This tasks outputs a message to the standard output when running Verga in standalone mode, and to the simulation console when running under TkGate. A newline is appended to the output. It is similar to the printf() function used in C.
Unlike the C printf function, the first argument is not necessarily a control string. The $display function can have from zero up to an arbitrary number of control strings. For example:
integer r1,r2;
r1 = 7; r2 = 12;
$display(r1,r2);
will print:
7 12The same output could have been produced with a control string using the statement:
$display("%d %d",r1,r2);
The general rule is that if an argument is a string, then it is
treated as a control string and consumes as many arguments after it as
there are conversions (e.g., "%d", etc.). If an argument is
not a string, its value is printed. The format in which it is printed
depends on the variable type. Unsized values are printed in decimal,
sized values are printed using the Verilog hexadecimal notation. For
example:
integer r1;
reg [16:0] r2;
r1 = 7; r2 = 12;
$display(r1,r2);
will print
7 16'hcWhen printing constants and expressions, $display will use the base specified in any constants. For example $display(1'o0+r2) would produce 16'o14.
You can get greater control of the output using control strings with conversions. Strings are enclosed by the double quote character ("). Use a backslash (\) before a double quote to output the double quote itself and use \\ to output a single backslash character. In addition, you can use "\n" to insert extra newlines into the output.
The conversions supported in Verga are (case is ignored):
| Conversion | Description |
|---|---|
| %% | Output a single percent character. |
| %m | Output the name of the current module instance. |
| %t | Output a simulation time value. The raw simulation time in epochs is converted to the units specified in any `timescale declaration for the current module. If a precision smaller than the units is specified, then the time value will be output with a decimal point and the appropriate number of digits after the decimal point. |
| %d | Output a value in decimal format. |
| %b | Output a value in binary format. |
| %h | Output a value in hexadecimal format. |
| %o | Output a value in octal format. |
| %c | Output the character with the specified ASCII value. |
| %s | Output the value as a string. |
| %f | Output the value in floating point format. |
| %g | Output the value in floating point format while minimizing the number of characters needed. |
$display("%h",8'h1e);
will output only "1e".
Like the C printf(), you can specify a number of spaces with the conversion. For example %4h will output a hexadecimal value in a four character field, padding with spaces if necessary. If you use leading zero as in %04h, then Verga will pad the output with zeros if necessary (e.g., "16'h2e" will be output as "002e").
As a final example including multiple control strings, consider the statement:
$display(9," a=%d b=%d ",42, 77, 12,13," c=%d d=%d ",5,9, 88);This will produce:
9 a=42 b=77 12 13 c=6 d=9 88
The $monitor task formats the output from its arguments in exactly the same way as the $display task. The difference is that instead of outputting the text immediately, it places a watch on all variables referenced in the statement and outputs its message any time a variable changes. This can be used to generate a trace of any variable changes.
As an example, consider this module:
module top;
reg [15:0] a,b,c;
initial
$monitor("%t: a=16'h%04h b=16'h%04h c=16'h%04h",$time,a,b,c);
initial
begin
#1 a = 'h4ef; b = 'h6def;
#1 c = 'h84ff;
#5 b = 42;
end
endmodule
When simulated, it will produce the following output:
1: a=16'h04ef b=16'h6def c=16'hxxxx 2: a=16'h04ef b=16'h6def c=16'h84ff 7: a=16'h04ef b=16'h002a c=16'h84ffEach time any of the variables a, b or c change, the $monitor task will output the stored line. In this example, we set a and b after one time step. The value of c was printed as 16'hxxxx because it is still uninitialized. One time step later, we set c, then five time steps later we set b. This results in output from $monitor at times steps 1, 2 and 7.
Only one $monitor may be active at a time. If you call $monitor more than once, only the last call will remain in effect.
This task temporarily disables a $monitor that has been placed. The monitor remains registered, but all output from it is disabled.
This task re-enables a $monitor that has been disabled by $monitoroff.
