Registers. Flip-flops are available in a variety of configurations. A simple one with two independent D flip-flops with clear and preset signals is illustrated on the following slide. Although packaged together, the two flip-flops are unrelated.
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Flip-flops are available in a variety of configurations.
A simple one with two independent D flip-flops with clear and preset signals is illustrated on the following slide.
Although packaged together, the two flip-flops are unrelated.
The second arrangement shows an octal flip-flop in which the eight D flip-flops are not only missing the Q’ and preset lines, but all the clock lines are ganged together and driven by pin 11 so the flip-flops are loaded on the rising transition.
We could have designed a circuit in which the three OR lines were just fed into the three output lines, but in practice the same lines are used for both input and output.
What is needed is an electronic switch that can make or break a connection in a few nanoseconds. Such a switch is called a noninverting buffer. It has a data in, data out, and a control line. When control is high, the buffer acts like a wire. When control is low, the buffer acts like an open circuit.
The memory shown previously easily extends to larger sizes.
For example, to extend to 4 words of 8 bits each we add five more columns of four flip-flops each, as well as five more input and output lines. To extend to eight words of three bits each, we add four more rows of three flip-flops each and one more address line.
For maximum efficiency, the number of words in memory should be a power of 2, but the size of words can be anything.
For any given memory size, there are various ways of organizing the chip.
A 4-Mbit chip could be organized as 512K words of 8 bits each or 4096K words of 1 bit each.
Aside - on some pins a high voltage causes an action to happen while on others a low voltage causes the action. Thus, we will say that a signal is asserted or negated to avoid this issue. A signal S is asserted high, but S’ is asserted low.
Dynamic RAMs (DRAMs) do not use flip-flops. A DRAM is an array of cells, each cell contains one transistor and one capacitor. The capacitors can be charged or discharged, allowing data to be stored.
Since the electric charge tends to leak away, every bit in a DRAM must be refreshed every few milliseconds. DRAMs have a high capacity, thus they are almost always used for main memory. On the other hand, they are slow.
The EPROM (Erasable PROM) can be field-erased as well by exposing the EPROM to a strong ultraviolet light for 15 minutes.
The EEPROM can be erased by applying pulses to it rather than UV light. It can also be programmed in place (an EPROM requires a special programming device). EEPROMs are slow compared to DRAMs and SRAMs.
Flash memory is block erasable and rewritable. They are often used in digital cameras, but they wear out after about 10,000 erasures.