K20 FTM , deadtimes in Combine Mode and ProcessorExpert

[timward]

The documentation for the FlexTimer says deadtimes can be used in combined mode.

ProcessorExpert flags an error if the combined channel isn't set to complementary when deadtime is enabled.

In fact, I can't find a spot in ProcessorExpert to set the length of the deadtimes.

Do I just need to hand code the initialization?

[adriansc · Accepted solution]

Hi,

When you try to configure the combine mode and deadtime Processor Expert shows the next message:

"The Complementary mode must be enabled for deadtime insertion"

This is because Processor Expert is based in the documentation that Freescale offers. As the Reference Manual for Kinetis products says:

Hence it is necessary to set Combine and Complementary modes to use deadtime. Different configurations are out of specification.

Hope this helps,

Regards.

[timward]

I parsed that as the deadtime feature working in Combined mode, and the deadtime feature working in Complementary mode.

If indeed it won't work in Combined mode, it would be a lot more clear if the manual simply said:

"The deadtime feature should only be used in Complementary mode."

I say this because Complementary mode is a subset of Combined mode.  You can be in Combined mode without having the Complementary bit set, but you can't be in Complementary mode without having the Combined bit set.

Why even bring up Combined?  It just confuses the issue.

On the other hand, having the deadtime feature working in Combined mode makes a lot of sense.  For a half-bridge with P and N type FETs,  you need to raise one type's gate to turn it off, and you need to raise the other type's gate to turn it on.

The documentation says both channels output the same signal in Combined mode if the COMP bit is not set.

So Combined will work for a complementary pair, with both pins driving the same direction, but you'll get shoot through.  So even though both pins need to go in the same direction, you'd like a delay between switching off one type and switching on the other. That is to say, deadtime.

It just seemed really unlikely to me that the engineers designing the module wouldn't be aware of this very common use case, and accomodated it.

But if they didn't, they didn't.