Everybody wants ERC. Everybody hates ERC.

Electrical rules checking (ERC) looks for errors in how your circuit is constructed. It’s like running lint, but for hardware. And as with lint, you get a whole bunch of warnings that don’t matter but which obscure the ones that do.

SKiDL tries to help by allowing you to selectively turn off ERC for nets, pins, and parts. That fixes part of the problem.

The other part is the inflexibility of ERC: it has fixed rules for common errors (like two outputs driving against each other), but adding rules for special cases isn’t easy.

The erc_assert() function was added to SKiDL to make customizing ERC easier. With erc_assert(), you can add specialized rules that are checked only in particular instances. In the example below, erc_assert() is used to flag nets that have too much fanout:

from skidl import *

# Function to get the number of inputs on a net.
def get_fanout(net):
    fanout = 0
    for pin in net.get_pins():
        if pin.func in (Pin.INPUT, Pin.BIDIR):
            fanout += 1
    return fanout

net1, net2 = Net('IN1'), Net('IN2')

# Place some assertions on the fanout of each net.
# Note that the assertions are passed as strings.
erc_assert('get_fanout(net1) < 5', 'failed on net1')
erc_assert('get_fanout(net2) < 5', 'failed on net2')

# Attach some pins to the nets.
net1 += Pin(func=Pin.OUTPUT)
net2 += Pin(func=Pin.OUTPUT)
net1 += Pin(func=Pin.INPUT) * 4  # This net passes the assertion.
net2 += Pin(func=Pin.INPUT) * 5  # This net fails because of too much fanout.

# When the ERC runs, it will also evaluate any erc_assert statements.
ERC()
ERC ERROR: get_fanout(net2) < 5 failed on net2 in <ipython-input-20-766848d35dca>:16:<module>.

0 warnings found during ERC.
1 errors found during ERC.

You might ask: “Why not just use the standard Python assert statement?” The reason is that a standard assertion is evaluated as soon as the assert statement is encountered so the result might be incorrect if the nets are not yet completely defined. But the statement passed to the erc_assert() function is a string that isn’t evaluated until all the circuitry has been connected and ERC() is called. Note in the code above that when the erc_assert() function is called, no pins are even attached to the net1 or net2 nets. The erc_assert() function just places the statement to be checked into a queue that gets evaluated when ERC() is run. By then, the nets will have pins attached by then.

You can perform more complicated checks by creating a function and then placing a call to it in the assertion string:

from skidl import *

def get_fanout(net):
    fanout = 0
    for pin in net.get_pins():
        if pin.func in (Pin.INPUT, Pin.BIDIR):
            fanout += 1
    return fanout

# Function to check a fanout constraint on a net and drop into
# the debugger if the constraint is violated.
def check_fanout(net, threshold):
    fanout = get_fanout(net)
    if fanout >= threshold:
        # Report the net which violated the constraint."
        print(f'{net.name} fanout of {fanout} >= {threshold}.')

        # Drop into the debugger so you can query the circuit
        # and then continue.
        breakpoint()

        return False  # Return False to trigger the erc_assert().

    return True  # Return True if the constraint is not violated.

net1, net2 = Net('IN1'), Net('IN2')

# Place calls to the check_fanout() function into the assertions.
erc_assert('check_fanout(net1, 5)')
erc_assert('check_fanout(net2, 5)')

net1 += Pin(func=Pin.OUTPUT)
net2 += Pin(func=Pin.OUTPUT)
net1 += Pin(func=Pin.INPUT) * 4
net2 += Pin(func=Pin.INPUT) * 5

ERC()
IN2 fanout of 5 >= 5.
> <ipython-input-18-800a83e11624>(22)check_fanout()
-> return False  # Return False to trigger the erc_assert().
(Pdb) c
ERC ERROR: check_fanout(net2, 5) FAILED in <ipython-input-18-800a83e11624>:30:<module>.

0 warnings found during ERC.
1 errors found during ERC.

You can detect if a subcircuit is being used correctly by embedding calls to erc_assert() to check inputs, outputs, and internal circuitry:

from skidl import *

# Return True if the net has a pullup on it, False if not.
def has_pullup(net):
    for pin in net.get_pins():
        if pin.func == Pin.PULLUP:
            return True

    print(f'No pullup on net {net.name}')
    return False

@subcircuit
def some_circuit(in1, in2):
    # Check the subcircuit inputs to see if they have pullups.
    erc_assert('has_pullup(in1)')
    erc_assert('has_pullup(in2)')

    # OK, this subcircuit doesn't really do anything, so you'll
    # just have to imagine that it did.
    pass

net1, net2 = Net('IN1'), Net('IN2')

# Instantiating the subcircuit automatically checks the input nets.
some_circuit(net1, net2)

net1 += Pin(func=Pin.OUTPUT)
net1 += Pin(func=Pin.INPUT)  # No pullup -> this net fails.
net2 += Pin(func=Pin.OUTPUT)
net2 += Pin(func=Pin.PULLUP)

ERC()
No pullup on net IN1
ERC ERROR: has_pullup(in1) FAILED in <ipython-input-16-6e72ef7d46c8>:15:some_circuit.

0 warnings found during ERC.
1 errors found during ERC.

You can even use the erc_assert() function to add global checks that scan the entire circuit for violations of custom rules:

from skidl import *

# Global function that checks all the nets in the circuit and
# flags nets which have pullups when they shouldn't.
def my_erc():
    erc_result = True
    for net in default_circuit.get_nets():
        if getattr(net, 'disallow_pullups', False) == True:
            pin_types = [pin.func for pin in net.get_pins()]
            if Pin.PULLUP in pin_types:
                print(f'Pull-up not allowed on {net.name}.')
            erc_result = False
    return erc_result

net1, net2 = Net('IN1'), Net('IN2')

net1.disallow_pullups = True  # Don't allow pullups on this net.
net1 += Pin(func=Pin.INPUT)
net1 += Pin(func=Pin.PULLUP) # This net will fail ERC. 
net2 += Pin(func=Pin.INPUT)
net2 += Pin(func=Pin.PULLUP)

# Call your own customized global ERC.
erc_assert('my_erc()')

ERC()
Pull-up not allowed on IN1.
ERC ERROR: my_erc() FAILED in <ipython-input-12-888102e7799c>:24:<module>.

0 warnings found during ERC.
1 errors found during ERC.

That’s about it for the erc_assert() function. Since it’s new, it hasn’t seen a lot of use so there could be modifications that are needed to make it more useful. If you have questions or problems, please ask on the SKiDL forum or raise an issue.