Debugging cells

The $print cell is used to log the values of signals, akin to (and translatable to) the $display and $write family of tasks in Verilog. It has the following parameters:

FORMAT

The internal format string. The syntax is described below.

ARGS_WIDTH

The width (in bits) of the signal on the ARGS port.

TRG_ENABLE

True if triggered on specific signals defined in TRG; false if triggered whenever ARGS or EN change and EN is 1.

If TRG_ENABLE is true, the following parameters also apply:

TRG_WIDTH

The number of bits in the TRG port.

TRG_POLARITY

For each bit in TRG, 1 if that signal is positive-edge triggered, 0 if negative-edge triggered.

PRIORITY

When multiple $print or $check cells fire on the same trigger, they execute in descending priority order.

Ports:

TRG

The signals that control when this $print cell is triggered.

If the width of this port is zero and TRG_ENABLE is true, the cell is triggered during initial evaluation (time zero) only.

EN

Enable signal for the whole cell.

ARGS

The values to be displayed, in format string order.

yosys> help $check

Simulation model (verilog)

Listing 210 simlib.v

module \$check (A, EN, TRG, ARGS);

    parameter FLAVOR = "";
    parameter PRIORITY = 0;

    parameter FORMAT = "";
    parameter ARGS_WIDTH = 0;

    parameter TRG_ENABLE = 1;
    parameter TRG_WIDTH = 0;
    parameter TRG_POLARITY = 0;

    input A;
    input EN;
    input [TRG_WIDTH-1:0] TRG;
    input [ARGS_WIDTH-1:0] ARGS;

endmodule

yosys> help $print

Simulation model (verilog)

Listing 211 simlib.v

module \$print (EN, TRG, ARGS);

    parameter PRIORITY = 0;

    parameter FORMAT = "";
    parameter ARGS_WIDTH = 0;

    parameter TRG_ENABLE = 1;
    parameter TRG_WIDTH = 0;
    parameter TRG_POLARITY = 0;

    input EN;
    input [TRG_WIDTH-1:0] TRG;
    input [ARGS_WIDTH-1:0] ARGS;

endmodule

yosys> help $scopeinfo

Simulation model (verilog)

Listing 212 simlib.v

module \$scopeinfo ();

    parameter TYPE = "";

endmodule

Format string syntax

The format string syntax resembles Python f-strings. Regular text is passed through unchanged until a format specifier is reached, starting with a {.

Format specifiers have the following syntax. Unless noted, all items are required:

{

Denotes the start of the format specifier.

size

Signal size in bits; this many bits are consumed from the ARGS port by this specifier.

:

Separates the size from the remaining items.

justify

> for right-justified, < for left-justified.

padding

0 for zero-padding, or a space for space-padding.

width?

(optional) The number of characters wide to pad to.

base

• b for base-2 integers (binary)

• o for base-8 integers (octal)

• d for base-10 integers (decimal)

• h for base-16 integers (hexadecimal)

• c for ASCII characters/strings

• t and r for simulation time (corresponding to $time and $realtime)

For integers, this item may follow:

+?

(optional, decimals only) Include a leading plus for non-negative numbers. This can assist with symmetry with negatives in tabulated output.

signedness

u for unsigned, s for signed. This distinction is only respected when rendering decimals.

ASCII characters/strings have no special options, but the signal size must be divisible by 8.

For simulation time, the signal size must be zero.

Finally:

}

Denotes the end of the format specifier.

Some example format specifiers:

• {8:>02hu} - 8-bit unsigned integer rendered as hexadecimal, right-justified, zero-padded to 2 characters wide.

• {32:< 15d+s} - 32-bit signed integer rendered as decimal, left-justified, space-padded to 15 characters wide, positive values prefixed with +.

• {16:< 10hu} - 16-bit unsigned integer rendered as hexadecimal, left-justified, space-padded to 10 characters wide.

• {0:>010t} - simulation time, right-justified, zero-padded to 10 characters wide.

To include literal { and } characters in your format string, use {{ and }} respectively.

It is an error for a format string to consume more or less bits from ARGS than the port width.

Values are never truncated, regardless of the specified width.

Note that further restrictions on allowable combinations of options may apply depending on the backend used.

For example, Verilog does not have a format specifier that allows zero-padding a string (i.e. more than 1 ASCII character), though zero-padding a single character is permitted.

Thus, while the RTLIL format specifier {8:>02c} translates to %02c, {16:>02c} cannot be represented in Verilog and will fail to emit. In this case, {16:> 02c} must be used, which translates to %2s.