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bea3ec2724
Firmware2/Marlin/src/HAL/HAL_LPC1768/LPC1768_PWM.h
Christopher Pepper bea3ec2724 M355 S0, S1 fixes & faster LCD, SD card 
fix Travis error
2017-09-27 19:26:00 -05:00

514 lines
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/**
* Marlin 3D Printer Firmware
* Copyright (C) 2017 MarlinFirmware [https://github.com/MarlinFirmware/Marlin]
*
* Based on Sprinter and grbl.
* Copyright (C) 2011 Camiel Gubbels / Erik van der Zalm
*
* This program is free software: you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation, either version 3 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program. If not, see <http://www.gnu.org/licenses/>.
*
*/
/**
* The class Servo uses the PWM class to implement its functions
*
* All PWMs use the same repetition rate - 20mS because that's the normal servo rate
*/
/**
* This is a hybrid system.
*
* The PWM1 module is used to directly control the Servo 0, 1 & 3 pins. This keeps
* the pulse width jitter to under a microsecond.
*
* For all other pins the PWM1 module is used to generate interrupts. The ISR
* routine does the actual setting/clearing of pins. The upside is that any pin can
* have a PWM channel assigned to it. The downside is that there is more pulse width
* jitter. The jitter depends on what else is happening in the system and what ISRs
* prempt the PWM ISR. Writing to the SD card can add 20 microseconds to the pulse
* width.
*/
/**
* The data structures are setup to minimize the computation done by the ISR which
* minimizes ISR execution time. Execution times are 2.2 - 3.7 microseconds.
*
* Two tables are used. active_table is used by the ISR. Changes to the table are
* are done by copying the active_table into the work_table, updating the work_table
* and then swapping the two tables. Swapping is done by manipulating pointers.
*
* Immediately after the swap the ISR uses the work_table until the start of the
* next 20mS cycle. During this transition the "work_table" is actually the table
* that was being used before the swap. The "active_table" contains the data that
* will start being used at the start of the next 20mS period. This keeps the pins
* well behaved during the transition.
*
* The ISR's priority is set to the maximum otherwise other ISRs can cause considerable
* jitter in the PWM high time.
*
* See the end of this file for details on the hardware/firmware interaction
*/
#ifdef TARGET_LPC1768
#include <lpc17xx_pinsel.h>
#define NUM_PWMS 6
typedef struct { // holds all data needed to control/init one of the PWM channels
uint8_t sequence; // 0: available slot, 1 - 6: PWM channel assigned to that slot
uint8_t logical_pin;
uint16_t PWM_mask; // MASK TO CHECK/WRITE THE IR REGISTER
volatile uint32_t* set_register;
volatile uint32_t* clr_register;
uint32_t write_mask; // USED BY SET/CLEAR COMMANDS
uint32_t microseconds; // value written to MR register
uint32_t min; // lower value limit checked by WRITE routine before writing to the MR register
uint32_t max; // upper value limit checked by WRITE routine before writing to the MR register
bool PWM_flag; // 0 - USED BY sERVO, 1 - USED BY ANALOGWRITE
uint8_t servo_index; // 0 - MAX_SERVO -1 : servo index, 0xFF : PWM channel
bool active_flag; // THIS TABLE ENTRY IS ACTIVELY TOGGLING A PIN
uint8_t assigned_MR; // Which MR (1-6) is used by this logical channel
uint32_t PCR_bit; // PCR register bit to enable PWM1 control of this pin
uint32_t PINSEL3_bits; // PINSEL3 register bits to set pin mode to PWM1 control
} PWM_map;
#define MICRO_MAX 0xffffffff
#define PWM_MAP_INIT_ROW {0, 0xff, 0, 0, 0, 0, MICRO_MAX, 0, 0, 0, 0, 0, 0, 0, 0}
#define PWM_MAP_INIT {PWM_MAP_INIT_ROW,\
PWM_MAP_INIT_ROW,\
PWM_MAP_INIT_ROW,\
PWM_MAP_INIT_ROW,\
PWM_MAP_INIT_ROW,\
PWM_MAP_INIT_ROW,\
};
PWM_map PWM1_map_A[NUM_PWMS] = PWM_MAP_INIT;
PWM_map PWM1_map_B[NUM_PWMS] = PWM_MAP_INIT;
PWM_map *active_table = PWM1_map_A;
PWM_map *work_table = PWM1_map_B;
PWM_map *ISR_table;
#define IR_BIT(p) (p >= 0 && p <= 3 ? p : p + 4 )
#define COPY_ACTIVE_TABLE for (uint8_t i = 0; i < 6 ; i++) work_table[i] = active_table[i]
#define PIN_IS_INVERTED(p) 0 // place holder in case inverting PWM output is offered
/**
* Prescale register and MR0 register values
*
* 100MHz PCLK 50MHz PCLK 25MHz PCLK 12.5MHz PCLK
* ----------------- ----------------- ----------------- -----------------
* desired prescale MR0 prescale MR0 prescale MR0 prescale MR0 resolution
* prescale register register register register register register register register in degrees
* freq value value value value value value value value
*
* 8 11.5 159,999 5.25 159,999 2.13 159,999 0.5625 159,999 0.023
* 4 24 79,999 11.5 79,999 5.25 79,999 2.125 79,999 0.045
* 2 49 39,999 24 39,999 11.5 39,999 5.25 39,999 0.090
* 1 99 19,999 49 19,999 24 19,999 11.5 19,999 0.180
* 0.5 199 9,999 99 9,999 49 9,999 24 9,999 0.360
* 0.25 399 4,999 199 4,999 99 4,999 49 4,999 0.720
* 0.125 799 2,499 399 2,499 199 2,499 99 2,499 1.440
*
* The desired prescale frequency comes from an input in the range of 544 - 2400 microseconds and the
* desire to just shift the input left or right as needed.
*
* A resolution of 0.2 degrees seems reasonable so a prescale frequency output of 1MHz is being used.
* It also means we don't need to scale the input.
*
* The PCLK is set to 25MHz because that's the slowest one that gives whole numbers for prescale and
* MR0 registers.
*
* Final settings:
* PCLKSEL0: 0x0
* PWM1PR: 0x018 (24)
* PWM1MR0: 0x04E1F (19,999)
*
*/
#define LPC_PWM1_MR0 19999 // base repetition rate minus one count - 20mS
#define LPC_PWM1_PR 24 // prescaler value - prescaler divide by 24 + 1 - 1 MHz output
#define LPC_PWM1_PCLKSEL0 0x00 // select clock source for prescaler - defaults to 25MHz on power up
// 0: 25MHz, 1: 100MHz, 2: 50MHz, 3: 12.5MHZ to PWM1 prescaler
#define MR0_MARGIN 200 // if channel value too close to MR0 the system locks up
void LPC1768_PWM_init(void) {
#define SBIT_CNTEN 0 // PWM1 counter & pre-scaler enable/disable
#define SBIT_CNTRST 1 // reset counters to known state
#define SBIT_PWMEN 3 // 1 - PWM, 0 - timer
#define SBIT_PWMMR0R 1
#define PCPWM1 6
#define PCLK_PWM1 12
LPC_SC->PCONP |= (1 << PCPWM1); // enable PWM1 controller (enabled on power up)
LPC_SC->PCLKSEL0 &= ~(0x3 << PCLK_PWM1);
LPC_SC->PCLKSEL0 |= (LPC_PWM1_PCLKSEL0 << PCLK_PWM1);
LPC_PWM1->MR0 = LPC_PWM1_MR0; // TC resets every 19,999 + 1 cycles - sets PWM cycle(Ton+Toff) to 20 mS
// MR0 must be set before TCR enables the PWM
LPC_PWM1->TCR = _BV(SBIT_CNTEN) | _BV(SBIT_CNTRST)| _BV(SBIT_PWMEN);; // enable counters, reset counters, set mode to PWM
LPC_PWM1->TCR &= ~(_BV(SBIT_CNTRST)); // take counters out of reset
LPC_PWM1->PR = LPC_PWM1_PR;
LPC_PWM1->MCR = (_BV(SBIT_PWMMR0R) | _BV(0)); // Reset TC if it matches MR0, disable all interrupts except for MR0
LPC_PWM1->CTCR = 0; // disable counter mode (enable PWM mode)
LPC_PWM1->LER = 0x07F; // Set the latch Enable Bits to load the new Match Values for MR0 - MR6
// Set all PWMs to single edge
LPC_PWM1->PCR = 0; // single edge mode for all channels, PWM1 control of outputs off
NVIC_EnableIRQ(PWM1_IRQn); // Enable interrupt handler
// NVIC_SetPriority(PWM1_IRQn, NVIC_EncodePriority(0, 10, 0)); // normal priority for PWM module
NVIC_SetPriority(PWM1_IRQn, NVIC_EncodePriority(0, 0, 0)); // minimizes jitter due to higher priority ISRs
}
bool PWM_table_swap = false; // flag to tell the ISR that the tables have been swapped
bool PWM_MR0_wait = false; // flag to ensure don't delay MR0 interrupt
bool LPC1768_PWM_attach_pin(uint8_t pin, uint32_t min = 1, uint32_t max = (LPC_PWM1_MR0 - MR0_MARGIN), uint8_t servo_index = 0xff) {
while (PWM_table_swap) delay(5); // don't do anything until the previous change has been implemented by the ISR
COPY_ACTIVE_TABLE; // copy active table into work table
uint8_t slot = 0;
for (uint8_t i = 0; i < NUM_PWMS ; i++) // see if already in table
if (work_table[i].logical_pin == pin) return 1;
for (uint8_t i = 1; (i < NUM_PWMS + 1) && !slot; i++) // find empty slot
if ( !(work_table[i - 1].set_register)) slot = i; // any item that can't be zero when active or just attached is OK
if (!slot) return 0;
slot--; // turn it into array index
work_table[slot].logical_pin = pin; // init slot
work_table[slot].PWM_mask = 0; // real value set by PWM_write
work_table[slot].set_register = PIN_IS_INVERTED(pin) ? &LPC_GPIO(pin_map[pin].port)->FIOCLR : &LPC_GPIO(pin_map[pin].port)->FIOSET;
work_table[slot].clr_register = PIN_IS_INVERTED(pin) ? &LPC_GPIO(pin_map[pin].port)->FIOSET : &LPC_GPIO(pin_map[pin].port)->FIOCLR;
work_table[slot].write_mask = LPC_PIN(pin_map[pin].pin);
work_table[slot].microseconds = MICRO_MAX;
work_table[slot].min = min;
work_table[slot].max = MIN(max, LPC_PWM1_MR0 - MR0_MARGIN);
work_table[slot].servo_index = servo_index;
work_table[slot].active_flag = false;
//swap tables
PWM_MR0_wait = true;
while (PWM_MR0_wait) delay(5); //wait until MR0 interrupt has happend so don't delay it.
