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--- en:multiasm:paarm:chapter_5_9 [2024/09/27 20:27] – created pczekalski
+++ en:multiasm:paarm:chapter_5_9 [2025/12/04 14:59] (current) – [Pulse Width Modulation] eriks.klavins
@@ Line 1: / Line 1: @@
-====== Peripheral Management RPi ======
+====== Peripheral Management in RPi ======
+The Raspberry Pi has ported out some General-Purpose Input/Output (GPIO) lines. Several lines share functionality across the board's peripherals. To access peripherals like GPIO, they must be enabled in the Raspberry Pi 5 OS. Many things can be done with the Raspberry Pi's available GPIOs. In the picture below, the GPIOs and their alternative functions (in scope) are shown.
+{{:en:multiasm:paarm:2025-12-04_15_54_09-.jpg|}}
+For example, to enable UART communication on GPIO 14 and 15, the “./boot/config.txt” file must be edited by adding these two lines:
+  * ''dtoverlay=uart0-pi5''
+  * ''dtparam=uart0=on''
+There are libraries designed to work with GPIO and/or communication interfaces such as UART, SPI, and I2C. For example, to work with GPIO, a special library in C or Python can be used, or GPIO can be accessed via memory mapping of IO registers. Note that GPIO parameters must be set before using them as outputs or inputs, like setting pull-up or pull-down resistors, setting direction, etc. On Linux, using libraries such as libgpiod or wiringPi/pigpio can save a lot of time. But these libraries are primarily available in C or even higher-level programming languages. The hardest part is to find proper hardware addresses. The Raspberry Pi5 installer RP1 peripheral controller documentation contains two addresses: System-Address, which is 40-bit long, and Proc-Address, which is 32-bit long. Taking the example of peripherals on the APB0 internal bus, the address in Proc-Address is ''0x40000000'', but in System-Address it is ''0x<fc #008000>C0</fc>40000000''. The difference is coloured out. Similar differences appear across all other peripheral buses, and these differences may occur in the code depending on the OS version, installed libraries, and drivers.
+===== General Purpose Inputs and Outputs (GPIO) =====
+Usable GPIO pins on the Raspberry Pi 5 are in IO_BANK0, and this provides information on the addresses available for GPIO programming. Other banks, IO_BANK1 and IO_BANK2, are reserved for internal use inside the board. The base address for IO_BANK0 is 0x400D0000, and the remaining addresses for GPIO programming are documented in the section “3.1.4.Registers” at the [[https://datasheets.raspberrypi.com/rp1/rp1-peripherals.pdf|RP1 datasheet]].
+<note>To be sure that the code will work as desired, the program code must be adjusted accordingly, and the addresses must be verified.</note>
+In the previous section, the basic GPIO pin toggling was implemented. GPIOs can be used for many purposes, such as replacing hardware components that are not available on the current board. The technique is called bit-banging. A Raspberry Pi has only one I2C interface, and let's assume that two or more sensors must be connected to the board and share the same I2C device address. This means that only one sensor per I2C channel can be connected, and as the Raspberry Pi has only one interface, only one sensor can be used. With the bit-bang technique, it is possible to create many communication interfaces if at least two GPIO pins are available. Note that the bit-banging technique uses processor power, so there are also limits for the number of interfaces. That’s because I2C must maintain a 100kHz clock rate for successful communication, and bit-banging techniques require significant processor power. Under heavy CPU load, the emulated I2C interfaces may run slower than necessary.
+The following code example will be executed in the User space and may require root permissions to access the “''/dev/gpiomem''” device.
