In Verilog, the strength of driving a net refers to the relative power or capability of a driver to influence the value of a net.
Two types of strengths can be specified in a net declaration
Charge Strength
Charge strength is specifically used with trireg nets to model charge storage. It indicates the relative size of the capacitance associated with the net indicated by either small, medium or large.
This strength determines how quickly a charge can decay on the net when it is not actively driven, allowing for more accurate simulation of real-world behavior in circuits that involve capacitive elements. The default charge strength of a trireg net is medium.
The simulation time for charge decay should be defined in the delay specification for the trireg net.
trireg a_net; // strength medium by default
trireg (medium) #(0, 0, 100) cap1; // strength medium, charge decay time of 100 time units
trireg (large) [3:0] cap2; // strength large, no decay time
Drive Strength
Drive strength refers to the capability of a driver to influence the value of a net. It indicates how strongly a signal is driven on the output terminals of a gate or net.
Drive strength is crucial in resolving conflicts when multiple drivers attempt to control a net. The net will take on the value from the strongest driver, and if there are conflicting values from drivers of the same strength, the result will be unknown (x).
When using the assign statement, you can specify the driving strength explicitly. The syntax for this is:
assign (strength1, strength0) net = expression;
- strength1: The strength when the net is driven to logic 1.
- strength0: The strength when the net is driven to logic 0.
If no strengths are specified, the default drive strength is typically strong, which means that the net will take on the value from a strong driver if multiple drivers are present.
If multiple drivers with different strengths attempt to drive a net, the net will take on the value of the strongest driver. If two or more drivers have the same strength but different values, the result will be unknown (x).
wire out;
assign (strong1, weak0) out = a & b; // Drives 'out' with strong1 when true
In this example, if a & b evaluates to 1, out will be driven with a strong signal; if it evaluates to 0, it will be driven weakly.
Verilog Strength Keywords
supply0
The supply0 net is a net that is always driven to a logic low (0) value. It is typically used to represent a ground connection or a negative power supply in a circuit. When connected to other components, it ensures that those components see a consistent low voltage level. If no other driver is present, the value of a supply0 net remains 0. It can be used in simulations to model scenarios where certain parts of the circuit are grounded.
supply1
The supply1 net is a net that is always driven to a logic high (1) value. It represents a positive power supply connection, ensuring that connected components receive a consistent high voltage level. Similar to supply0, if no other driver is present, the value of a supply1 net remains 1. This is useful for modeling scenarios where certain parts of the circuit are powered.
strong0
The strong0 keyword indicates that the driver will actively drive the net to a logic low (0) with strong strength. When a net is assigned a value using strong0, it signifies that the driver has a robust capability to pull the net down to 0, overriding weaker drivers.
strong1
The strong1 keyword signifies that the driver will actively drive the net to a logic high (1) with strong strength. When a net is assigned a value using strong1, it indicates that the driver can effectively pull the net up to 1, overpowering any weaker drivers.
If multiple drivers are connected to the same net, the net will take on the value from the strongest driver. If there are conflicting values from drivers of equal strength, the result will be unknown (x).
assign (strong1, weak0) my_net = some_signal; // Drives my_net high with strong strength
assign (weak1, strong0) my_net = another_signal; // Drives my_net low with strong strength
pull0
The pull0 strength indicates that a net has a resistive pull-down device connected to it. When a net is assigned the pull0 strength, it will be driven to a logic low (0) when no other drivers are actively driving it high. This ensures that the net defaults to 0 if left floating. If no drivers are present or if all drivers are in a high-impedance state (z), the net will resolve to 0 due to the pull-down effect.
pull1
The pull1 strength signifies that a net has a resistive pull-up device connected to it. When assigned the pull1 strength, the net will be driven to a logic high (1) when no other drivers are actively driving it low. This ensures that the net defaults to 1 if left floating. Similar to pull0, if no drivers are present or all are in high-impedance state (z), the net will resolve to 1 due to the pull-up effect.
If a net with pull strengths experiences conflicting values from active drivers, the strongest driver will take precedence. If two drivers of equal strength drive different values, the result will be unknown (x).
assign (pull1, pull0) my_net = some_signal; // Pulls up to 1 unless driven low
weak0
The weak0 strength indicates that a net will be driven to a logic low (0) with a weak driving capability. When a net is assigned the weak0 strength, it signifies that the driver can pull the net down to 0, but it is not as strong as other driving strengths like strong0 or pull0. This is useful in situations where you want to allow for the possibility of other stronger drivers to take precedence. If no stronger drivers are present, the net will resolve to 0 when driven by weak0.
weak1
The weak1 strength indicates that a net will be driven to a logic high (1) with a weak driving capability. When assigned the weak1 strength, it means that the driver can pull the net up to 1, but again, it is weaker than other driving strengths like strong1 or pull1. This allows for potential overriding by stronger drivers. If no stronger drivers are present, the net will resolve to 1 when driven by weak1.
