Jameco-electronics Rabbit 3000 Manual do Utilizador descarregar pdf (Página 229)

100

101

220 Rabbit 3000 Microprocessor

Figure 16-4 illustrates the sources that create memory access time delays.

Figure 16-4. Sources of Memory Access Time Delays

The gross memory access time is 2T, where T is the clock period. To calculate the actual

memory access time, subtract the clock to address output time, the data in setup time, and

the clock period shortening due to the clock spectrum spreader from 2T.

Example

• clock = 29.49 MHz,

• T = 34 ns,

• operating voltage is 3.3 V,

• bus loading is 60 pF,

• address to output time = 8 ns (see Table 16-2),

• data setup time = 1 ns,

• the spectrum spreader is on in normal mode, resulting in a loss of 3 ns.

The access time is given by

access time = 2T - (clock to address) - (data setup) - (spreader delay)

= 68 ns - 8 ns - 1 ns - 3 ns

= 56 ns

data out

address

memory access

time

clock period shortening

output enable

memory output enable

time

clock

data in setup time

clock to

(early)

output

address

due to spectrum spreader

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