{[ promptMessage ]}

Bookmark it

{[ promptMessage ]}

# hw2s - ENGRD2300 Introduction to Digital Logic Fall 2008...

This preview shows pages 1–2. Sign up to view the full content.

ENGRD2300: Introduction to Digital Logic Fall 2008 Homework 2 Solutions Problem 1. You need to drive a large number of 74HC00-like inputs with 74HC00 NAND gates. What is the maximum number of 74HC00-like inputs that you can connect to one single 74HC00 output without exceeding its worst-case loading specifications (fanout)? Use Wakerly Table 3-3. Hint: Calculate BOTH LOW-state and HIGH-state DC fanouts. The maximum amount of current that the output of the 74HC00 can sink or source, I OLmax and I OHmax , cannot be lower than the sum of the maximum input currents of the 74HC00, I ILmax and I IHmax . There are two possible solutions to this problem, depending on which value of I OLmax and I OHmax you choose. If you choose the typical value for connections to “CMOS” inputs, then from Table 3-3 we obtain I OLmax = 20 μ A, I OHmax = -20 μ A, I ILmax = -1 μ A and I IHmax = 1 μ A. The LOW-state DC fanout is: Fanout LOW = | I OLmax / I ILmax | = 20 μ A / 1 μ A = 20 The HIGH-state DC fanout is: Fanout HIGH = | I OHmax / I IHmax | = 20 μ A / 1 μ A = 20 The device fanout is min( Fanout LOW , Fanout HIGH ) = 20, so one should theoretically connect no more than 20 74HC00-like inputs to one output of a 74HC00.

This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document
This is the end of the preview. Sign up to access the rest of the document.

{[ snackBarMessage ]}