In cases where multiple drivers are connected to a single net, the net will take on the value from the strongest driver. If two or more drivers of equal strength drive conflicting values, the result will be unknown (x).
assign (weak1, weak0) my_net = some_signal; // Drives my_net with weak strengths
highz0
The highz0 strength indicates that a net is in a high-impedance state while being driven to a logic low (0). It is typically used when modeling nets that can be disconnected or tri-stated, allowing the net to effectively "float" at 0 when no active drivers are present. When a net is assigned the highz0 strength, it means that it can be driven to 0 but will also enter a high-impedance state if no active drivers are present.
highz1
The highz1 strength signifies that a net is in a high-impedance state while being driven to a logic high (1). Similar to highz0, this strength is used in scenarios where the net can be tri-stated and should float at 1 when not actively driven. When assigned the highz1 strength, the net can be driven to 1 but will enter a high-impedance state if there are no active drivers.
assign (highz1, pull0) my_net = some_signal; // Drives my_net with high impedance when not driven
The following two rules shall constrain the use of drive strength specifications:
- The strength specifications (highz1, highz0) and (highz0, highz1) shall be treated as illegal constructs.
- If drive strength is not specified, it shall default to (strong1, strong0).
Verilog Strength Example
module tb;
// Declare nets with different strengths
reg a, b, c, d;
wire strong0_net, strong1_net;
wire pull0_net, pull1_net;
wire weak0_net, weak1_net;
wire highz0_net, highz1_net;
// Assign strong drivers
assign (strong1, weak0) strong1_net = a; // Strongly drives high if 'a' is 1
assign (weak1, strong0) strong0_net = b; // Strongly drives low if 'b' is 0
// Assign pull drivers
assign (pull1, pull0) pull1_net = c; // Pulls up to 1 unless driven low
assign (pull0, pull1) pull0_net = d; // Pulls down to 0 unless driven high
// Assign weak drivers
assign (weak1, weak0) weak1_net = a; // Weakly drives high if 'a' is 1
assign (weak0, weak1) weak0_net = b; // Weakly drives low if 'b' is 0
// Assign high impedance drivers
assign (highz1, pull0) highz1_net = a; // High impedance when not driven
assign (highz0, pull1) highz0_net = b; // High impedance when not driven
initial begin
reg [1:0] values = {1'b1, 1'b0};
repeat (10) begin
integer idx;
#10;
idx = $random % 2;
a = values[idx];
idx = $random % 2;
b = values[idx];
idx = $random % 2;
c = values[idx];
idx = $random % 2;
d = values[idx];
end
end
initial
$monitor("[%0t] a=%0b b=%0b c=%0b d=%0b strong1=%0b strong0=%0b pull1=%0b pull0=%0b weak1=%0b weak0=%0b highz1=%0b highz0=%0b", $time, a, b, c, d, strong1_net, strong0_net, pull1_net, pull0_net, weak1_net, weak0_net, highz1_net, highz0_net);
endmodule
xcelium> run [0] a=x b=x c=x d=x strong1=x strong0=x pull1=x pull0=x weak1=x weak0=x highz1=x highz0=x [10] a=0 b=x c=x d=x strong1=0 strong0=x pull1=x pull0=x weak1=0 weak0=x highz1=0 highz0=x [20] a=1 b=1 c=x d=0 strong1=1 strong0=1 pull1=x pull0=0 weak1=1 weak0=1 highz1=z highz0=1 [30] a=1 b=1 c=0 d=1 strong1=1 strong0=1 pull1=0 pull0=1 weak1=1 weak0=1 highz1=z highz0=1 [40] a=1 b=0 c=1 d=0 strong1=1 strong0=0 pull1=1 pull0=0 weak1=1 weak0=0 highz1=z highz0=z [50] a=x b=0 c=1 d=x strong1=x strong0=0 pull1=1 pull0=x weak1=x weak0=0 highz1=x highz0=z [60] a=0 b=1 c=0 d=0 strong1=0 strong0=1 pull1=0 pull0=0 weak1=0 weak0=1 highz1=0 highz0=1 [70] a=0 b=x c=0 d=x strong1=0 strong0=x pull1=0 pull0=x weak1=0 weak0=x highz1=0 highz0=x [80] a=x b=x c=x d=0 strong1=x strong0=x pull1=x pull0=0 weak1=x weak0=x highz1=x highz0=x [90] a=0 b=0 c=0 d=x strong1=0 strong0=0 pull1=0 pull0=x weak1=0 weak0=0 highz1=0 highz0=z [100] a=0 b=x c=x d=x strong1=0 strong0=x pull1=x pull0=x weak1=0 weak0=x highz1=0 highz0=x xmsim: *W,RNQUIE: Simulation is complete.
