Imprecise result after bitwise operation on GPIO registers

Question

I am using a Raspberry Pi(Model 1 B+) and an oscilloscope in order to program SPI with C. I am doing this by using direct access to the GPIO register.

Like this: https://elinux.org/RPi_GPIO_Code_Samples#Direct_register_access.

The datasheet of BCM2835: https://www.raspberrypi.org/app/uploads/2012/02/BCM2835-ARM-Peripherals.pdf

The problem is that if I do access the register using a bitwise operation as following :
GPSET0 |= 1 << 8;
it also modifies the GPIO pin 11. I think I heard some of GPIO pins are hard-wired together. Is it because of that? But if I don't use bitwise operation, i.e:
GPSET0 = 1 << 8
It only modifies the GPIO pin 8.

I need advice. Thank you.

P.P.S This is how I initialised GPSET0 and gpiomem is the pointer stored after mapping the memory.

#define GPFSEL0 gpiomem[0]
#define GPFSEL1 gpiomem[1]
#define GPFSEL2 gpiomem[2]
#define GPFSEL3 gpiomem[3]
#define GPSET0 gpiomem[7]
#define GPSET1 gpiomem[8]
#define GPCLR0 gpiomem[10]
#define GPCLR1 gpiomem[11]
#define GPLEV0 gpiomem[13]
#define GPLEV1 gpiomem[14]
#define SEL_FNC(gpfsel, pin, fnc) gpfsel = (gpfsel & (~((0x7)<<(pin*3)))) | (fnc << (pin * 3))
#define SEL_OUTPUT 0x1
#define SEL_INPUT 0x0
extern volatile uint32_t *gpiomem;

P.S.

While the SPI library offered is precise, it seems rather slow because of the delay between the chip select and the data transmission. a few tens of micro-seconds. (To be precise 33.6us but this doesn't work for me because I need to refresh a 128x160 LCD module using ST7735. Taking account of this alone gives me a calculation of 600ms to refresh a screen, approximately) There is a bit longer delay until next transmission.

Maybe I am doing this wrong. If someone can give me an insight into this, that'd be great.

Answer 1

You have misunderstood the usage of the set and clear registers.

If bit x is 1 in the set register then GPIO x is set high. If bit x is 0 then the level of GPIO x is not affected (i.e. if high it stays high, if low it stays low).

If bit x is 1 in the clear register then GPIO x is set low. If bit x is 0 then the level of GPIO x is not affected (i.e. if high it stays high, if low it stays low).

So there is no need to read the set/clear register and or in the bits. Just set the bits of the GPIO you want to affect.

Or'ing in the bits wouldn't necessarily be harmful until you consider that the set/clear registers are WRITE only. So when you read the register you will be reading gibberish. So the end result is you are randomly setting and clearing bits by the very act of or'ing.