今天小编给大家分享下英伟达NVIDIA近两年数字IC后端笔试真题（ASIC Physical Design）
- 请使用OR门和INV反相器来搭建下面所示F逻辑表达式的电路图。
数字IC后端设计如何从零基础快速入门？(内附数字IC后端学习视频）
- 异步复位&同步复位
- What is synchronous reset and asynchronous reset?
- Please list at least 1 disadvantage each for synchronous and asynchronous reset respectively. How to resolve these issues? How to solve these issues?
Synchronous reset means reset is sampled with respect to clock. In other words, when reset is enabled, it will not be effective until the next active clock edge.
In asynchronous reset, reset is sampled independent of clk. That means, when reset is enabled it will be effective immediately and will not check or wait for the active clock edges.
- Disadvantages of synchronous reset
Reset signal needs to be stretched, if it is not long enough to be seen at the active clock edge.
Requires a clock to be present if reset is to occur
If there are internal tri-state buffers, separate asynchronous reset may still be required
If there are gated clocks for power saving, this type of reset is not suitable.
Synthesis will not be able to easily differentiate reset from other signals. So this has to be taken care while doing synthesis. Otherwise it may lead to timing issues.
- Disadvantages of asynchronous reset
Reset line is sensitive to glitches.
There may be metastability issues in the circuits.
- Solution
Asynchronous reset and synchronous release（异步复位同步释放）.
- 数字后端名词解释
Please briefly describe below terminologies in digital circuit design：combination logic, gray code, metastability, lockup latch，setup, static power, logic synthesis, time borrow, power gating, process variation
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combinational logic: A type of digital logic implemented by Boolean circuit, where the output is a pure function of the present input only. Combinational logic does not require clock to realize the function.
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gray code: Gray code is an ordering of the binary numeral system such that two successive values differ in only one bit.
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metastability: States where the signals can settle to an intermediate value between logic 0 and logic 1, this is called metastable state.
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lockup latch: A lockup latch is a transparent latch used to avoid large clock skew and mitigate the problem in closing hold timing due to large uncommon clock path. Lockup latch is an important element in scan-based designs, especially for hold timing closure of shift modes. Lockup latches are necessary to avoid skew problems during shift phase of scan-based testing.
吾爱IC：原来负沿Latch可以用来修hold（Timing borrowing及其应用）
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setup: The setup time is the time during which a data signal must remain stable before the active clock edge.
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static power: Power consumption caused by leakage currents including sub-threshold leakage, gate leakage, gate induced drain leakage, reverse bias junction leakage.
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logic synthesis: The process of converting a functional behavioral model of a system represented as RTL model to structural (logical gate netlist) description model is called logic synthesis. It is a process of creating logic circuit from circuit description.
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time borrow: Time borrow, also called cycle stealing, occurs at a latch. Since a latch is transparent when the clock is active, the data can arrive later than the active clock edge, that is, it can borrow time from the next cycle.
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power gating: Shut down the power supply to a block of logic when it is not active.
- process variation: Operating conditions like process, temperature, and voltage define the process variations, which affect the functionality and performance of the SOC design.
11）Please briefly introduce what is physical synthesis. What are the benefits compared with logical only synthesis? （考查逻辑综合和物理综合基本概念和各自的优缺点）
4.利用脚本处理文件
There is a file named "scores.txt" which has below format: name score. Please read this file and print out the passing list (score >= 60) & the failing list (score < 60), both list ranking by scores from highest to lowest. You can use Perl/Tcl/Python/C to implement the code.
File scores.txt:
Joe 90
Tom 58
Jim 70
James 95
Alan 82
John 89
Mike 85
Sandy 55
Nancy 88
Sherry 72
Alice 80
Tony 60
脚本处理题记得借助chatGPT来完成。
open IN,'<','scores.txt';
my %passing_list,%failing_list;
while(<IN>){
if(/(\w+)\s+(\d+)/){
$name = $1;
$score = $2;
if($score>=60){
$passing_list{$name} = $score;
}
if($score<60){
$failing_list{$name} = $score;
}
}
}
print "Passing list:\n";
foreach $key (sort {$passing_list{$b} <=> $passing_list{$a}} keys %passing_list){
print "\t$key\t$passing_list{$key}\n";
};
print "\n";
print "Failing list:\n";
foreach $key (sort {$failing_list{$b} <=> $failing_list{$a}} keys %failing_list){
print "\t$key\t$failing_list{$key}\n";
};
close IN;
英伟达Nvidia几乎每年都会考到利用tcl等脚本语言来处理文本文件。
5.crosstalk知识点考查
Below figure shows a circuit that include cross-coupled capacitances. Please answer the following questions: 1) The waveforms on A, B and clk are shown as below. Considering the impact of crosstalk, please draw the waveform on pin D. And analyze the timing impact on pin D under below condition. 2) Please list at least 3 methods to reduce the impact of crosstalk and analyze its disadvantage.
1)Considering the impact of crosstalk, the transition time of signal on D pin will be reduced compared with waveform on B. This is because aggressor switching (net a) in same direction can result in a smaller delay for the switching net b.
2)if required, shielding wires between the nets, with Vdd and Vss nets to be run between two parallel long nets.（Disadvantages: shielding consumes more power, increases routing area and add interconnect complexity.）
not to run two long parallel interconnects on the same layer（Disadvantages: add interconnect complexity.)
increase spacing of parallel interconnect nets ( Disadvantages: need more routing resources and may increase die area.)
increase drive of victim net and decease drive of aggressor net.
use different routing layers (Disadvantages: need more routing resources.)
6.时序计算题
时序计算题也是英伟达NV每年必考的题目。大家一定要都能做对。
According to below timing report (the unit is ns), please answer following questions:
1）What check is this report for? Please point out the clues in the timing report.
2）What's the max frequency this path can achieve?
3）What's the main reason for this violation? List at least 3 ways that may fix this violation.
7.画一个两输入或门的版图
8.低功耗设计实现技术Clock Gating
秒杀数字后端实现中clock gating使能端setup violation问题
