博文速递:STA Numericals
作者:Feroz Ahmed Choudhary
STA Numericals
Problem 1:
In the following circuit
Each flip flop has:
Setup time of 60ps
Hold time of 20ps
Clock-to-Q maximum delay of 70ps
Clock-to-Q minimum delay of 50ps
Each XOR gate has:
Propagation delay of 100ps
Contamination delay of 55ps
a. If there is no clock skew, what is the maximum operating frequency of this circuit?
b. How much clock skew can the circuit tolerate before it might experience a hold time violation?
c. Redesign the circuit so that it can be operated at 3GHz frequency. How much clock skew can your circuit tolerate before it might experience a hold time violation?
Solution
a.
Tc ≥ Tpcq + Tpd + Tsetup
Longest path:
Tc ≥ Tpcq + 3*Tpd + Tsetup
Tc ≥ 70 + 3*100 + 60 = 430 ps
Max Frequency = 1/Tc = 2.33 GHz
b.
Tccq + Tcd≥Thold + Tskew
Shortest Path:
Tccq + Tcd ≥ Thold + Tskew
50 + 55 ≥ 20 + Tskew
Tskew ≤ 85 ps
c.
Tc ≥ Tpcq + 2*Tpd + Tsetup + Tskew
Tc ≥ 330 + Tskew
Tccq + 2Tcd ≥ Thold + Tskew
Tskew≤ 140 ps
Problem 2:
Q. Determining the Max. Clock Frequency for a Sequential Circuit.
Solution
Before starting timing analysis, consider the flow of data in this circuit in response to a rising clock edge, starting at flip-flop A.
1. Following the rising clock edge on Clk, a valid output appears on signal X after tClk−Q = 10 ns.
2. A valid output Y appears at the output of inverter F, tpd = 5 ns after a valid X arrives at the gate. 3. Signal Y is clocked into flip-flop B on the next rising clock edge. This signal must arrive at least ts = 2ns before the rising clock edge.
As a result, the minimum clock period, Tmin of the circuit is:
Tmin = tClk − Q(A) + tpd(F) + ts(B)
= 10ns + 5ns + 2ns = 17ns
maximum clock frequency of the circuit is 1/Tmin = 1/17ns = 58.8 MHz
Problem 3:
Q. Determining the Max. Clock Frequency the Sequential circuit shown below.
In a typical sequential circuit design there are often millions of flip-flop to flip-flop paths that need to be considered in calculating the maximum clock frequency. This frequency must be determined by locating the longest path among all the flip-flop paths in the circuit. For example, consider the circuit shown in above. there are three flip-flop to flip-flop paths (flop A to flop B, flop A to flop C, flop B to flop C),the delay along all three paths are:
TAB = tClk−Q(A) + ts(B) = 9 ns + 2 ns = 11 ns
TAC = tClk−Q(A) + tpd(Z) + ts(C) = 9 ns + 4 ns + 2 ns = 15 ns
TBC = tClk−Q(B) + tpd(Z) + ts(C) = 10 ns + 4 ns + 2 ns = 16 ns
Since the TBC is the largest of the path delays, the minimum clock period for the circuit is Tmin = 16 ns and the maximum clock frequency is 1/Tmin = 62.5 MHz.
Problem 4:
Q. For the circuit given below calculate.
• Maximum clock frequency for reliable operation.
• The amount of clock skew the circuit can tolerate if it needs to operate at 5 Ghz.
• How much clock skew the circuit can tolerate before it experiences a hold time violation?
Flip-Flop (clock-to-q) propagation delay (tpcq) = 35 ps
Flip-Flop (clock-to-q) contamination delay (tccq) = 20 ps
Flip-Flop data setup time (ts) = 30 ps
Flip-Flop data hold time (th) = 10 ps
Solution
a.
Period > (FF propagation delay) + (max combination circuit delay) + (FF Setup time) + (max clock skew)
Period > 35 + (60+20) + 30 +0 ps
Period > 145 ps
F < 1/(145 ps)
F < 6.8965 GHz.
b.
At F= 5 GHz Period = 1/(5 Ghz) = 200 ps.
Max clock skew = Clock period – (FF propagation delay + max combination circuit delay + FF Setup time)
Max clock skew = 200 – (35 + (60+20) + 30) = 200 – 145 = 55 ps.
c.
For hold time violation to NOT occur.
Hold time <= (FF contamination delay) + (min combinational circuit delay) - (max clock skew)
So hold time will get violated when
Max clock skew > (FF contamination delay) + (min combinational circuit delay) – (Hold Time)
Max clock skew > 20 + (20+10) – 10
Max clock skew > 40 ps
Problem 5:
Q. For the circuit given below calculate.
a. setup slack
b. hold slack
Solution
before proceeding with the solution we should know :
setup slack = RTmin(minimum required time) - ATmax(maximum arrival time)
where; RTmin ≥ ATmax to satisfy setup time
Hold Slack = ATmin(minimum arrival time) - RTmax(maximum arrival time)
where; ATmax ≥ RTmin to satisfy hold time
Let's solve this..
a. Setup Slack
ATmax = 2 + 11 + 2 + 9 + 2 + 3 = 29ns
RTmin = 2+ 5 + 2 + 15(here 15ns is the time perid) = 24ns
Setup slack = ATmax - RTmin = 24 - 29 = -5ns
we can see setup is violating as ATmax is less than RTmin.
b. Hold Slack
ATmin = 1 + 9 + 1 + 6 + 1 - 2 (here Thold is considered and subtracted) = 16ns
RTmax = 3 + 9 + 3 = 15ns
Hold Slack = ATmin - RTmax = 16 - 15 = 1ns
here ATmax ≥ RTmin i.e 16 ≥ 15 (no violation)
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