NVIC_DisableIRQ(PWM1_IRQn);
PWM_map *pointer_swap = active_table;
active_table = work_table;
work_table = pointer_swap;
PWM_table_swap = true; // tell the ISR that the tables have been swapped
NVIC_EnableIRQ(PWM1_IRQn); // re-enable PWM interrupts
return 1;
}
#define pin_11_PWM_channel 2
#define pin_6_PWM_channel 3
#define pin_4_PWM_channel 1
// used to keep track of which Match Registers have been used and if they will be used by the
// PWM1 module to directly control the pin or will be used to generate an interrupt
typedef struct { // status of PWM1 channel
uint8_t map_used; // 0 - this MR register not used/assigned
uint8_t map_PWM_INT; // 0 - available for interrupts, 1 - in use by PWM
uint8_t map_PWM_PIN; // logical pin number for this PwM1 controlled pin / port
volatile uint32_t* MR_register; // address of the MR register for this PWM1 channel
uint32_t PCR_bit; // PCR register bit to enable PWM1 control of this pin
uint32_t PINSEL3_bits; // PINSEL3 register bits to set pin mode to PWM1 control
} MR_map;
MR_map map_MR[NUM_PWMS];
void LPC1768_PWM_update_map_MR(void) {
map_MR[0] = {0, (uint8_t) (LPC_PWM1->PCR & _BV(8 + pin_4_PWM_channel) ? 1 : 0), 4, &LPC_PWM1->MR1, 0, 0};
map_MR[1] = {0, (uint8_t) (LPC_PWM1->PCR & _BV(8 + pin_11_PWM_channel) ? 1 : 0), 11, &LPC_PWM1->MR2, 0, 0};
map_MR[2] = {0, (uint8_t) (LPC_PWM1->PCR & _BV(8 + pin_6_PWM_channel) ? 1 : 0), 6, &LPC_PWM1->MR3, 0, 0};
map_MR[3] = {0, 0, 0, &LPC_PWM1->MR4, 0, 0};
map_MR[4] = {0, 0, 0, &LPC_PWM1->MR5, 0, 0};
map_MR[5] = {0, 0, 0, &LPC_PWM1->MR6, 0, 0};
}
uint32_t LPC1768_PWM_interrupt_mask = 1;
void LPC1768_PWM_update(void) {
for (uint8_t i = NUM_PWMS; --i;) { // (bubble) sort table by microseconds
bool didSwap = false;
PWM_map temp;
for (uint16_t j = 0; j < i; ++j) {
if (work_table[j].microseconds > work_table[j + 1].microseconds) {
temp = work_table[j + 1];
work_table[j + 1] = work_table[j];
work_table[j] = temp;
didSwap = true;
}
}
if (!didSwap) break;
}
LPC1768_PWM_interrupt_mask = 0; // set match registers to new values, build IRQ mask
for (uint8_t i = 0; i < NUM_PWMS; i++) {
if (work_table[i].active_flag == true) {
work_table[i].sequence = i + 1;
// first see if there is a PWM1 controlled pin for this entry
bool found = false;
for (uint8_t j = 0; (j < NUM_PWMS) && !found; j++) {
if ( (map_MR[j].map_PWM_PIN == work_table[i].logical_pin) && map_MR[j].map_PWM_INT ) {
*map_MR[j].MR_register = work_table[i].microseconds; // found one of the PWM pins
work_table[i].PWM_mask = 0;
work_table[i].PCR_bit = map_MR[j].PCR_bit; // PCR register bit to enable PWM1 control of this pin
work_table[i].PINSEL3_bits = map_MR[j].PINSEL3_bits; // PINSEL3 register bits to set pin mode to PWM1 control} MR_map;
map_MR[j].map_used = 2;
work_table[i].assigned_MR = j +1; // only used to help in debugging
found = true;
}
}
// didn't find a PWM1 pin so get an interrupt
for (uint8_t k = 0; (k < NUM_PWMS) && !found; k++) {
if ( !(map_MR[k].map_PWM_INT || map_MR[k].map_used)) {
*map_MR[k].MR_register = work_table[i].microseconds; // found one for an interrupt pin
map_MR[k].map_used = 1;
LPC1768_PWM_interrupt_mask |= _BV(3 * (k + 1)); // set bit in the MCR to enable this MR to generate an interrupt
work_table[i].PWM_mask = _BV(IR_BIT(k + 1)); // bit in the IR that will go active when this MR generates an interrupt
work_table[i].assigned_MR = k +1; // only used to help in debugging
found = true;
}
}
}
else
work_table[i].sequence = 0;
}
LPC1768_PWM_interrupt_mask |= (uint32_t) _BV(0); // add in MR0 interrupt
// swap tables
PWM_MR0_wait = true;
while (PWM_MR0_wait) delay(5); //wait until MR0 interrupt has happend so don't delay it.
NVIC_DisableIRQ(PWM1_IRQn);
LPC_PWM1->LER = 0x07E; // Set the latch Enable Bits to load the new Match Values for MR1 - MR6
PWM_map *pointer_swap = active_table;
active_table = work_table;
work_table = pointer_swap;
PWM_table_swap = true; // tell the ISR that the tables have been swapped
NVIC_EnableIRQ(PWM1_IRQn); // re-enable PWM interrupts
}
bool LPC1768_PWM_write(uint8_t pin, uint32_t value) {
while (PWM_table_swap) delay(5); // don't do anything until the previous change has been implemented by the ISR
COPY_ACTIVE_TABLE; // copy active table into work table
uint8_t slot = 0xFF;
for (uint8_t i = 0; i < NUM_PWMS; i++) // find slot
if (work_table[i].logical_pin == pin) slot = i;
if (slot == 0xFF) return false; // return error if pin not found
LPC1768_PWM_update_map_MR();
switch(pin) {
case 11: // Servo 0, PWM1 channel 2 (Pin 11 P1.20 PWM1.2)
map_MR[pin_11_PWM_channel - 1].PCR_bit = _BV(8 + pin_11_PWM_channel); // enable PWM1 module control of this pin
map_MR[pin_11_PWM_channel - 1].map_PWM_INT = 1; // 0 - available for interrupts, 1 - in use by PWM
map_MR[pin_11_PWM_channel - 1].PINSEL3_bits = 0x2 << 8; // ISR must do this AFTER setting PCR
break;
case 6: // Servo 1, PWM1 channel 3 (Pin 6 P1.21 PWM1.3)
map_MR[pin_6_PWM_channel - 1].PCR_bit = _BV(8 + pin_6_PWM_channel); // enable PWM1 module control of this pin
map_MR[pin_6_PWM_channel - 1].map_PWM_INT = 1; // 0 - available for interrupts, 1 - in use by PWM
map_MR[pin_6_PWM_channel - 1].PINSEL3_bits = 0x2 << 10; // ISR must do this AFTER setting PCR
break;
case 4: // Servo 3, PWM1 channel 1 (Pin 4 P1.18 PWM1.1)
map_MR[pin_4_PWM_channel - 1].PCR_bit = _BV(8 + pin_4_PWM_channel); // enable PWM1 module control of this pin
map_MR[pin_4_PWM_channel - 1].map_PWM_INT = 1; // 0 - available for interrupts, 1 - in use by PWM
map_MR[pin_4_PWM_channel - 1].PINSEL3_bits = 0x2 << 4; // ISR must do this AFTER setting PCR
break;
default: // ISR pins
pinMode(pin, OUTPUT); // set pin to output but don't write anything in case it's already in use
break;
}
work_table[slot].microseconds = MAX(MIN(value, work_table[slot].max), work_table[slot].min);
work_table[slot].active_flag = true;
LPC1768_PWM_update();
return 1;
}
bool LPC1768_PWM_detach_pin(uint8_t pin) {
while (PWM_table_swap) delay(5); // don't do anything until the previous change has been implemented by the ISR
COPY_ACTIVE_TABLE; // copy active table into work table
uint8_t slot = 0xFF;
for (uint8_t i = 0; i < NUM_PWMS; i++) // find slot
if (work_table[i].logical_pin == pin) slot = i;
if (slot == 0xFF) return false; // return error if pin not found
LPC1768_PWM_update_map_MR();
// OK to make these changes before the MR0 interrupt
switch(pin) {
case 11: // Servo 0, PWM1 channel 2 (Pin 11 P1.20 PWM1.2)
LPC_PWM1->PCR &= ~(_BV(8 + pin_11_PWM_channel)); // disable PWM1 module control of this pin
map_MR[pin_11_PWM_channel - 1].PCR_bit = 0;
LPC_PINCON->PINSEL3 &= ~(0x3 << 8); // return pin to general purpose I/O
map_MR[pin_11_PWM_channel - 1].PINSEL3_bits = 0;
