Asynchronous reset

Here is an example of a Mealy finite state machine. Variable MealyState holds the machine state, while NextState is used to pass information from the combinational logic always statement to the sequential logic always statement. Input Reset asynchronously resets the state to STO. 

Designs often require a synchronous or an asynchronous reset signal to be inferred. In this section we show how asynchronous (Example 2.3) and synchronous (Example 2.4) resets can be described. The only difference between inferring synchronous and... 

Example 2.3 shows how an asynchronous reset can be incorporated into the design. (Figure 2.2). Example 2.3 is similar to Example 2.4, except that an additional input for reset (RST) is declared in the entity and the reset condition is added before the dock event and clock = 1 statement, implying that the reset signal is asynchronous. 

Example 2.16 describes a parametrizable multi bit register with asynchronous reset and clock enable signal. 

You have a flop with both asynchronous reset and preset. If both of these inputs are active, how would the DC handle this design assuming that the library does not have any information on the output when both are active. In other words, you wish to know whether will DC ensure that such condition does not occur or will DC ignore it. 

Clock clock Sense rising edge Asynchronous Reset Unspecified... 

Asynchronous Reset Unspecified Encoding Bit Length 4 Encoding style one hot... 

In the testmode, it is important that all clocks and asynchronous reset signals be controllable from the primary inputs. For internally generated clocks in the design (clock dividers, for instance), one approach is to synthesize a mux in the HDL code such that, in the testmode, die test clock from the primary input is selected. The same approach can be followed for the reset line. 

You are running DRC (check test) on the top level after integrating your ASIC. Finding that TC is inferring the a nchronous reset line as a clock and all the scan cells with asynchronous reset pins being classified as constant-logic black-box cells. 

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... 

The first block DIV is a frequency divider. This block has 2 modes of operation, the normal mode and the test mode. In the test mode, the UART chip runs 16 times faster than in the normal mode. Also, the transmission data rate of the UART chip is 16 times faster than the receiving rate. Each block is initialized by setting the reset line low by applying a 0 to port MR. The TX block accepts 8-bit parallel data from the microprocessor interface (MP) block and transmits it serially to the RS-232 port through port DOUT. Conversely, the RX block receives serial data input, and sends it in 8-bit parallel format to the MP block. Again, the transmitter runs at 16 times the speed of the receiver. The microprocessor interface (MP) block asynchronously controls the parallel data flow between the RX / TX blocks and the microprocessor data bus. 

TABLE 1.25 Asynchronous Set-Reset Latch Truth Table... 

An optional fifth pin called Test Reset (TRST ) is an asynchronous active low reset for the 1149.1 circuitry. Because any TAP can be reset by five clock pulses to TCK while TMS is held high, TRST is not actually needed for resetting an 1149.1 device. It is often included as a fail-safe measure with a board-level pull-down resistor providing a constant reset to the TAP. Many 1149.1-compliant ICs do not include the TRST pin because the extra pin required may be too costly. 

All racks contain independent 5V power supply units and communication between racks is via serial lines (and is asynchronous). All modules containing a CPU employ a relay watchdog refreshed by the on-card software. Diagnosed, on-line faults from any source cause all I/O modules to be reset to a known safe condition (all outputs low). 

XC4000 devices have dedicated resources for a Global Set/Reset net that can be utilized to initialize all the sequential elements inside the CLBs and lOBs on the device. This capability can be used, if your design has a global signal that effects every flop in your design, by instantiating a STARTUP component from the Xilinx library. However, to simulate the asynchronous behavior in the RTL code you must include the asynchronous capability in the code and later disconnect the asynchronous net and instead cormect it to the STARTUP component. 

At the start of simulation, all the sequential elements on the ASIC including those in the TAP controller are uninitialized. Since the TAP controller does not have a reset pin (either synchronous or asynchronous) the "U"s keeps propagating to the data pin of the flops thereby not being able to move the TAP controller to the Idle state. One way is to arbitrarily initialize the state vector of the TAP controller to a known value at the start of simulation and then simulate your regular functional vectors. If you are using Synopsys VHDL System Simulator for your gate-level simulation, the VSS commands hold and assign can be used to clock a known arbitrary value into the TAP controller. 

You have a design where the asynchronous pins of your flip-flops are controlled by an active low top level primary input RESET port. The stuck-at-one faults on your reset line are reported as untestable. Why ... 

Flow fields with velocities ranging from micrometers per second to supersonic speeds can be studied since interframe time separations down to few hundred nanoseconds can be obtained. One interesting feature of the cameras is that they can be reset asynchronously. This is particularly useful in conjunction with the special triggering options for synchronizing the measurements to external events, such as rotating machinery. 

The most common and definitely the best way of achieving this is to attach an external asynchronous or synchronous reset signal to each and every storage element in the circuit. When an external initialization signal is specified, the compiler will select an appropriate component from the library (if the elements are inferred). For example, if a flip flop is required to be initialized to V, the selected element will possess an input for a preset signal. A variety of designs based on this form of initialization are given in the examples in this chapter. 

The RESET signal is synchronous and is part of the combinational logic input to each flip flop. The circuit behaviour could have been specified as asynchronous by performing the reset sequence outside the clocked part of the process. This would have inferred flip flops with Preset and Clear inputs. The actual construction of a behavioural description to produce this circuit is left for the reader to attempt. Beware, oAer changes will also be required to the process ... 

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