GPIO

Introduction

GPIO, short for General-Purpose Input/Output, is a type of pin that can be dynamically configured and controlled during software execution. All GPIOs are in input mode after power-up, and can be set to pull-up or pull-down through software. They can also be configured as interrupt pins, and their drive strength is programmable. The core of GPIO operation involves populating the methods and parameters of the GPIO bank and registering it with the kernel using gpiochip_add.

GPIO Pin Calculation

The LKD3568 has 5 groups of GPIO banks: GPIO0~GPIO4, each group is further divided with identifiers A0~A7, B0~B7, C0~C7, D0~D7. The following formulas are commonly used to calculate the pins:

GPIO pin calculation formula: pin = bank * 32 + number

GPIO group number calculation formula: number = group * 8 + X

Here is a demonstration of the calculation method for GPIO4_D5 pin: bank = 4; //GPIO4_D5 => 4, bank ∈ [0,4]

group = 3; //GPIO4_D5 => 3, group ∈ {(A=0), (B=1), (C=2), (D=3)}

X = 5; //GPIO4_D5 => 5, X ∈ [0,7]

number = group * 8 + X = 3 * 8 + 5 = 29

pin = bank*32 + number= 4 * 32 + 29 = 157;

GPIO	Identifier	Pin	Identifier	Pin	Identifier	Pin	Identifier	Pin
GPIO0	A0	0	B0	8	C0	16	D0	24
	A1	1	B1	9	C1	17	D1	25
	A2	2	B2	10	C2	18	D2	26
	A3	3	B3	11	C3	19	D3	27
	A4	4	B4	12	C4	20	D4	28
	A5	5	B5	13	C5	21	D5	29
	A6	6	B6	14	C6	22	D6	30
	A7	7	B7	15	C7	23	D7	31
GPIO1	A0	32	B0	40	C0	48	D0	56
	A1	33	B1	41	C1	49	D1	57
	A2	34	B2	42	C2	50	D2	58
	A3	35	B3	43	C3	51	D3	59
	A4	36	B4	44	C4	52	D4	60
	A5	37	B5	45	C5	53	D5	61
	A6	38	B6	46	C6	54	D6	62
	A7	39	B7	47	C7	55	D7	63
GPIO2	A0	64	B0	72	C0	80	D0	88
	A1	65	B1	73	C1	81	D1	89
	A2	66	B2	74	C2	82	D2	90
	A3	67	B3	75	C3	83	D3	91
	A4	68	B4	76	C4	84	D4	92
	A5	69	B5	77	C5	85	D5	93
	A6	70	B6	78	C6	86	D6	94
	A7	71	B7	79	C7	87	D7	95
GPIO3	A0	96	B0	104	C0	112	D0	120
	A1	97	B1	105	C1	113	D1	121
	A2	98	B2	106	C2	114	D2	122
	A3	99	B3	107	C3	115	D3	123
	A4	100	B4	108	C4	116	D4	124
	A5	101	B5	109	C5	117	D5	125
	A6	102	B6	110	C6	118	D6	126
	A7	103	B7	111	C7	119	D7	127
GPIO4	A0	128	B0	136	C0	144	D0	152
	A1	129	B1	137	C1	145	D1	153
	A2	130	B2	138	C2	146	D2	154
	A3	131	B3	139	C3	147	D3	155
	A4	132	B4	140	C4	148	D4	156
	A5	133	B5	141	C5	149	D5	157
	A6	134	B6	142	C6	150	D6	158
	A7	135	B7	143	C7	151	D7	159

The device tree property description corresponding to GPIO4_D5 is: <&gpio4 29 IRQ_TYPE_EDGE_RISING>. As defined by the macros in kernel/include/dt-bindings/pinctrl/rockchip.h, GPIO4_D5 can also be described as <&gpio4 RK_PD5 IRQ_TYPE_EDGE_RISING>.

#define RK_PA0		0
#define RK_PA1		1
#define RK_PA2		2
#define RK_PA3		3
#define RK_PA4		4
#define RK_PA5		5
#define RK_PA6		6
#define RK_PA7		7
#define RK_PB0		8
#define RK_PB1		9
#define RK_PB2		10
#define RK_PB3		11
......

GPIO4_D5 may be occupied by other functions; the following is just an example. When the pin is not multiplexed by other peripherals, we can export the pin for use through export.

:/ # ls /sys/class/gpio/
export     gpiochip128  gpiochip32   gpiochip64  unexport
gpiochip0  gpiochip255  gpiochip500  gpiochip96
:/ # echo 157 > /sys/class/gpio/export
:/ # ls /sys/class/gpio/
export   gpiochip0    gpiochip255  gpiochip500  gpiochip96
gpio157  gpiochip128  gpiochip32   gpiochip64   unexport
:/ # ls /sys/class/gpio/gpio157
active_low  device  direction  edge  power  subsystem  uevent  value
:/ # cat /sys/class/gpio/gpio157/direction
in
:/ # cat /sys/class/gpio/gpio157/value
0

FAQs

Q1: How do I switch the MUX value of a PIN to a general GPIO?

A1: When using GPIO request, the MUX value of that PIN will be forcibly switched to GPIO. Therefore, ensure that the PIN is not being used by other modules when using it for GPIO functionality.

Q2: Why do I get a value of 0x00000000 when I read with IO commands?

A2: If you read a GPIO register with IO commands and get an abnormal value, such as 0x00000000 or 0xffffffff, please confirm whether the CLK of that GPIO is turned off. The CLK of GPIO is controlled by CRU. You can check whether the CLK is on by reading the CRU_CLKGATE_CON* registers in the datasheet. If it is not on, you can use the io command to set the corresponding register to turn on the CLK. After turning on the CLK, you should be able to read the correct register value.

Q3: What should I check if the voltage measured at the PIN is incorrect?

A3: If the voltage measured at the PIN is incorrect, excluding external factors, confirm whether the IO voltage source of that PIN is correct and whether the IO-Domain configuration is correct.

Q4: What is the difference between gpio_set_value() and gpio_direction_output()?

A4: If you are not going to dynamically switch between input and output when using the GPIO, it is recommended to set the GPIO output direction at the start. Later, when pulling high or low, use the gpio_set_value() interface instead of gpio_direction_output(). This is because the gpio_direction_output() interface contains a mutex lock, which can cause errors in interrupt context calls. Moreover, compared to gpio_set_value(), gpio_direction_output() does more things, which is wasteful.