/RCON pin external circuitry

[Dogbert256]

Could somebody please answer this VERY simple question????

I'm using the MCF5232 processor.  /RCON is asserted (low) at reset. 

Looking at the MCF523X Reference Manual, on page 9-9 it says "/RCON must also be negated within one cycle after /RSTOUT is negated."

Looking at the schematics for the MCF5235EVB (Evaluation Board), I find that /RCON is simply switched direct to ground using a dip-switch.  There is no external circuitry that would perform an /RCON negation (high) after /RSTOUT is negated.

So I'm confused... does /RCON need to be "negated within one cycle after /RSTOUT is negated" or not??  I've looked through all the latest errata and there is no mention of this.  It bugs me when the actual hardware design does not match the specs.  

Thanks in advance...

Dogbert256

[Dogbert256]

Hello, is there anybody out there??

[TomE]

> Hello, is there anybody out there??

Hello, are there any manners in there?

Typical Dogbert, demanding an answer within ONE DAY on the WEEKEND! :smileyhappy:

This is a USER forum, not a commercial 24*7 manned-on-the-phone-waiting operation.

The timing specifications are for the combination of RCON and the data pins that are being overridden. They have to be valid for specific setup and hold times relative to RSTOUT (looking at my MCF5329 Data Sheet).

Shorting RCON to ground FULLY meets those timing requirements. The chip is looking for a LEVEL on this pin, and not an EDGE.

There may be chips out there that multiplex other signals on RCON, but not on the MCF5329 and possibly not on your chip either, so in this case the link is fine.

What you should have noticed more than that are the instructions in the Reference Manuals of the form:

NOTE
It is recommended that the logic levels for reset configuration on D[9:86:1]
be actively driven when RCON is used. The rest of the data bus should be
allowed to float or be pulled high.

The Reference Boards use resistive pullups and pulldowns rather than using a bus driver like the above implies.

These questions have been asked before:

https://community.freescale.com/message/15792#15792

Tom

[Dogbert256]

Thanks for the answer:  /RCON if used can be connected directly to ground for chip configuration at /RESET.

However, it does say in the MCF523X Reference Guide, exact quote:  "/RCON must also be negated within one cycle after /RSTOUT is negated."  This statement is completely false, and is not corrected in any errata sheets that I have seen.

Dogbert can see people designing complex circuitry to achieve this goal, all for no reason.  Freescale really should document their products better than this. 

[TomE]

> However, it does say in the MCF523X Reference Guide, exact quote:

> "/RCON must also be negated within one cycle after /RSTOUT is negated."

> This statement is completely false, and is not corrected in any errata sheets that I have seen

If you read more than one Reference Manual you'll begin to see patterns that show how these manuals are written.

Chips are made from "modules" that are mix-and-matched to make a chip with a particular set of features. The modules come with their own reference manual chapters. These are put together to make the reference manual.

The chapter documents what the Module CAN do or MAY do. Sometimes it documents module features that aren't enabled in your particular chip. These should all be fixed on every edit, but some can slip though.

(Try reading ARM or PPC manuals sometimes and see how complicated this gets when the "core manuals" are somewhere else, and with optional features documented somewhere else again.)

Also, any problems in those chapters carry on for a very long time. Search for "freescale bear pit" to find an example of this:

https://community.freescale.com/message/71442#71442

which details how a PIT module from 1998 was modified to put into the next chip, and two "traps for the unwary" were added. These (and their non-or-mis-documentation) in the chapters have been copied word-for-word for 13 years. One App Note for one chip only warns about one of these traps.

The MCF5235 Reference Manual Errata has 9 items related to RCON, indicating the chapter was copied from another chip, and then not changed to properly match the MCF5235 chip until a bit later. You've found another "leftover" from a previous chip that didn't get fixed.

The phrase "RCON must also be negated within one cycle after RSTOUT is negated." shows up in the manuals for the 5235, 5271, 5275 and 52235 chips and others.

The MCF5235 manual also states "During reset, the external RCON pin assumes its RCON pin function, but this pin changes to the function defined by the chip operation mode immediately after reset. See Table 9-8."

That table doesn't list an alternate pin function for RCON for any of those chips. The phrase also shows up in the MMC2107 manual, but its RCON pin is single-function as well. Somewhere in the menagerie there may be a chip with a multifunction RCON pin, but I can't find it - but the leftover DNA, support and documentation of that vestigal feature lives on in the "Digital DNA".

Found it. The MMC2114 (Advance Information, 2002) has the pin "SHS/RCON/PE7". That's the common ancestor.

> Freescale really should document their products better than this.

Would you pay double for the chips to get more polished manuals? Maybe we all should.

You can always choose to buy chips from a different manufacturer if you like their manuals better.

Tom