topic Re: MCF5485 Flexbus lines level at start up? in ColdFire/68K Microcontrollers and Processors

MCF5485 Flexbus lines level at start up?

zwilcox — Wed, 13 Nov 2019 19:38:34 GMT

Hey! I have a question as to what level the Flexbus lines come up as on the MCF5485.

The Table 2-1 of the RM shows FBCS[5:0] and BE come up as high.

However, table 17-1 shows they come up as Hi-Z

Re: MCF5485 Flexbus lines level at start up?

Hui_Ma — Fri, 15 Nov 2019 09:37:02 GMT

Hi,

The Table 2-1 of the RM shows FBCS[5:0] and BE come up as high is correct.

best regards，

Mike

Re: MCF5485 Flexbus lines level at start up?

zwilcox — Fri, 15 Nov 2019 17:48:06 GMT

Thanks :smileyhappy:

Re: MCF5485 Flexbus lines level at start up?

TomE — Tue, 19 Nov 2019 22:37:54 GMT

Since it is easy to do, I would test this. Jam the chip in Reset and see if the pins read high with a resistive load on them. The 1M/10M resistance of an oscilloscope probe would be enough for an initial measure. Then remove Reset and see if they change.

Otherwise, if you have signals where it is important that they don't float at reset, add pull resistors to the hardware design.

There might be a difference between "State During Reset" and "State After Reset". With some chips, even dedicated output pins are tri-state when the reset pin is asserted. If this is important to you it is worth checking on the bench.

If you read the RM Errata, Table 2.1 has been updated 9 times, although changing the reset state isn't listed as a change. That may explain why it is correct.

Tom