Motorola MC68340 manual Calculating Frequency-Adjusted Output, Memory Access Times at 16.78 MHz

Freescale Semiconductor, Inc.

Freescale Semiconductor, Inc.

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.

Table 10-1. Memory Access Times at 16.78 MHz

Access Time	N = 2	N = 3	N = 4	N = 5	N = 6
tADV	54 ns	114 ns	173 ns	233 ns	292 ns
tCSDV	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.

10.2.3 Calculating Frequency-Adjusted Output

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

t d

OUTPUTS

t su t h

ASYNCHRONOUS

INPUTS

Figure 10-11. Signal Relationships to CLKOUT

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.

MOTOROLAMC68340 USER’S MANUAL10-7

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