Clock Skew 1
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Clock skew refers to the difference in arrival times of a clock signal at different devices. Excessive clock skew can be minimized by buffering the clock signal so that all signals experience equal delays, adding delay to the data path to account for skew, reversing the clock signal direction to clock the destination register before the source, or using alternate phase clocking with two clock phases or opposite clock edges between sequential registers.
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Rising and falling edge of the clock
For a +ve edge triggered design +ve (or rising) edge is called ‘leading edge’ whereas –ve (or falling) edge is called ‘trailing edge’.
For a -ve edge triggered design –ve (or falling) edge is called ‘leading edge’ whereas +ve (or rising) edge is called ‘trailing edge’.
basic clock
Minimum pulse width of the clock can be checked in PrimeTime by using commands given below:
set_min_pulse_width -high 2.5 [all_clocks]
set_min_pulse_width -low 2.0 [all_clocks]
These checks are generally carried out for post layout timing analysis. Once these commands are set, PrimeTime checks for high and low pulse widths and reports any violations.
Capture Clock Edge
The edge of the clock for which data is detected is known as capture edge.
Launch Clock Edge
This is the edge of the clock.Jack_Knutson_SNUG2003_ Copy
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A dual beam oscilloscope can display two signals simultaneously using a CRT that generates and deflects two separate electron beams. It avoids issues with dual trace oscilloscopes that time share a single beam. A dual trace oscilloscope displays two signals by rapidly switching a single beam between the two input channels. Sampling oscilloscopes convert fast signals to low frequency domains by taking samples over successive cycles. Digital storage oscilloscopes digitize input waveforms and store them in memory for display, allowing non-repetitive signals to be observed. Oscilloscope probes come in passive and active varieties, with 1x, 10x, and 100x attenuation ratios for passive probes and integrated circuits in active probes for improved performanceTiming notes 2006
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This document discusses timing parameters for combinational and sequential logic circuits. It defines propagation delay and contamination delay for combinational logic, and propagation delay, contamination delay, setup time, and hold time for sequential logic circuits like flip-flops. It also discusses determining maximum clock frequency for sequential circuits based on these timing parameters, modeling delays from gates and interconnect, using clock trees, and issues related to clock skew and jitter.Library Characterization Flow
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Clock Skew 1
- 1. Clock Skew
- 2. Clock Skew Definitions: The difference between arrival times of the clock at different devices is called clock skew. Example of clock skew:
- 3. Clock Skew Buffering the clock: In a large system, the single clock signal may not have adequate fanout to drive all of the devices, so it may be necessary to provide one or two copies of the clock signal. The buffering method of figure (a) produces excessive clock skew, since CLOCK1 and CLOCK2 are delayed through an extra buffer compared to CLOCK. A recommended method is shown in figure (b). All of the clock signals go through identical buffers, and thus have roughly equal delays.
- 4. Minimizing the clock skew Add Delay in Data Path: The amount of the inserted delay (number of BUFD or INVD macros) in the data path should be large enough so that the delay becomes sufficiently greater than the clock skew.
- 5. Minimizing the clock skew Clock Reversing: In this method, the clock signal arrives at the clock port of the destination register sooner than the source register. Therefore, the destination register will clock in the source register (current) value before the source register receives it’s clock edge. The clock reversing method will not be effective in circular structures such as Johnson counters because it is not possible to define the source register explicitly.
- 6. Minimizing the clock skew Clock Reversing in circular structure: In this example, the clock skew problem exists between flip-flops U1 and U3.
- 7. Minimizing the clock skew Alternate Phase Clocking: The following are the most common methods of alternate phase clocking: 1. Clocking on alternate edges 2. Clocking with two phases 1. Clocking on alternate edges: In this method, the sequentially adjacent registers are clocked on opposite edges of the clock. This method provides a short path-clock skew margin of about one-half clock cycle.
- 8. Minimizing the clock skew Signal propagation for previous circuit:
- 9. Minimizing the clock skew Clocking with two phases: In this method, the sequentially adjacent registers are alternatively clocked on two different phases of the same clock. In this case, between each two adjacent registers, there is a safety margin approximately equal to the phase difference of the two phases.
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