Slew rate explanation

[romainbenet]

Hi,

I use a MPC5604B and I don't understand the utility of the slew rate.

What is the difference between the medium and fast utilization and what are these pins which can be configured for medium and for fast?

What difference can I see on its output?

In your MPC5604BC rev 13 datasheet table 23 you precise the I/O weight but what is its definition? What do it mean values in per cent?

Best regards,

Romain

[lukaszadrapa]

Hi,

GPIOs on MPC5604B are medium (M), slow (S), input/output with analog feature (J) and input only pins (I)  . See the table "Functional port pin descriptions" in reference manual. We can set the slew rate for M, S and J pins in PCRn register. From datasheet:

Then read the chapter "2.15 I/O pad electrical characteristics" in the datasheet. Here you can find different specification.

Regarding your second question - if you are going to toggle more pins from the same supply segment at the same time, then sum up all IO weights (from Table 23 in datasheet) for that pins. The sum cannot exceed 100%.
Example: let’s say we have 144pin package and we are going to toggle first 7 pins from Table 23 (PB[3], PC[9], PC[14]…) which belongs to supply segment number 4. If we use 5V and SRC will be set to 0 then the sum of weights is 65% (10+10+9+9+9+9+9). So, this is OK because we will not exceed 100%, so we will not exceed dynamic current. 

In other words - the switching of outputs creates dynamic current consumption (it's just short peak). If too many output is switched at the same time, it can draw a lot of current from supply pins. So, we have to ensure that the dynamic current does not exceed this specification.

Regards,

Lukas