[LPC1347] PWMout freq on 16 bit timers wrong

[JojoS62]

Description

• Type: Bug
• Priority: Major

Bug

PWMout frequency is higher than set in pwm.period(). This happens on pwm pins with 16 bit counters CT16B0 / CT16B1, on CT32B0 it works as expected.

Target
LPC1347

Toolchain:
ARM, online compiler

mbed lib 64...125

Expected behavior
pwm.period(0.01) -> 10 ms

Actual behavior
pwm.period(0.01) -> 0.898 ms

Steps to reproduce

#include <mbed.h>

PwmOut pwm_ok(P0_18);
PwmOut pwm_not_ok(P0_9);

int main()
{
    pwm_ok.period(0.01f);       // 10 ms / 100 Hz
    pwm_ok = 0.25f;

    pwm_not_ok.period(0.01f);   // 10 ms / 100 Hz
    pwm_not_ok = 0.25f;

    while(1);
}

[JojoS62]

The problem is: clock of 72 MHz / 2^16 gives a max pwm period time of about 0.9 ms.
I've used the solution from LPC11XX that sets a prescaler, initial to 31 to get the default setting of 20 ms pwm period time working. For times >28 ms it is necessary to set a higher prescaler value.

[0xc0170]

cc @toyowata

[toyowata]

I got the LPCXpresso1347 board and just started to see detail. I hope same fix as the LPC11XX can be used. I will investigate more.

[JojoS62]

Great,

And thanks for your help.

I have attached the modified source, I don’t know if my pull request did work.

Another thing to note: there is a definition for CT32B1, but this is used for the internal mbed ticker and also for USB, so these PWM pins should not be used.

Then I use the LPC1347 with the LPCXpresso. I have created the startup code for this toolchain and modified the linker script to use the nano lib, otherwise the generated code will not fit into this controller.

Regards,

Johannes

Von: Toyomasa Watarai [mailto:notifications@github.com]
Gesendet: Dienstag, 20. September 2016 16:29
An: ARMmbed/mbed-os
Cc: JojoS62; Author
Betreff: Re: [ARMmbed/mbed-os] [LPC1347] PWMout freq on 16 bit timers wrong (#2660)

I got the LPCXpresso1347 board and just started to see detail. I hope same fix as the LPC11XX can be used. I will investigate more.

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/* mbed Microcontroller Library

• Copyright (c) 2006-2013 ARM Limited
  *
• Licensed under the Apache License, Version 2.0 (the "License");
• you may not use this file except in compliance with the License.
• You may obtain a copy of the License at
  *

• http://www.apache.org/licenses/LICENSE-2.0
  
• Unless required by applicable law or agreed to in writing, software
• distributed under the License is distributed on an "AS IS" BASIS,
• WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
• See the License for the specific language governing permissions and
• limitations under the License.
  */
  #include "mbed_assert.h"
  #include "pwmout_api.h"
  #include "cmsis.h"
  #include "pinmap.h"

#define TCR_CNT_EN 0x00000001
#define TCR_RESET 0x00000002

/* To have a PWM where we can change both the period and the duty cycle,

• we need an entire timer. With the following conventions:

• * MR3 is used for the PWM period

• • MR0, MR1, MR2 are used for the duty cycle
    /
    static const PinMap PinMap_PWM[] = {
    / CT16B0 /
    {P0_8 , PWM_1, 2}, {P1_13, PWM_1, 2}, / MR0 /
    {P0_9 , PWM_2, 2}, {P1_14, PWM_2, 2}, / MR1 /
    {P0_10, PWM_3, 3}, {P1_15, PWM_3, 2}, / MR2 */

  /* CT16B1 /
  {P0_21, PWM_4, 1}, / MR0 /
  {P0_22, PWM_5, 2}, {P1_23, PWM_5, 1}, / MR1 */

  /* CT32B0 /
  {P0_18, PWM_6, 2}, {P1_24, PWM_6, 1}, / MR0 /
  {P0_19, PWM_7, 2}, {P1_25, PWM_7, 1}, / MR1 /
  {P0_1 , PWM_8, 2}, {P1_26, PWM_8, 1}, / MR2 */

  /* CT32B1 /
  {P0_13, PWM_9 , 3}, //{P1_0, PWM_9 , 1}, / MR0 /
  {P0_14, PWM_10, 3}, //{P1_1, PWM_10, 1}, / MR1 /
  {P0_15, PWM_11, 3}, //{P1_2, PWM_11, 1}, / MR2 */

