The M or Minimum Speed grade device appears to be faster than the fastest speed grade in the I/O timing report and the term "Minimum Speed grade" can be a bit confusing.

For starters, it does NOT mean the slowest silicon device. The term refers to the propagation delay times (Tpd) within the device, so the "Minimum Speed grade" is actually the fastest silicon device; or in other words has the minimum Tpd. This is why you are seeing these results in the I/O timing report.

In reality, we guarantee that a device will meet a minimum frequency. So for a MachXO 256C-3T100 device, we guarantee that it can meet the minimum frequency as specified in the data sheet for the -3 speed grade. It may also meet the specifications for the -4 or -5 speed grade in some circumstances, but is not guaranteed to do so. In order to sell -5 speed grade devices, we must actually exceed the -5 minimum frequency specification for the majority of the devices to ensure that there is a comfortable margin above the specification. This means that most devices will actually run faster than their speed rating in practice.

The M speed grade device is listed in the software to allow the customer to look at the data hold time analysis from a perspective of how fast the device may operate in a worst case situation. If for example they received a -3 speed grade device that actually exceeds the -5 speed rating, then there could be hold time violations that would not be noticed if the timing analysis only looked at the -3 speed ratings.