The $strobe task formats the output from its arguments in exactly the same way as the $display task. The primary difference is that instead of outputting the message immediately, it waits until the end of the time step and does the output with the values at that time. It can be used when you want to ensure that you are outputting the final value of a variable for the time step. As an example, consider the module:
(1) module top;
(2) reg [15:0] a, b, c;
(3)
(4) initial
(5) begin
(6) a = 4; b = 7; c = a + b;
(7) $strobe("%t: strobe a=%d b=%d c=%d",$time,a,b,c);
(8) $display("%t: display a=%d b=%d c=%d",$time,a,b,c);
(9) a = c*b;
(10) c = a + 9;
(11) #1 b = a + 99;
(12) end
(13)
(14) endmodule
when this module is simulated, the following output is produced:
0: display a=4 b=7 c=11 0: strobe a=77 b=7 c=86This $display tasks output the values as they are when $display is called, but $strobe waits until the end of the time step at Line 11 before it generates its output.
The $strobe task formats the output from its arguments in exactly the same way as the $display task. The only difference is that it does not append a newline. Use this task when you want to use multiple $write statements to construct a line of output. You can use a \n in a string to generate a newline.
The $open task opens the specified file and returns a descriptor. The descriptor should be declared as an integer. Files are always open for writing, so any file you specify will be overwritten. You may have at most 31 files open at once. Below is an example, of how to use $open to open a file.
integer d;
initial
begin
d = $fopen("myfile.log");
...
You may use the bit-wise OR operator "|" to combine two or
more descriptors. This will cause an write operations on that
descriptor to output to all of the files combined in this way. You
may also OR in the value "1" to include output to the console. For
example, given the statements:
d1 = $fopen("myfile1.log");
d2 = $fopen("myfile2.log");
d = d1 | d2 | 1;
writing to d will cause that output to be written to
myfile1.log, myfile2.log and the standard output.
Closes the file or files referenced by the descriptor. If descriptor has been formed by ORing together two or more descriptors, all of the referenced files will be closed.
These tasks operate in exactly the same way as their counter-parts without the leading "f", except that they take a descriptor as the first argument and write their output to the file (or files) referenced by that descriptor.
The $hold task generates a warning message when a hold-time constraint is not satisfied. clock specifies a clock event after which a data line must be held constant. data specifies the data signal that must not change for the hold period, and limit specifies the time periods for which changes in data are forbidden. The $hold task may only be used in a specify block.
The $setup task generates a warning message when a setup-time constraint is not satisfied. data specifies the data signal that must not change for the setup period. clock specifies a clock event that ends the setup period, and limit specifies the length of the setup period. If the clock event occurs less than limit time steps from the last change of data, a warning message is generated. The $setup task may only be used in a specify block.
The $width task generates a warning message when a pulse width is shorter than a specified limit. event specifies the the leading edge of the pulse that is to be tested. Use the posedge or negedge keyword to indicate which edge is to be tested. limit is the minimum allowable pulse width. If the opposite transition is detected in fewer than limit time steps, then warning message will be generated. The $width task may only be used in a specify block.
This task terminates the simulation. If called while running Verga in stand-along mode, Verga will terminate immediately. If called from a description running under the TkGate, the simulator will terminate, and TkGate will revert to edit mode.
This task suspends the simulation and puts it into pause mode. If called while running Verga in stand-along mode, it will have the same effect as $finish, terminating the simulation immediately. If called from a description running under the TkGate, the simulator will enter paused mode allowing you to examine the values of variables before unpausing the simulation.
This task returns the current simulation time. When assigning it to a variable, that variable should be declared as a time variable. This is normally a 64-bit integer. One of the most useful places to use $time is in conjunction with a $display or $monitor task to show the simulation time at which the output is produced. When using $time in this way, you should use the %t conversion in the control string.
This task returns the low 32 bits of the current simulation time. You may assign it to an integer variable.
When called without an argument, this task returns a random number. When called with an argument, that argument is used as the seed value for the random number generator.
