Motorola MC68VZ328 18.1.3 Clock and Layout Considerations, 18.1.4 Bus and I/O Considerations

18-2 MC68VZ328 User’s Manual 
Application Guide
18.1.3 Clock and Layout Considerations
This section covers layout considerations affecting DragonBall timing issues during operation and also
during the initial power up.
• Place the crystal within 0.5 inches of the MC68VZ328. The crystal and the capacitors must be as
close to the chip as possible.
• If an RC reset circuit is being used, place the resistor and capacitor within 0.5 inches of the
MC68VZ328. The RESET pin is a Schmitt trigger input signal. A simple power -up RC rese t circuit
can be used. Since the internal module takes time to complete the reset operation, a minimum
250 ms power-up reset pulse is required.
• Use multiple power and ground planes. It is strongly recommended to use at le ast one ground pl ane,
one 3.3 V VDD plane, and one 5V VSS plane (if 5 V parts exist in the system ). Thi s helps improve
the power stability and enhance the noise immunity of the system.
18.1.4 Bus and I/O Considerations
Several of the items that are warned against in this section appear to be good design practice. However,
experience has demonstrated that not heeding the following suggestions can lead to problems.
• Do not leave unused input pins floating. Unused inputs should be tied high or low, but not left
floating. Unused inputs can be tied directly to VSS or VDD or through pull-ups or pul l-downs t o VSS
or VDD.
• Use the port pins efficiently. When port pins ar e not used, t he y sh ould be conf i gured a s input s wit h
pull-up enabled or as an output with pull-up disabled to reduce power consumption.
• Apply internal pull-ups to dedicated function pins. Many pins are mixed wit h a dedicat ed functi on.
The internal pull-up or pull-down resistors apply to bot h the dedi cated fu nction an d the ge neral I/ O
function. For instance, when using the RXD/PE4 signal and the RXD function, the associated
internal pull-up resistor can be used to pull up the RXD input signal.
• Do not rely solely on the value of interna l pull-up resistors. The internal resistors are nominally
1 megaohm, but their deviation is large.
• Always provide a development interface port on your design. The MC68VZ3 28 has bootstrap mode
and a bootstrap utility program that can be used to download programs and data to a target system
and perform simple hardware debugging functions. However, bootstrap mode only uses the RXD
and TXD signals of the UART port, so it is recommended that a UART port be included in the
design for system debugging and flash memory updating.