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Error (10137): Verilog HDL Procedural Assignment error at expr.v(15): object "pwm" on left-hand side of assignment must have a variable data type
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If you don't manage to learn Verilog syntax from the bottom, you should at least read error messages thoroughly.
An output port that is assigned in a procedural statement must be defined as reg, e.g.
output reg pwm; or
output pwm;
reg pwm;