  {NC, NC, 0}
  };

typedef struct {
uint8_t timer;
uint8_t mr;
} timer_mr;

static timer_mr pwm_timer_map[11] = {
{0, 0}, {0, 1}, {0, 2},
{1, 0}, {1, 1},
{2, 0}, {2, 1}, {2, 2},
{3, 0}, {3, 1}, {3, 2},
};

static LPC_CTxxBx_Type *Timers[4] = {
LPC_CT16B0, LPC_CT16B1,
LPC_CT32B0, LPC_CT32B1
};

static unsigned int pwm_clock_mhz;

void pwmout_init(pwmout_t* obj, PinName pin) {
// determine the channel
PWMName pwm = (PWMName)pinmap_peripheral(pin, PinMap_PWM);
MBED_ASSERT(pwm != (uint32_t)NC);

obj->pwm = pwm;

// Timer registers
timer_mr tid = pwm_timer_map[pwm];
LPC_CTxxBx_Type *timer = Timers[tid.timer];

// Disable timer
timer->TCR = 0;

// Power the correspondent timer
LPC_SYSCON->SYSAHBCLKCTRL |= 1 << (tid.timer + 7);

/* Enable PWM function */
timer->PWMC = (1 << 3)|(1 << 2)|(1 << 1)|(1 << 0);

/* Reset Functionality on MR3 controlling the PWM period */
timer->MCR = 1 << 10;

if (timer == LPC_CT16B0 || timer == LPC_CT16B1) {
/* Set 16-bit timer prescaler to avoid timer expire for default 20ms */
/* This can be also modified by user application, but the prescaler value */
/* might be trade-off to timer accuracy. max Period time is 28 ms */
    timer->PR = 30;
} 

pwm_clock_mhz = SystemCoreClock / 1000000;

// default to 20ms: standard for servos, and fine for e.g. brightness control
pwmout_period_ms(obj, 20);
pwmout_write    (obj, 0);

// Wire pinout
pinmap_pinout(pin, PinMap_PWM);

}

void pwmout_free(pwmout_t* obj) {
// [TODO]
}

void pwmout_write(pwmout_t* obj, float value) {
if (value < 0.0f) {
value = 0.0;
} else if (value > 1.0f) {
value = 1.0;
}

timer_mr tid = pwm_timer_map[obj->pwm];
LPC_CTxxBx_Type *timer = Timers[tid.timer];
uint32_t t_off = timer->MR3 - (uint32_t)((float)(timer->MR3) * value);

timer->MR[tid.mr] = t_off;

}

float pwmout_read(pwmout_t* obj) {
timer_mr tid = pwm_timer_map[obj->pwm];
LPC_CTxxBx_Type *timer = Timers[tid.timer];

float v = (float)(timer->MR3 - timer->MR[tid.mr]) / (float)(timer->MR3);
return (v > 1.0f) ? (1.0f) : (v);

}

void pwmout_period(pwmout_t* obj, float seconds) {
pwmout_period_us(obj, seconds * 1000000.0f);
}

void pwmout_period_ms(pwmout_t* obj, int ms) {
pwmout_period_us(obj, ms * 1000);
}

// Set the PWM period, keeping the duty cycle the same.
void pwmout_period_us(pwmout_t* obj, int us) {
int i = 0;
uint32_t period_ticks;

timer_mr tid = pwm_timer_map[obj->pwm];
LPC_CTxxBx_Type *timer = Timers[tid.timer];
uint32_t old_period_ticks = timer->MR3;
period_ticks = (pwm_clock_mhz * us) / (timer->PR + 1);

timer->TCR = TCR_RESET;
timer->MR3 = period_ticks;

// Scale the pulse width to preserve the duty ratio
if (old_period_ticks > 0) {
    for (i=0; i<3; i++) {
        uint32_t t_off = period_ticks - (uint32_t)(((uint64_t)timer->MR[i] * (uint64_t)period_ticks) / (uint64_t)old_period_ticks);
        timer->MR[i] = t_off;
    }
}
timer->TCR = TCR_CNT_EN;

}

void pwmout_pulsewidth(pwmout_t* obj, float seconds) {
pwmout_pulsewidth_us(obj, seconds * 1000000.0f);
}

void pwmout_pulsewidth_ms(pwmout_t* obj, int ms) {
pwmout_pulsewidth_us(obj, ms * 1000);
}

void pwmout_pulsewidth_us(pwmout_t* obj, int us) {
timer_mr tid = pwm_timer_map[obj->pwm];
LPC_CTxxBx_Type *timer = Timers[tid.timer];
uint32_t t_on = (uint32_t)((((uint64_t)SystemCoreClock * (uint64_t)us) / (uint64_t)1000000) / (timer->PR + 1)); ;

timer->TCR = TCR_RESET;
if (t_on > timer->MR3) {
    pwmout_period_us(obj, us);
}
uint32_t t_off = timer->MR3 - t_on;
timer->MR[tid.mr] = t_off;
timer->TCR = TCR_CNT_EN;

}

[toyowata]

@JojoS62 Thanks. I created a PR here: #2810