![]() ![]() Within each instance of the “unrolled” generate loop, an implicit localparam is created with the same name and type as the loop index variable. The genvar declaration can be inside or outside the generate region, and the same loop index variable can be used in multiple generate loops, as long as the loops don’t nest. The genvar is used as an integer to evaluate the generate loop during elaboration. The loop index variable must first be declared in a genvar declaration before it can be used. The syntax for a generate loop is similar to that of a for loop statement. For readability, I like to use the generate and endgenerate keywords. Generate regions can only occur directly within a module, and they cannot nest. If they are used, then they define a generate region. Use of the keywords generate and endgenerate (and begin/ end) is actually optional. This sometimes causes confusion when trying to write a hierarchical reference to signals or modules within a generate block, so it is something to keep in mind. For example, generate constructs can be affected by values from parameters, but not by dynamic variables.Ī Verilog generate block creates a new scope and a new level of hierarchy, almost like instantiating a module. Therefore all expressions within generate constructs must be constant expressions, deterministic at elaboration time. Verilog generate constructs are evaluated at elaboration, which occurs after parsing the HDL (and preprocessor), but before simulation begins. Conditional generate constructs include if-generate and case-generate forms. Conditional generate constructs select at most one block of code between multiple blocks. Generate loop constructs allow a block of code to be instantiated multiple times, controlled by a variable index. There are two kinds of Verilog generate constructs. In this article, I will review the usage of three forms of Verilog generate-generate loop, if-generate, and case-generate. However, many Verilog programmers often have questions about how to use Verilog generate effectively. It can be used to create multiple instantiations of modules and code, or conditionally instantiate blocks of code. Take a look now at the state diagram with the transitions labeled.Verilog generate statement is a powerful construct for writing configurable, synthesizable RTL. If it seems to make your circuit a lot simpler if you break some rules, you probably should. This naming convention is a bit arbitrary and you shouldn't worry about it too much when designing your own FSMs. The FSM portion of circuit simply supplies signals to reset the counter and the data path provides signals like the counter has reached a certain value. The counter can't be part of the FSM because a counter requires some flip-flops to store the counter value! Instead, the counter is part of what is known as the data path. To keep the state the same for a longer amount of time, we need to introduce a counter. We want them to just flow to the next state, but we can't just have them flow immediately to the next state otherwise the robot won't have any time to backup or turn! It will happen so fast that it will appear that nothing happened. The first one is easy, when the FSM is in the FORWARD state it should change to the BACKUP_RIGHT state when the left switch is pressed, or to the BACKUP_LEFT state when the right switch is pressed. We now need to figure out under what conditions we would like the state to change. Take a look at the basic flow of states to help clear this up. If we didn't have separate states, we would have to read the state of the switch after it backed up and at that point the switch would not be pressed anymore. This is because we need to encode in the states which one will turn right after it backs up and which will turn left. Notice that there are two versions of the BACKUP state, BACKUP_RIGHT and BACKUP_LEFT. When the robot hasn't hit anything it will just drive forward. For some basic object avoidance, we will make the robot back up when it hits something then turn right if the left switch was pressed or turn left if the right switch was pressed. Remember, the Mojo's IO pins are not 5V tolerant, however, because this robot is using the Servo Shield, this is exactly what we want.īy using these switches, the robot can realize when it runs into something. These are wired up so that when they are not being pressed, the signal line is connected to ground, but when pressed, it get connected to +5V. ![]()
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