Basic Flip-Flop Circuit

Coinmon emitter current amplifier using an npn transistor. 

Basic flip-flop circuit using pnp BJTs. If Tj is conducting, T2 is turned off and vice versa. A negative pulse at Vci will turn on T2 and turn off Tj. This tj pe of circuit is the basic element in counters and serves as the memory element in a SRAM. 

If Ti happens to be on, the state may be changed by applying a negative pulse to Vci-This will cause T2 to start conducting, shutting off Ti. 

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The flip-flop circuit described previously is the basic digital counter and makes wide use of CMOS technology in applications such as digital watches where current drain must be held to a minimum. A quartz oscillator provides the basic clock whose pulses are repeatedly divided by two by a series of flip-flop circuits in order to count seconds, minutes, and hours. 

There are two general types of RAM, static RAM (SRAM), and dynamic RAM (DRAM). SRAM is based on the flip-flop circuit discussed previously. Two cross-coupled transistors comprise the basic storage unit, a logical "1" if one transistor is "on" and a logical "0," if the other is "on." Two more transistors sense the state, and two more transistors control the access to the storage location, so six transistors are required to store one bit of information. Access time ranges from 10 to 30 ns. The term "static" refers to the fact that the information remains stored as long as the system is powered. When power is removed, the state of the flip-flops becomes indeterminate hence, data is lost unless stored on the hard drive. Such a memory is called a volatile memory. 

The basic device for counting and storage operations is the flip-flop or bistable multivibrator. The flip-flop, in its many forms, can be constructed from individual gates, but it is usually purchased as a single unit in integrated-circuit form. 

Often one needs counters in an interface to divide down the clock frequencies and to count such events as the number of data points taken and the number of times data exceed a predetermined threshold. The flip-flop used in modern integrated-circuit counters is the master-slave JK flip-flop. It is used to construct two basic types of counters, asynchronous and synchronous, that will count up or down in a variety of counting schemes. 

In the standard cell approach, the IC designer selects from a library of predefined logic circuits or cells to build the desired circuit. In addition to the basic gates, the cell library usually includes more complex logic circuits such as exclusive-OR, AND-OR-INVERT, flip-flops, adders, read only memory (ROM), etc. 

Below the top level (inside the process), most of the synthesized circuits are different. The process in SIMPLE constructs the circuit shown in Figure 5.14. This is the most basic demonstration of how a signal assignment inside a process with a clock expression template will infer flip flops. The synthesizer has used the most basic storage elements available - edge-triggered D-t)rpes. 

Table 5.1 shows a comparison of the synthesis statistics for these architectures with the SIMPLE architecture, which contains no initialization function. In terms of transistors, synchronous initialization makes the basic circuit approximately one-third larger. The additional gate between each flip flop also increases the latch to latch delay. 

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