Clock edge

Hip-flop inference rule is simple If a variable is assigned a value under the control of a clock edge, a flip-flop is generated an exception to this rule is when a variable is assigned and used only locally within an always statement as an intermediate variable. 

If a falling-edge-triggered flip-flop are to be inferred, then the clock edge event posedge Clk needs to be replaced by ... 

Integer variables assigned under the control of a clock edge are also inferred as flip-flops. Here is an example where an integer variable is assigned under clock control. Four flip-flops are inferred for the variable IntState) the other high-order bits of the variable are optimized away (since they are not used). 

No flip-flops are inferred for Temp since it is locally declared within the always statement and a value is assigned to the variable and used immediately in the same clock edge. Flip-flops are inferred for NextState (as this... 

The synthesized netlist is the same as in Figure 2-48. Notice that on every clock edge, NextState always get the value of Temp assigned in the previous clock cycle, but not so in the synthesized netlist. The recommendation here is to avoid using locally declared variables in this fashion. Hopefully a synthesis tool will issue a warning if no flip-flops are inferred for Temp. 

A restriction usually imposed by a synthesis system in this case is that a variable cannot be assigned under two different clock conditions or for that matter, under different clock edges. For example, it would be illegal to assign a value to D in the first always statement. 

Since NextState is assigned a value under the control of a clock edge (Stmt C) and it is also assigned asynchronously (Stmt A and B), a falling-edge-triggered flip-flop with asynchronous preset and clear is synthesized. This is shown in Figure 2-53. Note that four flip-flops are required. The first flip-flop (the leftmost bit of NextState) has both asynchronous preset and clear terminals since it needs to be preset on Reset and cleared on Set. Similarly, the fourth flip-flop has both asynchronous preset and clear terminals since it needs to be preset on Set and cleared on Reset. The... 

Next is an example of a task call that occurs under the control of a clock edge. 

In a Moore finite state machine, the output of the circuit is dependent only on the state of the machine and not on its inputs. This is described i pictorially in Figure 3-5. Since the outputs are dependent only on the j state, a good way to describe a Moore machine is to use an always state- j ment with a case statement. The case statement is used to switch between j the various states and the output logic for each state is described in the appropriate branch. The always statement can have the clock event in its] event list to indicate that it is a clocked always statement. This models the] condition of a finite state machine going from state to state synchronously j on every clock edge. The machine state itself is modeled using a reg vari-] able (a variable of reg data type). 

Here is an example of a Moore finite state machine. A reg variable] MooreState is used to model the machine state which can have either of] the four states. The event list indicates that the state transitions occur synchronously on every rising clock edge. 

Here is a model of a modulo-binary up-counter. This counter has only a synchronous preclear control and all transitions occur on the rising clock edge. 

Synthesis infers four flip-flops for this model, three for variable Previous and one for SeqFound. However, optimization reveals that one of the flip-flops for Previous is not necessary and hence it is removed. In this model, the output is latched since it is assigned a value under the control of a clock edge. If a latched output is not desired, then the assignment to SeqFound must be done outside the always statement. Such a module is shown next. 

Fig. D.6. (a) A simple T-flip flop implemented using NAND gates (b) the symbol for a T flip-flop. Q is the output, Q is the inverse of the output. When a rising clock edge is applied to T, the state switches from on to off or vice versa. More sophisticated latches may be necessary to prevent race conditions depending on the transistors used.

The memory elements of the circuit (flip-flops and latches), called registers, are all loaded synchronously with a clock edge at the clock edge, a register loads its next value, which then becomes its current value until the next clock edge registers hold the state of the machine. 

The time span between two clock edges is long enough, and the environment provides stable values on the circuit inputs soon enough, to let all signals stabilize in the combinational part of the circuit before the next clock edge. 

The Verilog and VHDL synchronous subsets that we consider refer to the second modeling option. As a consequence, these subsets exclude the synchronization primitives that would introduce state changes independent of a clock edge the wait for and after clauses of VHDL, and the wait and " " of Verilog, must be forbidden. 

Statements synchronised with the clock edge for these the 5 function is calculated, and possibly part of the X function. 

To understand the behavior of these processes, we associate one or more subscripted abstract variables to each declared object x in the HDL text, to hold current(x), the value of X when the description has stabilized (just before the clock edge), and the successively assigned values to the object in the process or always statement. 

Ugurdag, H. F. Fuhrman, T. E. (1996), Autocircuit A clock edge general behavioral system with a direct path to physical datapaths, in Proc. ICCD 96 , pp. 514-523. 

The module just stimulated is a multiply-accumulator with two unsigned 4-bit inputs, b and c. The product P gets added (accumulated) to the current register value S every positive clock edge. 

The minimum delay requirements are set by the hold constraints for the sequential cells. Hold time problems are caused due to short delay paths between registers which cause the data signal to propagate through two adjacent flip-flops on a single clock edge. Since path delays are the shortest under best-case operating conditions, hold time problems are maximum in these conditions. Hence, hold violations have to be fixed under these conditions. 

If there is an ASIC vendor restriction that test default bidir delay occur before the active clock edge, you have the following options ... 

Use a strobe before the clock protocol. This protocol is automatically inferred by TC when the test default.strobe is set before the active clock edge. In this protocol file the parallel measure and the parallel capture cycles are combined into one. 

These preclude a signal from being assigned on both a rising and falling clock edge or on different clocks. 

If the initialization signal, INTT, is high the normal shift operation occurs. The register will be initialized if INIT goes low. These operations inside a process are synchronous when a Wait statement is used with the dock expression. The trigger in this case is a rising clock edge. See Box 5.7 for more details on this construct. 

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