Accessing the design¶
When cocotb initializes it finds the toplevel instantiation in the simulator
and creates a handle called
dut. Toplevel signals can be accessed using the
“dot” notation used for accessing object attributes in Python. The same mechanism
can be used to access signals inside the design.
# Get a reference to the "clk" signal on the toplevel clk = dut.clk # Get a reference to a register "count" # in a sub-block "inst_sub_block" count = dut.inst_sub_block.count
Assigning values to signals¶
Values can be assigned to signals using either the
value property of a handle object
or using direct assignment while traversing the hierarchy.
# Get a reference to the "clk" signal and assign a value clk = dut.clk clk.value = 1 # Direct assignment through the hierarchy dut.input_signal <= 12 # Assign a value to a memory deep in the hierarchy dut.sub_block.memory.array <= 2
sig <= new_value is a short form of
sig.value = new_value.
It not only resembles HDL syntax, but also has the same semantics:
writes are not applied immediately, but delayed until the next write cycle.
sig.setimmediatevalue(new_val) to set a new value immediately
Signed and unsigned values¶
Both signed and unsigned values can be assigned to signals using a Python int.
Cocotb makes no assumptions regarding the signedness of the signal. It only
considers the width of the signal, so it will allow values in the range from
the minimum negative value for a signed number up to the maximum positive
value for an unsigned number:
-2**(Nbits - 1) <= value <= 2**Nbits - 1
Note: assigning out-of-range values will raise an
BinaryValue object can be used instead of a Python int to assign a
value to signals with more fine-grained control (e.g. signed values only).
module my_module ( input logic clk, input logic rst, input logic [2:0] data_in, output logic [2:0] data_out );
# assignment of negative value dut.data_in <= -4 # assignment of positive value dut.data_in <= 7 # assignment of out-of-range values dut.data_in <= 8 # raises OverflowError dut.data_in <= -5 # raises OverflowError
Reading values from signals¶
Values in the DUT can be accessed with the
property of a handle object.
A common mistake is forgetting the
.value which just gives you a reference to a handle
(useful for defining an alias name), not the value.
The Python type of a value depends on the handle’s HDL type:
logicand subtypes of that (
unsigned, etc.) are of type
Integer nets and constants (
natural, etc.) return
Floating point nets and constants (
Boolean nets and constants (
String nets and constants (
>>> # Read a value back from the DUT >>> count = dut.counter.value >>> print(count.binstr) 1X1010 >>> # Resolve the value to an integer (X or Z treated as 0) >>> print(count.integer) 42 >>> # Show number of bits in a value >>> print(count.n_bits) 6
We can also cast the signal handle directly to an integer:
>>> print(int(dut.counter)) 42
Concurrent and sequential execution¶
await will run an
async coroutine and wait for it to complete.
The called coroutine “blocks” the execution of the current coroutine.
Wrapping the call in
fork() runs the coroutine concurrently,
allowing the current coroutine to continue executing.
At any time you can
await the result of the forked coroutine,
which will block until the forked coroutine finishes.
The following example shows these in action:
# A coroutine async def reset_dut(reset_n, duration_ns): reset_n <= 0 await Timer(duration_ns, units="ns") reset_n <= 1 reset_n._log.debug("Reset complete") @cocotb.test() async def parallel_example(dut): reset_n = dut.reset # Execution will block until reset_dut has completed await reset_dut(reset_n, 500) dut._log.debug("After reset") # Run reset_dut concurrently reset_thread = cocotb.fork(reset_dut(reset_n, duration_ns=500)) # This timer will complete before the timer in the concurrently executing "reset_thread" await Timer(250, units="ns") dut._log.debug("During reset (reset_n = %s)" % reset_n.value) # Wait for the other thread to complete await reset_thread dut._log.debug("After reset")
See Coroutines for more examples of what can be done with coroutines.
Forcing and freezing signals¶
In addition to regular value assignments (deposits), signals can be forced to a predetermined value or frozen at their current value. To achieve this, the various actions described in Assignment Methods can be used.
# Deposit action dut.my_signal <= 12 dut.my_signal <= Deposit(12) # equivalent syntax # Force action dut.my_signal <= Force(12) # my_signal stays 12 until released # Release action dut.my_signal <= Release() # Reverts any force/freeze assignments # Freeze action dut.my_signal <= Freeze() # my_signal stays at current value until released
Accessing Identifiers Starting with an Underscore¶
The attribute syntax of
dut._some_signal cannot be used to access
an identifier that starts with an underscore (
_, as is valid in Verilog)
because we reserve such names for cocotb-internals,
thus the access will raise an
A workaround is to use indirect access using
_id() like in the following example: