I was setting up a Nordic BLE chip to the UART connection from SparkFun and went to the schematics to figure out how to wire the UART up:


enter image description here

Personally, I was confused if P0.27 is the pin connect to the UART RX or if it was actually "Pin 39" as per the schematic.

I don't layout lots of boards, but for laying out nets I don't leave any numbers like that. Seems confusing.

I had to monkey around and figure in a project build from Nordic which pins are the UART pins, and spent 15 minutes building and re-building the project till I found it indeed the pin is 27 I needed.

(This is the C code, I needed to guess through a defines list! Is it pin 38/39 or 26/27, UART not working, swap values, check connections and power, blah blah blah, etc.)

#define RX_PIN_NUMBER  26
#define TX_PIN_NUMBER  27

What is the point of the convention with numbers for each net? Does it go back to the physical pin order? Is one convention more common than another on this?

For a smaller footprint I can see leaving the numbers for like a SOT, but for a QFN like this is this really typical?

  • \$\begingroup\$ I think P0.27 means port 0, channel 27 or something like that. So the pin function which stays the same across packages, but not pins. Haven't looked at datasheet. \$\endgroup\$
    – DKNguyen
    Mar 7 '20 at 18:47
  • \$\begingroup\$ So what purpose does the 39 serve? \$\endgroup\$
    – Leroy105
    Mar 7 '20 at 18:47
  • \$\begingroup\$ 39 is the physical pin number on the package - all chips in the same package have pin number 39 at that standardized position. What piece of silicon is inside the package determines what is the pin name/functionality of pin 39. In case of that chip, the pin 39 has name "P0.27" just like pin 30 has name "ANT". Some packages are not even limited to numbers, for example a BGA package first pin is called A1. \$\endgroup\$
    – Justme
    Mar 7 '20 at 19:37

The point is the name of pin function, not the pin number. For all packages, the physical pin number might be different. called P0.27 which (most likely) means bit 27 of GPIO port 0. Another debate is whether that is a good name to name things anyway, but they are. For other MCUs, the name could be PA13 for example, for GPIO port A, bit 13.


"P0.27" and "P27" are the net names. They are probably abbreviated from "Port 0, bit 27". They are not really numbers.

In general when a bus of similar signals appears in a schematic there will be bit number associated with it, this is very common. It is just unfortunate in this example that Port and Pin both begin with 'P' and the numbers are in the same range.


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