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spring: set boost PWM frequency to 32Khz

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Push the PWM frequency outside the audio band to avoid audible whining.

Also fix the off-by-one in the PWM frequency computation.

Signed-off-by: Vincent Palatin <vpalatin@chromium.org>

BRANCH=spring
BUG=chrome-os-partner:17583
TEST=manual : probe the PWM output with a scope.

Change-Id: Ie1c8c5aed72bef46c8bac16c1bf3007e8e9573bb
Reviewed-on: https://gerrit.chromium.org/gerrit/45584
Reviewed-by: Vic Yang <victoryang@chromium.org>
Commit-Queue: Vincent Palatin <vpalatin@chromium.org>
Tested-by: Vincent Palatin <vpalatin@chromium.org>
This commit is contained in:
Vincent Palatin
2013-03-15 09:53:07 -07:00
committed by ChromeBot
parent 05f496759f
commit f2df09d602
1 changed files with 5 additions and 5 deletions
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10
board/spring/usb_charging.c
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@@ -21,7 +21,7 @@
#include "tsu6721.h"
#include "util.h"
#define PWM_FREQUENCY 10000 /* Hz */
#define PWM_FREQUENCY 32000 /* Hz */
/* Console output macros */
#define CPUTS(outstr) cputs(CC_USBCHARGE, outstr)
@@ -116,14 +116,14 @@ static void board_ilim_use_pwm(void)
STM32_TIM_CR1(3) = 0x0000;
/*
* CPU_CLOCK / PSC determines how fast the counter operates.
* CPU_CLOCK / (PSC + 1) determines how fast the counter operates.
* ARR determines the wave period, CCRn determines duty cycle.
* Thus, frequency = CPU_CLOCK / PSC / ARR.
* Thus, frequency = CPU_CLOCK / (PSC + 1) / ARR.
*
* Assuming 16MHz clock and ARR=100, PSC needed to achieve PWM_FREQUENCY
* is: PSC = CPU_CLOCK / PWM_FREQUENCY / ARR
* is: PSC = CPU_CLOCK / PWM_FREQUENCY / ARR - 1
*/
STM32_TIM_PSC(3) = CPU_CLOCK / PWM_FREQUENCY / 100; /* pre-scaler */
STM32_TIM_PSC(3) = CPU_CLOCK / PWM_FREQUENCY / 100 - 1; /* pre-scaler */
STM32_TIM_ARR(3) = 100; /* auto-reload value */
STM32_TIM_CCR1(3) = 100; /* duty cycle */