Question

Why is I2C1_SDA available on pins PC4 and PB11 of the 64-pin TQFP for the Giant, Leopard, and Wonder Gecko devices while I2C1_SCL is only available on PC5? The I2C1_ROUTE register sets the location for both I2C1 signals, so having only SDA available on PB11 serves no purpose.

Answer

Giant, Leopard, and Wonder Gecko are pin-compatible devices from the standpoint of power and ground pins as well as the RESETn input and the general location of GPIO pins. Each of these EFM32 family members is also available in a range of packages from a 64-pin TQFP to a 120-ball BGA. To support a range of application needs, the Giant, Leopard, or Wonder Gecko die is bonded in a variety of packages with varying sizes and pin counts.

 

Consider, for example, the Leopard Gecko (EFM32LG232) 64-pin QFP pinout:

 

64pinTQFP.png

 

Notice on the bottom side of the package that only port B pins 11, 13, and 14 are bonded. The absence of PB12 is suspicious, so now examine the 64-pin QFN (EFM32LF230):

 

64pinQFN.png

 

Lo and behold, this device in a 64-pin package has PB12, and a quick check of 4.2 Alternate Functionality Pinout in the datasheet shows the missing I2C1_SCL:

 

I2C1pins.png

 

A comparison of both pinout diagrams reveals that the 64-pin TQFP has VSS pins on the top and bottom of the package that are replaced by PA15 and PB12 on the 64-pin QFN. Why is this the case even though both packages ostensibly have 64 pins?

 

The answer is simply that the 64-pin QFN is not a 64-pin package but a 65-pin package. That extra pin is the exposed pad on the underside of the package and labeled as pin 0 in the diagram above.

 

Per industry standardization, the exposed pad on a QFN package is always used as a device ground. Furthermore, its large size allows it to carry all of the ground return current that must otherwise travel through bond wires connected to individual pins on a leaded package like a TQFP. This is especially the case for ICs in DFN (dual flat no leads) and QFN (quad flat no leads) packages like audio power amplifiers.

 

Ultimately, this is just a documentation error as the PB11 option for I2C1_SDA should not be listed in the alternate functionality table. Forthcoming revisions to the Giant, Leopard, and Wonder Gecko datasheets will fix this.

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