Applications Information
10-6
MC68307 USER’S MANUAL
MOTOROLA
10.1.5 EPROM Timing
The EPROM interface offers zero wait state accesses with a 100ns EPROM. A timing worth
special note is the EPROM enable to EPROM data out time. CS0 is asserted on the rising
edge of S2, and the selected EPROM has to return valid data a setup time before the falling
edge of S6 (an access time of approximately 2.5 clocks). At 5V and 16.67MHz, the equation
for the EPROM access time is as follows:
2.5 clocks – data setup time – clock to CS asserted time (maximum)= 150 – 5 – 30 = 115ns
Programming CS0 for 1 wait state would increase this access time to 175ns, while 6 wait
states permit an access time up to 475ns at 16.67MHz, and 5V.
For slow memories, the EPROM chip enable high to output float time should be confirmed
to ensure the EPROM-driven data is removed before the start of the next bus cycle (next
rising edge of S2). Although the exact timing is dependent upon the system, a maximum
time of 1 clock from CS0 negated to EPROM data floating is a good guideline for use with
the MC68307.
10.1.6 RAM Timing
The R/W signal is always setup prior to the RAM chip enable, so the timing is always RAM
enable (E) controlled rather than write (W) controlled. Besides the additional delay intro-
duced by the OR gate for decoding the RAM upper/lower byte enables, the RAM read timing
is the same as that of the EPROM. It is the RAM write timing that dictates the RAM access
time requirements.
The RAM chip enable to end of write time determines the necessary speed of RAM. The chip
enable time begins on the rising edge of S4 when the data strobes (UDS, LDS) are asserted,
and ends when the data strobes/CS1 negate, approximately 1 clock later. This typically
includes RAMs with access times of up to 60–70ns, for zero wait states at 16.67MHz and
5V (check specific RAM manufacturer’s data sheets). The MCM6206s used have a 35ns
access time.
For proper timing of write cycles, it is also important to know how much time elapses
between CS negated and the data output hold time. With a 5V, 16.67MHz MC68307, the
data out hold time is a 15ns minimum. Thus, on the enable controlled RAM write cycle, the
RAM data-in hold time requirement must be
≤15ns OR gate delay. When using a 74F32 OR
gate, the RAM data-in hold time must be
≤ 7ns and while using a 74AC32 it must be ≤ 5ns.
For the MCM6206, it is 0.