This library makes it possible to control servos with the Raspberry Pi without using much CPU power. It is heavily inspired by richardghirst's ServoBlaster.
- written purely in C
- easy to use C functions
- very compact (< 200 lines of code)
- very accurate:
- (nearly) no CPU usage (3 servos running in the picture below):
You will first of all need to install mipea, another library I wrote which grants the DMA (and physical memory) and PWM access.
Because there is no Makefile (yet) you will need to directly incorporate it into your project or build a library yourself.
- Include the header file:
#include "servo.h". - You need to initialize the library, else there will be a segmentation fault:
servo_init(); - Use the
servo_add(uint32_t pin);function to add as many servos as you want. - Use
servo_set(uint32_t pin, unsigned int pulsewidth);to set the pulsewidth (in µs) of the servo on pinpin. - Use
servo_update_cbs(void);to update the PWM pulse. - (optional) Remove a servo you do not need anymore:
servo_remove(uint32_t pin); - It is also very important to uninit the library again, because else the peripherals stay mapped and the PWM pulse will keep running:
servo_uninit(); - Compile:
gcc -o [your program] program.c servo.c -lmipea
First, in servo_init();, PWM is configured to trigger the DMA after 1µs
every time data is written to the FIFO.
Every time servo_update_cbs(void); is called the code block structure is set up
(for a visualization have a look at diagram.txt).
We differentiate between "time keeping code blocks" (TKCB) and
"gpio code blocks" (GCP). The function allocates space for 20000 TKCB plus two GCB
for every servo (two turn on and off). The code block structure is set up
so that for every µs there is a TKCB. At the position where a servo needs to
be turned on or off a GCP is inserted. A GCP does not wait 1µs.
- richardghirst's ServoBlaster where I took most of my inspiration
- BCM2835 ARM Peripherals Datasheet
- The rpi-gpio-dma-demo helped me understand how DMA works