If-Elif-Else

you cannot use the standard python if-elif-else statements to create statements. Instead, using nMigen branching.

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with m.If(condition1):
    m.d.comb += statements1
with m.Elif(condition2):
    m.d.comb += statements2
with m.Else():
    m.d.comb += statements3

If you use regular Python if-elif-else, then those will be evaluated during generation of the logic, not the logic itself. This can be useful if you want a flag to cause different logic implement to be generated, and this is a good use of platform string pass to elaborate().

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if (platform== "KC705")
    m.d.comb += statement1
else:
    m.d.comb += statement2

If platform is "KC705" then statement1 will be implement in generated hardware, otherwise only statement2 will be implement in the design.

Conditions

The conditions in If-Elif-Else are cimparisons, for example a == 1 or (a >= b) & (a <= c). Note that each comparison will be one-bit comparison.
If you have a signal with more than one bit and use it as the condition, use with m.If(a):, then the condition will be true if any bit in a is 1.

Switch-Case-Default

You can use Switch-Case-Default just as in standard HDLs using the following with constructs:

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with m.Switch(expression):
    with m.Case(value1):
        statements1
    with m.Case(value2):
        statements2
    with m.Default():
        statements3

Although it is suggested that using full-case switch that we used to do in verilog. You can use multiple values in one case statement.

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with m.Switch(expression):
    with m.Case(value1,value2):
        statements1
    with m.Default():
        statements3

You can leave out the Defalt(), but not suggest you to do that.

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m.d.comb += x.eq(1)
with m.Switch (y):
    with m.Case(0,1,2):
        m.d.comb += x.eq(2)

In this example, if y is in [0, 1, 2] then x is assigned 2. Otherwise x retains its value of 1.

• Recall the section on overriding statements. One signal can only assigned in only one clock domain.

Specify a bit patterns

The way to specify a matching pattern in a Case is with a Python string of binary digits. For example, "0011101011". A don’t-care bit is specified using a dash -, so for example "00111-----". The number of bits in the string must exactly the same as the number of bits in the expression it is being compared to.

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with m.If(a.matches("11---",3,b)):
    statement1

Is equivelent to

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with m.Switch(a):
    with m.Case("11---",3,b):
        statement1