Motorola MC68340 User Manual

10-8

MOTOROLA

Additionally, the relationship between the asynchronous inputs and the clock edge, as
shown in Figure 10-11, does not change as frequency changes.

A second type of specification indicates the minimum amount of time a signal will be
asserted. This type of specification is illustrated in Figure 10-12.

OUTPUT

CLKOUT

t w

N

T/2

t d

The method for calculating a frequency-adjusted tw is as follows:

tw' = tw + N (Tf'/2 – Tf/2) + (Tf'/2 – td)

where:

tw' = the frequency-adjusted signal width

tw = the signal width at 16.78 MHz

N = the number of full one-half clock periods in tw

Tf'/2 = one-half the new clock period

Tf/2 = one-half the clock period at full speed

td = the propagation time from the clock edge

The following calculation uses a 16.78-MHz part, specification 14, 

AS

 width asserted, at

12.5 MHz as an example:

tw = 100 ns

N = 3

Tf'/2 = 80/2 = 40 ns

Tf/2 = 60/2 = 30 ns

td = 30 ns maximum

therefore:

tw' = 100 + 3(40 – 30) + (40 – 30) = 140 ns

The third type of specification used is a skew between two outputs (see Figure 10-13).

F

re

e

sc

a

le

 S

e

m

ic

o

n

d

u

c

to

r,

 I

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

 

Freescale Semiconductor, Inc.

For More Information On This Product,

   Go to: www.freescale.com

n

c

.

..