These tasks initialize one or more memories in a design from the contents of a file. The forms ending in "b" assume binary formatted data, while the forms ending in "h" assume hexadecimal formatted data. If only a file name is specified, then the name(s) of the memories to be initialized must be encoded in the file. If a memory is specified, than the file is loaded into that memory. You may also specify a start and stop address.
The file should contain a space separated list of values for the memory. The unknown (x) and floating (z) values may also be used in one or more bits. There are also several special control lines beginning with "@" that can be used to specify the memory, the address, or the radix of the file. An example of a memory file is shown below:
@memory top.m @0 3 4 9 16 25 36 49 64 81 @C 100 x x 99 z 12 x8 z12 @100 4 3 9x x x11 @1000000 12 13 14 z 19 20 x x x 30 31The @memory top.m specifies the name of the memory to be loaded. It should be a fully qualified path name. The @0 line indicates that the following data is to be load beginning at address 0, the @C line indicates data to be loaded beginning at address hexadecimal C, and so forth. You can also use a line such as:
@radix 8To change the radix for data in the file to binary (2), octal (8), decimal (10) or hexadecimal (16).
These tasks write the contents of one or more memories to a file. The forms ending in "b" write the data in binary, while the forms ending in "h" write the data in hexadecimal. The files written are of a format suitable for being read by the $readmemb and $readmemh tasks. If only a file name is given, the contents of all memories in the design will be written. If a memory name is given, only the contents of that memory will be saved. You can also specify a start and stop address to save a portion of a memory. If there are large regions of uninitialized values in a memory, those values will not be explicitly saved. Only memory pages in which data has been stored will be saved to the file.
When the simulator is running under the TkGate GUI, TkGate and the simulator communicate over a pipe. This task writes a command from the simulator to TkGate directly on that pipe. The arguments are combined into a string in the same way as for $display, and the resulting string is feed directly to the TkGate GUI. When running Verga directly, this task performs identically to $display.
This task is subject to security settings set through the TkGate Security Options dialog box.
This task executes a Tcl/Tk command in the context of the TkGate GUI. It can be used to control a Virtual Peripheral Device (VPD), although its use for this purpose is currently discouraged. The $tkg$post, $tkg$send and $tkg$recv tasks are now the preferred tasks to use for interacting with VPDs.
When executing this task, the arguments are combined into a string in the same way as for $display, and the resulting string is sent to the TkGate GUI to be executed as a Tcl/Tk command. The command will only be executed if it is enabled through the TkGate Security Options dialog box. There are three possible security settings: fully enabled, enabled for registered functions, and disabled. When enabled only for registered functions, only Tcl commands that have been registered by a VPD on loading can be executed. Furthermore, the "[" and "]" characters are not allowed in the string. For more information on how to use this task in designing VPDs, see the section on Creating Virtual Peripheral Devices.
If running Verga directly, this task will simply print the simulator command requesting a Tcl/Tk command to be executed.
This task is used to start/post a Virtual Peripheral Device (VPD). The name argument specifies the name of the VPD. The id argument specifies the name of a unique identifier for this instance of the VPD. Normally, the string "%m" should be specified as the id. The "%m" will be replaced with the instance name of the calling module in the same way that "%m" is expanded in the $display task. For more information on how to use this task in designing VPDs, see the section on Creating Virtual Peripheral Devices.
If running Verga directly, this task will simply print the simulator command requesting a VPD to be posted.
This task is used to set a probe on a signal or signals. You can list an arbitrary number of signals specified either with their local names, or with fully qualified path names. This task will cause the TkGate GUI to display the probed signals in the logic analyzer window and begin displaying value changes on those signals. You can use this task in a simulator script to setup the signals you want to probe at the beginning of a simulation.
If running Verga directly, this task will simply print the simulator command requesting a probe to be set, and print value changes on that signal until the probe is removed with and $unprobe.
If running Verga directly, this task will simply print the simulator command setting a probe and reporting the values on the probed signals.
