# SAKS hat for raspberry_pi
SAKS = Swiss Army Knife Shield for Raspberry Pi
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## How other library control GPIO
wiringpi system but uses the /sys/class/gpio interface **rather than accessing the hardware directly**(softPwmWrite)
[sprintf(fName, "/sys/class/gpio/gpio%d/value", i)](https://github.com/WiringPi/WiringPi/blob/093e0a17a40e064260c1f3233b1ccdf7e4c66690/gpio/gpio.c#L428)
1. /sys/class/gpio: sysfs_gpio(deprecated), gpio(deprecated), maybe not working in pi4B
2. /dev/gpiomem or /dev/gpiochip0: rppal, wiringpi, gpio-cdev
3. /dev/pigpio and pigpio daemon process: pigpio
## Some questions need to solve
- [ ] Why gpio access from /dev/gpiomem via mmap syscall is faster than `format!("/sys/class/gpio/gpio{}/active_low", gpio_num)`
- [ ] Why gpio pin number has three encoding?(BOARD, BCM, wiringpi)
- [ ] About the onboard_led(/sys/class/leds/led0/) like arduino's LED_BUILTIN
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## My raspberry_pi notes
### CPU temperature
> vcgencmd measure_temp
### Setup raspberry_pi
How to connect raspberry_pi without internet/display/GUI Desktop?
1. put empty ssh file to SD card root directory in order to enable SSH on first time boot
2. SSH to pi via ethernet cable or serial(RS-232 with RXD+TXD+GND, don't connect 5V pin!) port