Motorola MC68340 用户手册

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10-7

An equation for the chip select access time, tCSDV, can be developed as follows:

tCSDV = tcyc(Nc – 1) – ts9 – ts27

where:

tcyc = system clock period
Nc = number of clocks per access
ts9 = CLKOUT low to 

CS

≈

 asserted = 30 ns maximum at 16.78 MHz

ts27 = data-in valid to CLKOUT low setup = 5 ns minimum at 16.78 MHz

Using these equations, the memory access times at 16.78 MHz are shown in Table 10-1.
See Section 11 Electrical Characteristics for more timing information.

t

ADV

54 ns

114 ns

173 ns

233 ns

292 ns

t

CSDV

24 ns

84 ns

143 ns

203 ns

263 ns

The values can be used to determine how many clock cycles an access will take, given
the access time of the memory devices and any delays through buffers or external logic
that may be needed.

The general relationship between the CLKOUT and most input and output signals is
shown in Figure 10-11. Most outputs transition off of a falling edge of CLKOUT, but the
same principle applies to those outputs that transition off of a rising edge.

CLKOUT

OUTPUTS

ASYNCHRONOUS

INPUTS

t d

t su

h

t

For outputs that are referenced to a clock edge, the propagation delay (td ) does not
change as the frequency changes. For instance, specification 6 in the electrical
characteristics, shown in Section 11 Electrical Characteristics, shows that address,
function code, and size information is valid 3 to 30 ns after the rising edge of S0. This
specification does not change even if the device frequency is less than 16.78 MHz.

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Freescale Semiconductor, Inc.

For More Information On This Product,

   Go to: www.freescale.com

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