The $tkg$recv task is used receive value on a channel from a Virtual Peripheral Device (VPD). A channel is essentially a queue that can be read to or written from either from the Tcl/Tk side of a VPD, or the Verilog side. The $tkg$recv task dequeues and returns the next value in the queue. If the queue is empty, the task blocks until a value is placed on the queue.
The channel name is an arbitrary string. Channels are dynamically created when referenced by a $tkg$recv or $tkg$send task. When used for communicating with a VPD, the channel name should be constructed from the VPD instance name (the same name used as the id in $tkg$post), followed by a "." character and a local name. This convention ensures that each instance of a VPD has its own set of channels to use. You can use "%m" in place of the instance name which will be replaced with the instance name for the current module.
For more information on how to use this task in designing VPDs, see the section on Creating Virtual Peripheral Devices.
While this task is primarily intended to be used for communication with VPDs, you can use it in conjunction with the $tkg$send task to send values from one Verilog thread to another.
This task pauses the simulation and sends a command indicating that a
breakpoint has fired. id is the name of the breakpoint, and
value is the value that has caused the break. This task is
primarily used for TkGate testing purposes.
The $tkg$send task is used send a value on a channel from the
Verilog description to a Virtual Peripheral
Device (VPD). A channel is essentially a queue that can be read
to or written from either from the Tcl/Tk side of a VPD, or the
Verilog side. The $tkg$send task enqueues a value on the
queue for the specified channel.
The channel name is an arbitrary string. Channels are dynamically
created when referenced by a $tkg$recv or $tkg$send
task. When used for communicating with a VPD, the channel name should
be constructed from the VPD instance name (the same name used as the
id in $tkg$post), followed by a "." character and a
local name. This convention ensures that each instance of a VPD has
its own set of channels to use. You can use "%m" in place of
the instance name which will be replaced with the instance name for
the current module.
For more information on how to use this task in designing VPDs, see
the section on Creating Virtual Peripheral
Devices.
While this task is primarily intended to be used for communication
with VPDs, you can use it in conjunction with the $tkg$recv
task to send values from one Verilog thread to another.
This task returns the current system time as a 64-bit time
value with the number of milliseconds since January 1, 1970.
This task removes one or more probes set either with the
$probe system task, or manually through the TkGate GUI.
This task will block the calling thread for the specified number of
milliseconds. The simulation itself will continue, and all other
threads will advance normally (unless the calling thread is the only
unblocked thread). As an example, consider the code fragment:
This task blocks a thread in the same way as $tkg$wait until
a specified absolute time. The $tkg$systime task should be
used to get a reference time to be used with $tkg$waituntil.
If the time specified by value is less than the current time,
$tkg$waituntil will return immediately and do nothing. This
task can be used to create real time clocks. For example, the
following module generates a clock signals that oscillates once per
second:
The advantage of using $tkg$waituntil instead of
$tkg$wait to generate the clock signal is to avoid drift. If
$tkg$wait were used instead, there would be additional real
time delay as the simulation was running between calls to
$tkg$wait. This small error would accumulate over time
causing the number of oscillations to diverge from the proper value
over a long time period.
Real time clocks such as this can be used to create real-time
simulations of devices such as digital clocks, or other simulation
using LEDs where you wish to run them synchronized with real time.
This task sets the zoom level in the TkGate scope window reletive to
the default zoom factor. The factor parameter is an integer
representing the zoom factor. A factor of 0 sets the default zoom
factor. Negative values represent the number of zoom-out steps
(showing more time on the x-axis), and positive values represent the
number of zoom-in steps (showing less time on the x-axis).
B.6.7 $tkg$send
B.6.8 $tkg$systime
B.6.9 $tkg$unprobe
B.6.10 $tkg$wait
always
begin
$tkg$wait(1000)
$display("Hello World");
end
This will cause the message "Hello World" to be printed once per
second.
B.6.11 $tkg$waituntil
module realclock(z);
output reg z;
time t;
initial
z = 1'b0;
initial
begin
t = $tkg$systime;
forever
begin
t = t + 500;
$tkg$waituntil(t);
z = ~z;
end
end
endmodule
B.6.12 $tkg$zoom