+<codeblock code_label>
+<caption>I2C with bit-bang technique</caption>
+<code>
+.global _start
+.section .text
+@ Constants
+.equ GPIO_BASE,  0x400D0000
+.equ GPFSEL0,    0x00
+.equ GPSET0,     0x1C
+.equ GPCLR0,     0x28
+.equ GPLEV0,     0x34
+.equ SDA_BIT,    (1 << 2)
+.equ SCL_BIT,    (1 << 3)
+_start:
+    @ Open /dev/gpiomem (fd=3)
+    MOV X8, #56              @ syscall openat
+    MOV X0, #-100            @ AT_FDCWD
+    LDR X1, =path
+    MOV X2, #2               @ O_RDWR
+    MOV X3, #0
+    SVC #0                   @ open -> fd in X0
+    MOV X19, X0              @ save fd
+    @ mmap GPIO
+    MOV X8, #222             @ syscall mmap
+    MOV X0, #0               @ addr
+    MOV X1, #4096            @ length
+    MOV X2, #3               @ PROT_READ|PROT_WRITE
+    MOV X3, #1               @ MAP_SHARED
+    MOV X4, X19              @ fd
+    MOV X5, #0               @ offset
+    SVC #0
+    MOV X20, X0              @ base addr
+    @ configure pins 2,3 as outputs
+    LDR W1, [X20, #GPFSEL0]
+    BIC W1, W1, #(0b111 << 6)    @ clear bits for GPIO2
+    ORR W1, W1, #(0b001 << 6)    @ set output
+    BIC W1, W1, #(0b111 << 9)    @ clear bits for GPIO3
+    ORR W1, W1, #(0b001 << 9)
+    STR W1, [X20, #GPFSEL0]
+    @ Send start condition: SDA low while SCL high
+    BL scl_high
+    BL delay
+    BL sda_low
+    BL delay
+    @ Send byte 0xA5 (1010 0101)
+    MOV W0, #0xA5
+    MOV W1, #8
+send_bit:
+    BL scl_low
+    TST W0, #0x80
+    BEQ send_zero
+    BL sda_high
+    B after_bit
+send_zero:
+    BL sda_low
+after_bit:
+    BL delay
+    BL scl_high
+    BL delay
+    BL scl_low
+    LSL W0, W0, #1
+    SUBS W1, W1, #1
+    B.NE send_bit
+    @ Stop: SDA high while SCL high
+    BL sda_low
+    BL scl_high
+    BL delay
+    BL sda_high
+    @ Exit
+    MOV X8, #93
+    MOV X0, #0
+    SVC #0
+@ ---- Subroutines ----
+sda_high:
+    MOV W2, #SDA_BIT
+    STR W2, [X20, #GPSET0]
+    RET
+sda_low:
+    MOV W2, #SDA_BIT
+    STR W2, [X20, #GPCLR0]
+    RET
+scl_high:
+    MOV W2, #SCL_BIT
+    STR W2, [X20, #GPSET0]
+    RET
+scl_low:
+    MOV W2, #SCL_BIT
+    STR W2, [X20, #GPCLR0]
+    RET
+delay:
+    MOV W3, #100
+delay_loop:
+    SUBS W3, W3, #1
+    B.NE delay_loop
+    RET
+.section .rodata
+path: .asciz "/dev/gpiomem"
+</code>
+</codeblock>
+<note>Note that the code is a minimalistic prototype; it does not check for ACK or byte reads, which are necessary for an I2C communication interface. With an oscilloscope, it is possible to investigate signals visually and, if needed, adjust the delay loop. Remember that the timing depends on CPU speed. </note>
+Overall, working with any peripheral requires studying its documentation to identify its base addresses and the offset addresses needed to control it.
+===== Communication interfaces =====
+The hardware can be used to send and receive a byte through I2C. The hardware itself performs all the control over digital signals. Everything that is needed again, find the base addresses for the hardware. Unfortunately, on the Raspberry Pi official homepage, the developers state that all available documentation on peripherals is intended for use with operating systems. On Raspberry Pi 5, the GPIO and the communication interfaces are on a separate chip, the RP1-C0.
+In chapter 2 of the [[https://datasheets.raspberrypi.com/rp1/rp1-peripherals.pdf|RP1-CO documentation]], it states that the board has multiple communication interfaces, including 7 I2C interfaces. All the base addresses are listed, but unfortunately, there is no information on how to control the I2C, SPI or UART communication interfaces. Only a few interfaces are available for user use because the Raspberry Pi 5 board has additional hardware that also requires some of those interfaces, or digital lines are used for special purposes. Sometimes, all the digital lines used for communication interfaces are repurposed for other purposes, rendering the communication interfaces useless.
