of on-board burst PSRAM
- explore the FPGA board
- build bitstream file in Gowin EDA
- run
./flash-fpga.shto flash bitstream file - then
./make-and-flash-os.shto flash program - connect with serial terminal to the tty (e.g.
/dev/ttyUSB1) at 115200 baud, 8 bit data, 1 stop bit, no parity, no flow control
- press button S1 to reset and view the greeting
oOo.o.
frameless osca oOo.oOo
__________________________ .oOo.
O\ -_ .. \ ___ \ ||
O \ \ \ \\ \ //\\
o /\ risc-v \ \|\\ \
. //\\ fpga \ || \
. \\/\\ overview \ \_\ \
. \\//\________________\________\
. \/_/, \\\--\\..\\ - /\_____ /
. \ \ . \\\__\\__\\./ / \__/ /
. \ \ , \ \\ ///./ ,/./ /
. \ \___\ sticky notes / /
. \/\________________/ /
./\. . / / /
/--\ .\/_________________/
___. .
|o o|. . . . . . . . .
/| |\ . .
____ . .
|O O| . .
|_ -_| . .
/||\
___
/- -\
/\_-_/\
| |
welcome to adventure #4
type 'help'
- an "operating system" in
/osinspired by text adventure games - other examples in
/notes/samples/
- see
configuration.py
- SystemVerilog adhering to most of style guide https://github.com/lowRISC/style-guides/blob/master/VerilogCodingStyle.md
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[ ] cat > /dev/ttyUSB1 should echo without dropping input
[ ] study why BAUD rate less than 2400 does not work
[ ] UART read 'short' and return 0xffff for no data available or 0xXX for byte read including 0
[ ] read LEDs
[ ] investigate and try to recreate the "// ??" issue in 'ramio'
=> the simulation works as expected
[ ] counter[highest_bit] == 1 in decreasing counters into negative instead of counter == 0
[ ] fix red hold and recovery paths (not involved in running CPU, ignored)
[ ] psram: CALIB
[ ] psram: RESET
next step
[ ] reduce execution cycle by one step by doing CpuExecute in CpuFetch when ramio_data_out_ready
=> registers module would be a 2 port read, 1 port write per cycle
step 12
[ ] always_comb based CPU
[ ] 1 cycle ALU op
[ ] 1+ cycle store op
[ ] 1+ cycle load op
[ ] FSM in always_comb?
[ ] dual channel 4 MB PSRAM
=> reasonable cache size does not fit in fpga resources
=> study cache synthesized as block ram
step 13
[ ] fully pipe-lined core
[ ] consider FIFO in UART
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[x] study why terminal drops characters
=> receive is being overrun but how can baud 9600 outpace 20 MHz?
=> due to 'uart_send_char()'?
=> UART overrun even when doing 'uart_read_char()' in a loop
=> moreover characters are dropped without UART being overrun
=> fixed end-to-end test without grasping why it did not previously worked
search for '// ??' in 'ramio'
[x] UART rx: if rx changes to low while in stop-bit assume drifting and switch to wait for go
=> stop bit is read for half a baud cycle giving half to acknowledge and wait for start bit
[x] fix truncation warnings
[x] enums for states in FSM
[x] review test benches
[x] testbenches: assert (condition) else $fatal("...");
=> else $error()
[x] make end-to-end test succeed without dropped input
[x] step 11: adapt riscv core (multi-cycle ad-hoc pipeline simplest way forward)
[x] RAMIO: read UART with 'lb' or 'lbu'
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[o] apply style guide https://github.com/lowRISC/style-guides/blob/master/VerilogCodingStyle.md
[x] use 'logic' instead of registers or wires where applicable?
=> reg -> logic, all outputs logic, all inputs wires
[x] parameters in UpperCamelCase
[ ] Main clock signal is named clk. All clock signals must start with clk_
[x] Reset signals are active-low and asynchronous, default name is rst_n
[x] Signal names should be descriptive and be consistent throughout the hierarchy
[ ] Add _i to module inputs, _o to module outputs or _io for bi-directional module signals
=> the ports have verbose names implying input/output
[x] Enumerated types should be suffixed with _e
[x] Use full port declaration style for modules, any clock and reset declared first
[x] Use named parameters for instantiation, all declared ports must be present, no .*
[x] Top-level parameters is preferred over `define globals
[x] Use symbolically named constants instead of raw numbers
[x] Local constants should be declared localparam, globals in a separate .svh file.
[x] logic is preferred over reg and wire, declare all signals explicitly
[x] always_comb, always_ff and always_latch are preferred over always
[x] Interfaces are discouraged
[x] Sequential logic must use non-blocking assignments
[x] Combinational blocks must use blocking assignments
[x] Use of latches is discouraged, use flip-flops when possible
[x] The use of X assignments in RTL is strongly discouraged, make use of SVAs to check invalid behavior instead.
[x] Prefer assign statements wherever practical.
[x] Use unique case and always define a default case
[x] Use available signed arithmetic constructs wherever signed arithmetic is used
[x] When printing use 0b and 0x as a prefix for binary and hex. Use _ for clarity
[x] Use logical constructs (i.e ||) for logical comparison, bit-wise (i.e |) for data comparison
[x] Bit vectors and packed arrays must be little-endian, unpacked arrays must be big-endian
[ ] FSMs: no logic except for reset should be performed in the process for the state register
[x] A combinational process should first define default value of all outputs in the process
[ ] Default value for next state variable should be the current state
[x] Use only ASCII characters with UNIX-style line endings("\n").
[x] Use the .sv extension for SystemVerilog files (or .svh for files that are included via the preprocessor).
