Can Registers Be Assigned Outside Of Always Block In Verilog

Signals of blazon wire or a similar wire like data type requires the continuous assignment of a value. For example, consider an electrical wire used to connect pieces on a breadboard. As long as the +5V battery is applied to ane end of the wire, the component connected to the other end of the wire volition get the required voltage.

In Verilog, this concept is realized by the assign statement where whatsoever wire or other like wire like data-types tin can be driven continuously with a value. The value can either be a constant or an expression comprising of a group of signals.

Assign Syntax

The assignment syntax starts with the keyword assign followed past the signal proper noun which can be either a unmarried betoken or a chain of dissimilar bespeak nets. The bulldoze strength and delay are optional and are by and large used for dataflow modeling than synthesizing into real hardware. The expression or point on the correct hand side is evaluated and assigned to the net or expression of nets on the left paw side.

                      assign <net_expression> = [drive_strength] [delay] <expression of different signals or constant value>                  

Delay values are useful for specifying delays for gates and are used to model timing beliefs in real hardware because the value dictates when the net should be assigned with the evaluated value.

Rules

There are some rules that demand to be followed when using an assign statement:

• LHS should always be a scalar or vector net or a concatenation of scalar or vector nets and never a scalar or vector register.
• RHS can contain scalar or vector registers and function calls.
• Whenever any operand on the RHS changes in value, LHS will be updated with the new value.
• assign statements are also called continuous assignments and are always active

Example #1

In the post-obit instance, a net called out is driven continuously by an expression of signals. i1 and i2 with the logical AND & form the expression.

If the wires are instead converted into ports and synthesized, nosotros will go an RTL schematic like the one shown beneath after synthesis.

Continuous assignment statement can exist used to correspond combinational gates in Verilog.

Example #2

The module shown beneath takes ii inputs and uses an assign argument to drive the output z using part-select and multiple bit concatenations. Care for each case as the just code in the module, else many assign statements on the aforementioned signal will definitely make the output become X.

                      module xyz (input [iii:0] 	ten,		// x is a four-scrap vector internet 						input 				y, 		// y is a scalar internet (1-bit) 						output [four:0] 	z ); 	// z is a five-bit vector net  wire [1:0] 	a; wire 				b; 						 // Assume 1 of the post-obit assignments are chosen in real design // If ten='hC and y='h1 let us see the value of z    // Case #1: iv-bits of x and one bit of y is concatenated to become a 5-bit net // and is assigned to the five-bit nets of z. So value of z='b11001 or z='h19 assign z = {x, y};  // Case #2: 4-$.25 of x and 1 bit of y is concatenated to become a v-bit net // and is assigned to selected 3-$.25 of internet z. Remaining 2 $.25 of z remains // undriven and will be loftier-imp. Then value of z='bZ001Z assign z[3:ane] = {10, y};  // Case #3: The same statement is used but at present bit4 of z is driven with a constant // value of i. Now z = 'b1001Z because simply bit0 remains undriven assign z[3:1] = {x, y}; assign z[4] = 1;  // Case #4: Assume bit3 is driven instead, merely now at that place are two drivers for bit3, // and both are driving the same value of 0. So at that place should be no contention and  // value of z = 'bZ001Z assign z[3:i] = {ten, y}; assign z[3] = 0;  // Case #5: Assume bit3 is instead driven with value one, so now there are two drivers // with different values, where the first line is driven with the value of X which // at the time is 0 and the 2nd assignment where it is driven with value 1, so // at present it becomes unknown which will win. So z='bZX01Z assign z[3:ane] = {ten, y}; assign z[iii] = 1;  // Instance #half-dozen: Partial selection of operands on RHS is also possible and say only 2-$.25 // are chosen from x, and then z = 'b00001 considering z[4:3] will be driven with 0 assign z = {x[i:0], y};  // Case #7: Say we explicitly assign only 3-bits of z and get out remaining unconnected // so z = 'bZZ001 assign z[2:0] = {x[ane:0], y};  // Case #viii: Aforementioned variable can be used multiple times also and z = 'b00111 // 3{y} is the same as {y, y, y} assign z = {3{y}};  // Case #9: LHS tin can too be concatenated: a is two-flake vector and b is scalar // RHS is evaluated to 11001 and LHS is iii-flake wide so get-go 3 bits from LSB of RHS // will be assigned to LHS. And so a = 'b00 and b ='b1 assign {a, b} = {x, y};  // Example #x: If we opposite guild on LHS keeping RHS same, nosotros become a = 'b01 and b='b0 assign {a, b} = {x, y};  endmodule                  

Assign reg variables

It is illegal to drive or assign reg blazon variables with an assign argument. This is considering a reg variable is capable of storing data and does non crave to be driven continuously. reg signals can but exist driven in procedural blocks like initial and always.

Implicit Continuous Consignment

When an assign statement is used to assign the given cyberspace with some value, it is called explicit assignment. Verilog also allows an assignment to be done when the net is declared and is chosen implicit assignment.

                      wire [i:0] a; assign a = x & y; 			// Explicit assignment  wire [1:0] a = x & y; 	// Implicit consignment                  

Combinational Logic Pattern

Consider the following digital circuit fabricated from combinational gates and the respective Verilog code.

Combinational logic requires the inputs to be continuously driven to maintain the output unlike sequential elements like flip flops where the value is captured and stored at the border of a clock. So an assign statement fits the purpose the well because the output o is updated whenever whatsoever of the inputs on the correct hand side alter.

                      // This module takes 4 inputs and performs a boolean // operation and assigns output to o. The combinational // logic is realized using assign statement.  module combo (	input 	a, b, c, d, 								output  o);      assign o = ~((a & b) | c ^ d);    endmodule                  

Hardware Schematic

Later on design elaboration and synthesis, we practise become to see a combinational circuit that would behave the same way as modeled by the assign statement.

Run into that the point o becomes i whenever the combinational expression on the RHS becomes true. Similarly o becomes 0 when RHS is false. Output o is Ten from 0ns to 10ns because inputs are X during the same time.

Click here for a slideshow with simulation example !

Source: https://www.chipverify.com/verilog/verilog-assign-statement

Posted by: bennettfactly.blogspot.com

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