+First, it is necessary to determine which interfaces are available on the Raspberry Pi and, of course, to find the device addresses. Another option is to use Linux. To use any communication interface, it must be enabled in the OS. The easiest way is to use the standard Linux i.e. i2c-dev interface, and it can be used with the following function calls:
+  * open("/dev/i2c-1", O_RDWR), close(fd) and exit(0) to access the file descriptor
+  * write(fd, &tx_byte, 1) to write 1 byte (used to send the byte)
+  * read(fd, &rx_byte, 1) to read 1 byte (used to receive the byte)
+In the assembler code, the I2C device must be opened by calling the OS system function “''openat(AT_FDCWD, "/dev/i2c-1", O_RDWR, 0)''”. In the assembler, it will look like :
+<codeblock code_label>
+<caption>open access to I2C</caption>
+<code>
+path_i2c:
+    .asciz "/dev/i2c-1"
+    .section .text
+    .global _start
+_start:
+	MOV     X0, #-100		@ #AT_FDCWD
+	LDR     X1, =path_i2c
+	MOV     X2, #2			@ O_RDWR - read and write
+	MOV     X3, #0
+	MOV     X8, #56			@ #SYS_openat
+	SVC     #0
+</code>
+</codeblock>
+After the system call is executed, the ''<fc #008000>X0</fc>'' register will hold the file descriptor that can be used for future system calls. It should be stored and in the example register ''<fc #008000>X19</fc>'' is used:”'' <fc #800000>MOV</fc> <fc #008000>X19</fc>, <fc #008000>X0</fc>''”. Later on, to send or read one byte, system calls WRITE or READ are used, like in the code below:
+<codeblock code_label>
+<caption>I2C Rx/TX</caption>
+<code>
+I2Cbyte:
+    .byte 0xAB
+    MOV     X0, X19
+    LDR     X1, =I2Cbyte
+    MOV     X2, #1			@ nuber of bytes
+    MOV     X8, #64			@#64=SYS_write #63=SYS_read
+    SVC     #0
+</code>
+</codeblock>
+The code sends the data; the device address is not set in these examples. The system call with number ''<fc #ffa500>#64</fc>'' generates the START signal, sends the device address (with the write bit set), sends the ''.byte'' value, and finally sends the STOP signal. After the system call is executed, the ''<fc #008000>X0</fc>'' register holds the number of bytes transmitted. After this example code executes, the value must be equal to 1.
+To unlock the full potential of any communication interface on Raspberry Pi, it will take a lot of effort to find the register addresses, digital lines, and their parameters, and even then, something will be missing. For example, the same code that works on Raspberry Pi version 3 or 4 will not work on version 5. The hardware addresses differ, and it seems the Operating System is translating them into 32-bit addresses. The communication interface depends heavily on the Operating System kernel version, kernel modules, and configuration. It is recommended to use system calls for other communication interfaces as well, as experimenting with the hardware without complete documentation is not recommended. As a result, the code may take harmful actions, damaging the Raspberry Pi.
+===== Pulse Width Modulation =====
+Basically, with a single GPIO line, you can do a lot: control almost any hardware, read or take measurements, generate wireless signals, and more. Of course, it is easier to control hardware designed for specific purposes, such as I2C communication in the previously described examples. As with the bit-banging technique, it is possible to generate periodic digital signals and control their duty cycle.
+In Raspberry Pi 5, the RP1 chip documentation contains much more information on pulse-width modulation than on basic communication interfaces. PWM registers are on the internal peripheral bus; the base addresses for PWM0 and PWM1 are ''0x40098000'' and ''0x4009C000'', respectively. This hardware is located in the RP1 chip and accessed through PCIe. The Linux OS sets up hardware address mapping, and this mapping is not exposed as a simple fixed physical address that can be accessed with just ''<fc #800000>LDR</fc>''/''<fc #800000>STR</fc>'' instructions from user space. To access the PWM registers, it is necessary to execute the code at least at the EL1 level and to know already the PCIe mapping, or the mapping can be implemented manually. Again, this is too advanced and carries a risk of breaking something.
