Date: 20230215

68HC11 FAQ for Unusual questions

When searching design resources for the 68hc11, you can find a lot of information on the web. However, all of these information are usually very similar and only describe usual use cases. Some questions are not answered by the multiple documents available on the internet.

This page attempts to answer questions I was not able to find answers for.

Q: In extended mode, are pull-ups required on the data bus?

Context: The 68HC11 external bus is multiplexed (except on the F1 variation). A 74xx373/74xx573 latch is used to keep the low address lines stable using the AS signal. Many 68HC11 boards have pull-ups on the AD0-AD7 lines that form the data bus. However, some old versions of the HC11 reference manual do not have them. What is the actual requirement?

A: No, pull ups are not required, the 68HC11 external bus will work fine without them. I know this, because I have tried it on my own board. For some reason, when reading addresses while no data is driven on the bus will return the low byte of the address. However, other documents indicate that the CMOS buffers on the 68HC11 input lines might oscillate if they are biased near Vcc/2. For stability reasons, the data bus, as well as on any other input signal of the 68HC11, should have pull resistors. But bus operation will not be prevented if you dont install them.

If you have more questions, or have accureate answers about unusual facts about the 68hc11, reach me via email at me@me.net where me is replaced by my ham callsign.