Confused about internal pull-ups/pull-downs on 5223x family

lambtron — Fri, 11 Jan 2008 02:30:46 GMT

I've read the MCF52235 ColdFire Integrated Microcontroller Reference Manual from cover to cover and I'm still a bit foggy about the meaning of the "pull-up/pull-down" column in Table 2-1.

The column heading cites Note 2, which says "all signals have a pull-up in gpio mode." I assume this to mean that the device unconditionally activates an internal pull-up for any multi-function pin that is operating as a gpio, but the internal pull-up is not activated for other modes unless there is an entry in the column that indicates otherwise. Is this correct?

All pins that have a gpio function are configured to operate in gpio mode upon reset, and thus all of these pins are pulled up by the internal pull-ups by default. Are the pull-ups active when reset is asserted, or only upon release of reset?

Some pins (e.g., IRQ2) indicate "pull-up" in the column and cite Note 6, which says "for primary and gpio functions only." I assume this to mean that the device activates an internal pull-up, but only when the pin is operating as either gpio or primary function, and in all other cases the pin has no internal pull-up. Correct?

The RSTI pin indicates "pull-up" and cites Note 9, which says "RSTI has an internal pull-up resistor, ..." Based on this, I assume that the TEST and JTAG_EN pins, which indicate "pull-down," have internal pull-downs, and thus it is not necessary to tie them to ground or supply external pull-downs. Correct?

Similarly, RCON indicates "pull-up," which is assumed to mean that it has an internal pull-up, and thus this pin may be left disconnected to permanently disable the ezport. Correct?

Re: Confused about internal pull-ups/pull-downs on 5223x family

DrSeuss — Sat, 12 Jan 2008 04:40:22 GMT

You are correct in all statements. There are a couple of things to think about though:
1) These are very weak pull devices - basically very small current sources equivalent to about 50K ohm. This is important for signals like RSTIN, which typ you want a nice edge on.
2) Redundancy, while external resistors are not required, it is suggest that at least for a critical signals - RSTIN, RCON, etc they be use for redundancy, if external fails you have the internal. etc.

topic Re: Confused about internal pull-ups/pull-downs on 5223x family in ColdFire/68K Microcontrollers and Processors

Confused about internal pull-ups/pull-downs on 5223x family

Re: Confused about internal pull-ups/pull-downs on 5223x family