+Before proceeding, the base addresses must be checked at least three times (**NO JOKES**) and, if needed, replaced. The PWM0_BASE and IO_BANK0_BASE addresses are already mapped and known for the Raspberry Pi 5. In the example, the GPIO line 18 will be used.
+<codeblock code_label>
+<caption>Address mapping</caption>
+<code>
+.equ PWM0_BASE,      0xXXXXXXXX      @ filled in by platform code
+.equ IO_BANK0_BASE,  0xYYYYYYYY      @ filled in by platform code
+.equ PWM_CHAN2_CTRL, 0x34
+.equ PWM_CHAN2_RANGE,0x38
+.equ PWM_CHAN2_DUTY, 0x40
+.equ GPIO18_CTRL,    0x094
+.equ FUNC_PWM0_2,    0xZZ           @ FUNCSEL value for PWM0[2] on GPIO18
+</code>
+</codeblock>
+These are the constants used later on in the code. Note that three constants are holding dummy values – these values depend on the hardware. The rest of the code will control GPIO18 and generate a pulse at the specified frequency and duty cycle. The frequency and duty cycle parameters can be passed to the code. The ''<fc #008000>X0</fc>'' register will hold the period value, and the ''<fc #008000>X1</fc>'' register will hold the duty cycle value.
+<codeblock code_label>
+<caption>PWM initalisation</caption>
+<code>
+.global pwm_init_chan2
+pwm_init_chan2:
+    LDR     X2, =IO_BANK0_BASE
+    ADD     X2, X2, #GPIO18_CTRL
+    LDR     W3, [X2]                @ read current CTRL
+    BIC     W3, W3, #0x1f  		@ clear FUNCSEL bits [4:0]
+    ORR     W3, W3, #FUNC_PWM0_2	@ set FUNCSEL to FUNC_PWM0_2
+    STR     W3, [X2]
+</code>
+</codeblock>
+At this moment, the GPIO line is ready and internally connected to the PWM generator. The following code lines provide the PWM generator with all the necessary parameters: period and duty cycle.
+<codeblock code_label>
+<caption>set PWM duty and frequency</caption>
+<code>
+    LDR     X4, =PWM0_BASE
+    ADD     X5, X4, #PWM_CHAN2_RANGE    @ X5=PWM0_BASE+PWM_CHAN2_RANGE
+    STR     W0, [X5]                    @ low 32 bits used
+    ADD     X5, X4, #PWM_CHAN2_DUTY
+    STR     W1, [X5]
+</code>
+</codeblock>
+Note that the code uses only 32-bit values of the ''<fc #008000>X0</fc>'' and ''<fc #008000>X1</fc>'' registers, where the input arguments are stored. The last step is to activate the PWM generator for Channel 2, setting its parameters, such as the PWM generation mode to trailing edge.
+<codeblock code_label>
+<caption>Enable PWM</caption>
+<code>
+    ADD     X5, X4, #PWM_CHAN2_CTRL   @ x5=PWM0_BASE+PWM_CHAN2_CTRL
+    LDR     W6, [X5]
+    @ Check the datasheet -> clear existing MODE bits (example)
+    BIC     W6, W6, #(0x7)
+    @ set mode to trailing-edge
+    ORR     W6, W6, #0x1
+    @ set enable bit (placeholder) check datasheet
+    ORR     W6, W6, #(1 << 8)
+    STR     W6, [X5]
+    RET
+</code>
+</codeblock>
+This code can be used as a kernel module, but it cannot be executed directly from a regular user program on Pi OS. That’s because the mapping is involved, and with that, the kernel is also involved (because of the PCIe mapping).
