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Showing posts from May, 2009

Synchronous Reset vs. Asynchronous Reset

Why Reset?

A Reset is required to initialize a hardware design for system operation and to force an ASIC into a known state for simulation.

A reset simply changes the state of the device/design/ASIC to a user/designer defined state. There are two types of reset, what are they? As you can guess them, they are Synchronous reset and Asynchronous reset.

Synchronous Reset

A synchronous reset signal will only affect or reset the state of the flip-flop on the active edge of the clock. The reset signal is applied as is any other input to the state machine.

The advantage to this type of topology is that the reset presented to all functional flip-flops is fully synchronous to the clock and will always meet the reset recovery time.Synchronous reset logic will synthesize to smaller flip-flops, particularly if the reset is gated with the logic generating the d-input. But in such a case, the combinational logic gate count grows, so the overall gate count savings may not be that significant…

VLSI Interview Questions with Answers - 1

1. Why does the present VLSI circuits use MOSFETs instead of BJTs?

Compared to BJTs, MOSFETs can be made very small as they occupy very small silicon area on IC chip and are relatively simple in terms of manufacturing. Moreover digital and memory ICs can be implemented with circuits that use only MOSFETs i.e. no resistors, diodes, etc.

2. What are the various regions of operation of MOSFET? How are those regions used?

MOSFET has three regions of operation: the cut-off region, the triode region, and the saturation region.
The cut-off region and the triode region are used to operate as switch. The saturation region is used to operate as amplifier.

3. What is threshold voltage?

The value of voltage between Gate and Source i.e. VGS at which a sufficient number of mobile electrons accumulate in the channel region to form a conducting channel is called threshold voltage (Vt is positive for NMOS and negative for PMOS).

4. What does it mean "the channel is pinched off"?