[x] All lines on non-empty files must end with a newline ("\n").
[x] Wrap the code at 100 characters per line.
=> formatting done automatically by extension
[x] Do not use tabs anywhere.
[x] Delete trailing whitespace at the end of lines.
[x] Use begin and end unless the whole statement fits on a single line.
[x] Indentation is two spaces per level.
[x] Keep branching preprocessor directives left-aligned and un-indented.
[x] For multiple items on a line, one space must separate the comma and the next character.
[x] Include whitespace on both sides of all binary operators.
[x] Add a space around packed dimensions.
[x] Add one space before type parameters, except when the type is part of a qualified name.
[x] When labeling code blocks, add one space before and after the colon.
[x] There must be no whitespace before a case item's colon; there must be at least one space after the case item's colon.
[x] Function and task calls must not have any spaces between the function name or task name and the open parenthesis.
[x] Include whitespace before and after SystemVerilog keywords.
[x] Use parentheses to make operations unambiguous.
[x] Ternary expressions nested in the true condition of another ternary expression must be enclosed in parentheses.
[x] C++ style comments (// foo) are preferred. C style comments (/* bar */) can also be used.
[x] Signals must be declared before they are used. This means that implicit net declarations must not be used.
[x] Declarations (module, class, package, interface) lower_snake_case
[x] Instance names lower_snake_case
[x] Signals (nets and ports) lower_snake_case
[x] Variables, functions, tasks lower_snake_case
[x] Named code blocks lower_snake_case
[x] `define macros ALL_CAPS
=> not used
[x] Tunable parameters for parameterized modules, classes, and interfaces UpperCamelCase
[x] Constants ALL_CAPS or UpperCamelCase
[x] Enumeration types lower_snake_case_e
[x] Other typedef types lower_snake_case_t
=> not used
[x] Enumerated value names UpperCamelCase
[x] Declare global constants using parameters in the project package file.
[x] `define ALL_CAPS Truly constant
[x] module parameter UpperCamelCase truly modifiable by instantiation, not constant
[x] derived localparam UpperCamelCase while not modified directly, still tracks module parameter
[x] tuneable localparam UpperCamelCase while not expected to change upon final RTL version,
is used by designer to explore the design space conveniently
[x] true localparam constant ALL_CAPS
[x] enum member true constant ALL_CAPS
[x] enum set member ALL_CAPS or UpperCamelCase
[x] Use parameter to parameterize, and localparam to declare module-scoped constants. Within a package, use parameter.
[x] Explicitly declare the type for parameters.
[ ] Suffixes are used in several places to give guidance to intent.
[x] Name enumeration types snake_case_e. Name enumeration values ALL_CAPS or UpperCamelCase.
[x] Use lower_snake_case when naming signals.
[x] Names should describe what a signal's purpose is.
[x] Use common prefixes to identify groups of signals that operate together.
[ ] The same signal should have the same name at any level of the hierarchy.
[ ] All clock signals must begin with clk
[x] Resets are active-low and asynchronous. The default name is rst_n
[x] Use these SystemVerilog constructs instead of their Verilog-2001 equivalents
[x] Packages must not have cyclic dependencies.
[x] Use the Verilog-2001 full port declaration style, and use the format below.
[x] Use named ports to fully specify all instantiations.
[x] Use named parameters for all instantiations.
[x] Do not instantiate recursively.
[x] It is recommended to use symbolicly named constants instead of raw numbers.
[x] Always be explicit about the widths of number literals.
[x] Do not use multi-bit signals in a boolean context.
[x] Only use the bit slicing operator when the intent is to refer to a portion of a bit vector.
[x] Beware of shift operations, which can produce a result wider than the operand.
[x] Sequential logic must use non-blocking assignments. Combinational blocks must use blocking assignments.
[x] Do not use #delay in synthesizable design modules.
[x] The use of latches is discouraged - use flip-flops when possible.
[x] Use the standard format for declaring sequential blocks.
[ ] Do not allow multiple non-blocking assignments to the same bit.
... the Verilog standard says that the second assignment will take effect, but this is a style violation.
[x] The use of X literals in RTL code is strongly discouraged.
[x] Avoid sensitivity lists, and use a consistent assignment type.
[x] Avoid case-modifying pragmas. unique case is the best practice. Always define a default case.
[ ] Use case inside if wildcard operator behavior is needed.
=> not supported by iverilog. using "unique casez(...)"
[x] Always name your generated blocks.
[x] Use the available signed arithmetic constructs wherever signed arithmetic is used.
[x] Prefix printed binary numbers with 0b. Prefix printed hexadecimal numbers with 0x. Do not use prefixes for decimal numbers.
[x] In synthesizable RTL the use of functions is allowed, provided they are declared automatic. Tasks should not be used.
=> non used
[x] The use of hierarchical references in synthesizable RTL code is prohibited.
[x] Do not rely on inferred nets.
[x] Use logic for synthesis. wire is allowed when necessary.
[x] Prefer logical constructs for logical comparisons, bit-wise for data.
[x] Bit vectors and packed arrays must be little-endian.
[x] Unpacked arrays must be big-endian.
[ ] State machines use an enum to define states, and be implemented with two process blocks: a combinational block and a clocked block.
[x] The _n suffix indicates an active-low signal.
[x] Use the _p and _n suffixes to indicate a differential pair.
=> non used
[x] Signals delayed by a single clock cycle should end in a _q suffix.
=> non used
[x] The wildcard import syntax, e.g. import ip_pkg::*; is only allowed where the package is part of the same IP as the module that uses that package.
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