Posted on July 11, 2012 at 09:25Hi, I trried to understand how to calculate pwm output from example that comes with Stm32f4-Discovery-FW1.1.0. The file is in Project > Peripheral_Examples > TIM_PWM_Output > main.c
This is the most confusing part in comment
/* -----------------------------------------------------------------------
TIM3 Configuration: generate 4 PWM signals with 4 different duty cycles.
In this example TIM3 input clock (TIM3CLK) is set to 2 * APB1 clock (PCLK1),
since APB1 prescaler is different from 1.
TIM3CLK = 2 * PCLK1
PCLK1 = HCLK / 4
=> TIM3CLK = HCLK / 2 = SystemCoreClock /2
To get TIM3 counter clock at 28 MHz, the prescaler is computed as follows:
Prescaler = (TIM3CLK / TIM3 counter clock) - 1
Prescaler = ((SystemCoreClock /2) /28 MHz) - 1
To get TIM3 output clock at 30 KHz, the period (ARR)) is computed as follows:
ARR = (TIM3 counter clock / TIM3 output clock) - 1
= 665
TIM3 Channel1 duty cycle = (TIM3_CCR1/ TIM3_ARR)* 100 = 50%
TIM3 Channel2 duty cycle = (TIM3_CCR2/ TIM3_ARR)* 100 = 37.5%
TIM3 Channel3 duty cycle = (TIM3_CCR3/ TIM3_ARR)* 100 = 25%
TIM3 Channel4 duty cycle = (TIM3_CCR4/ TIM3_ARR)* 100 = 12.5%
Note:
SystemCoreClock variable holds HCLK frequency and is defined in system_stm32f4xx.c file.
Each time the core clock (HCLK) changes, user had to call SystemCoreClockUpdate()
function to update SystemCoreClock variable value. Otherwise, any configuration
based on this variable will be incorrect.
----------------------------------------------------------------------- */
My problem are;
1) System Clock is 168 MHz and If I need to get 28Mhz, my prescaler should be 6 - 1. Isn't it? From this comment, prescaler is set to 3 - 1.
2) To calculate ARR value, we need to use this equation;
ARR = (TIM3 counter clock / TIM3 output clock) - 1
Where ''TIM3 counter clock'' is 28 MHz
and ''TIM3 output clock'' is 30 Khz
so we get ARR = (28MHz / 30 KHz) -1
= 933 - 1
= 932
why they get ARR = 655... ??????????? where is this magic number come from?
Thanks for your help
#stm32f4-pwm-output-example