+** The second approach **
+The second approach is similar to I2C communication: it uses system calls. The Raspberry Pi must be prepared with a device-tree overlay, for example, “''dtoverlay=pwm-2chan,pin=18,func=4''”. Note that to enable PWM, the string must be used, not just the value. Now it is time to create system calls. In the I2C example, three different system calls were made; the only difference between the code fragments is the system call value and arguments. Everything needed to activate PWM generation is to open the file, write some variables to it, and save it by closing it.
+<codeblock code_label>
+<caption>Write_file function</caption>
+<code>
+@ void write_file(const char *path, const char *str)
+write_file:
+    @ x0 = path, x1 = string
+    @ openat(AT_FDCWD, path, O_WRONLY, 0)
+    MOV     X2, #O_WRONLY
+    MOV     X3, #0
+    MOV     X8, #SYS_OPENAT
+    MOV     X4, #AT_FDCWD
+    MOV     X0, X4          @ AT_FDCWD
+    @ x1 already holds path
+    SVC     #0              @ X0 = fd
+    MOV     X19, X0         @ save fd
+    @ find string length
+    MOV     X0, X1          @ pointer to string
+:  LDRB    W2, [X0], #1
+    CBZ     W2, 2f          @ jump to label 2 (f means forward)
+    B       1b              @ jump to label 1 (b means backward)
+:
+    @ now X0 points past NUL; length = (X0 - 1) - str
+    SUB     X2, X0, X1      @ remove the NUL as it is not needed
+    SUB     X2, X2, #1
+    @ write(fd, str, len)
+    MOV     X0, X19
+    MOV     X8, #SYS_write
+    SVC     #0
+    @ close(fd)
+    MOV     X0, X19
+    MOV     X8, #SYS_close
+    SVC     #0
+    RET
+</code>
+</codeblock>
+Now, the code to activate the PWM generator sets the period and duty. It is necessary to know which file to edit and which values to write to the files. Required constants for this example are:
+<codeblock code_label>
+<caption>Constants</caption>
+<code>
+    .equ SYS_openat, 56
+    .equ SYS_write,  64
+    .equ SYS_close,  57
+    .equ SYS_exit,   93
+    .equ AT_FDCWD,  -100
+    .equ O_WRONLY,   1
+    .section .rodata
+path_export:
+    .asciz "/sys/class/pwm/pwmchip0/export"
+path_period:
+    .asciz "/sys/class/pwm/pwmchip0/pwm0/period"
+path_duty:
+    .asciz "/sys/class/pwm/pwmchip0/pwm0/duty_cycle"
+path_enable:
+    .asciz "/sys/class/pwm/pwmchip0/pwm0/enable"
+str_chan0:
+    .asciz "0\n"
+str_period:
+    .asciz "20000000\n" @ 20 ms period  = 20000000 ns  (for example, servo-style period)
+str_duty:
+    .asciz "5000000\n"  @ 5 ms high time = 5000000 ns  (25% duty)
+str_enable:
+    .asciz "1\n"          @ enable PWM
+</code>
+</codeblock>
+And the main code, which sets all values:
+<codeblock code_label>
+<caption>Main code</caption>
+<code>
+    LDR     X0, =path_export    @ export PWM channel 0
+    LDR     X1, =str_chan0
+    BL      write_file
+    LDR     X0, =path_period    @ set period
+    LDR     X1, =str_period
+    BL      write_file
+    LDR     X0, =path_duty      @ set duty_cycle
+    LDR     X1, =str_duty
+    BL      write_file
+    LDR     X0, =path_enable    @ enable PWM generation
+    LDR     X1, =str_enable
+    BL      write_file
+    MOV     X0, #0               @ exit
+    MOV     X8, #SYS_exit
+    SVC     #0
+</code>
+</codeblock>
+The code can be upgraded to accept the arguments: period and duty cycle. This would be similar to the write_file function, which takes two arguments.

en/multiasm/paarm/chapter_5_9.1727468877.txt.gz · Last modified: 2024/09/27 20:27 by pczekalski