读取GPIO的引脚电平？

Question

I want to read the level of a GPIO pin. The following is found in the documentation for the chip:

GPIO Pin Level Registers (GPLEVn)

SYNOPSIS    The pin level registers return the actual value of the pin.
            The LEV{n} field gives the value of the respective GPIO pin.

Bit(s)    Field Name        Description              Type    Reset
21-0      LEVn (n=32..53)   0 = GPIO pin n is low    R/W     0
                            1 = GPIO pin n is high

This is the code I have written:

ldr r0,=0x20200038    @ Load address of GPLEV into r0
mov r1,#1             @ Load value 1 into r1
lsl r1,#18            @ Left shift value in r1 eighteen places. This corresponds to GPIO50
str r1,[r0]           @ Store the content of r1 at the address in r0

I would now expect to find the pin level in r0 , but it does not seem to work. Are the above code a correct method for reading the pin level of a GPIO?

Answer 1

In your code, r0 holds the address of the GPLEV register but not the value of the GPLEV register. If you want to read the value from the GPLEV register into a different register then you need something like:

ldr r0,=0x20200038
ldr r2,[r0]

Your code puts the value 0x00040000 in r1 and then stores that value in GPLEV.