ARM's explanation of PINSEL

Concept: The pin connection module controls the multiplexer through the configuration register to connect the pins to the on-chip peripherals; (P0.1/RXD0/PWM3/EINT0)

Purpose: The purpose of the pin connection module is to configure the pins to the required functions, which can realize independent pin configuration;

Application: The reset values ​​of pin selection registers PINSEL0 and PINSEL1 are both 0x00000000; for specific settings, refer to the configuration table.

1. PINSEL0 has a total of 32 bits, in groups of 2 (implying a value range of 00 - 11, for example, the four functions P0.1/RXD0/PWM3/EINT0 are selected by 00/01/10/11), from P0.0 to P0.15;
2. PINSEL0 has a total of 32 bits, in groups of 2 (implying a value range of 00 - 11), from P0.0 to P0.15;
3. PINSEL1 has 32 bits in total, 2 bits in a group, from P0.16 to P0.31;
4. PINSEL2 is listed separately because it is special because its use requires access using the "read-modify-write" method.

Note: Accidental writes to bits 0 to 2 and/or 3 will result in loss of debug and/or trace functionality.

 Bit          Reset Value  Description                                                                                 

1:0        00          Reserved. Users are not allowed to write 1 to it.                                                   

  2                     When this bit is 0, P1.36:26 is used as GPIO;     P1.26 (with an overline, indicating a low level)

                       When this bit is 1, P1.36:26 is used as a debug port;  TRACE (with an overline, indicating a low level)

 3                      When this bit is 0, P1.25: 16 is used as GPIO;       P1.20 (with an overline, indicating low level)

                                         When this bit is 1, P1.25:16 is used as a tracking port; TRACESYNC (with an overline, indicating a low level)

31:4                   Reserved                      

   Generally, when setting the function of a pin, in order not to affect other pins, the “read-modify-write back” method is used, that is, first read the register value, then perform a logical “and” or “or” operation, and then write it back to this register.

If P0.8 and P0.9 are set to TxD1 and RxD1 functions (P0.8 and P0.9 both correspond to 2 bits, so they should be 17:16 and 19:18)

PINSEL0 = ( PINSEL0 & 0xFFF0FFFF ) | ( 0x05 << 16 ); //Set 2 pins at one time, corresponding values

                   Here 0 corresponds to bits 16-19        // need to refer to the register function table

Typically, when PINSEL2 is selected to connect the P1[25:16] pins to GPIO, the settings are as follows:

PINSEL2 = PINSEL2 & ( ~ 0x08 );     //Set the 3rd bit of PINSEL2 selection register 2 to 0