map_MR[pin_11_PWM_channel - 1].map_PWM_INT = 0; // 0 - available for interrupts, 1 - in use by PWM
break;
case 6: // Servo 1, PWM1 channel 3 (Pin 6 P1.21 PWM1.3)
LPC_PWM1->PCR &= ~(_BV(8 + pin_6_PWM_channel)); // disable PWM1 module control of this pin
map_MR[pin_6_PWM_channel - 1].PCR_bit = 0;
LPC_PINCON->PINSEL3 &= ~(0x3 << 10); // return pin to general purpose I/O
map_MR[pin_6_PWM_channel - 1].PINSEL3_bits = 0;
map_MR[pin_6_PWM_channel - 1].map_PWM_INT = 0; // 0 - available for interrupts, 1 - in use by PWM
break;
case 4: // Servo 3, PWM1 channel 1 (Pin 4 P1.18 PWM1.1)
LPC_PWM1->PCR &= ~(_BV(8 + pin_4_PWM_channel)); // disable PWM1 module control of this pin
map_MR[pin_4_PWM_channel - 1].PCR_bit = 0;
LPC_PINCON->PINSEL3 &= ~(0x3 << 4); // return pin to general purpose I/O
map_MR[pin_4_PWM_channel - 1].PINSEL3_bits = 0;
map_MR[pin_4_PWM_channel - 1].map_PWM_INT = 0; // 0 - available for interrupts, 1 - in use by PWM
break;
}
pinMode(pin, INPUT);
work_table[slot] = PWM_MAP_INIT_ROW;
LPC1768_PWM_update();
return 1;
}
bool useable_hardware_PWM(uint8_t pin) {
COPY_ACTIVE_TABLE; // copy active table into work table
for (uint8_t i = 0; i < NUM_PWMS; i++) // see if it's already setup
if (work_table[i].logical_pin == pin && work_table[i].sequence) return true;
for (uint8_t i = 0; i < NUM_PWMS; i++) // see if there is an empty slot
if (!work_table[i].sequence) return true;
return false; // only get here if neither the above are true
}
////////////////////////////////////////////////////////////////////////////////
#define HAL_PWM_LPC1768_ISR extern "C" void PWM1_IRQHandler(void)
// Both loops could be terminated when the last active channel is found but that would
// result in variations ISR run time which results in variations in pulse width
/**
* Changes to PINSEL3, PCR and MCR are only done during the MR0 interrupt otherwise
* the wrong pin may be toggled or even have the system hang.
*/
HAL_PWM_LPC1768_ISR {
if (PWM_table_swap) ISR_table = work_table; // use old table if a swap was just done
else ISR_table = active_table;
if (LPC_PWM1->IR & 0x1) { // MR0 interrupt
ISR_table = active_table; // MR0 means new values could have been loaded so set everything
if (PWM_table_swap) LPC_PWM1->MCR = LPC1768_PWM_interrupt_mask; // enable new PWM individual channel interrupts
for (uint8_t i = 0; (i < NUM_PWMS) ; i++) {
if(ISR_table[i].active_flag && !((ISR_table[i].logical_pin == 11) ||
(ISR_table[i].logical_pin == 4) ||
(ISR_table[i].logical_pin == 6)))
*ISR_table[i].set_register = ISR_table[i].write_mask; // set pins for all enabled interrupt channels active
if (PWM_table_swap && ISR_table[i].PCR_bit) {
LPC_PWM1->PCR |= ISR_table[i].PCR_bit; // enable PWM1 module control of this pin
LPC_PINCON->PINSEL3 |= ISR_table[i].PINSEL3_bits; // set pin mode to PWM1 control - must be done after PCR
}
}
PWM_table_swap = false;
PWM_MR0_wait = false;
LPC_PWM1->IR = 0x01; // clear the MR0 interrupt flag bit
}
else {
for (uint8_t i = 0; i < NUM_PWMS ; i++)
if (ISR_table[i].active_flag && (LPC_PWM1->IR & ISR_table[i].PWM_mask) ){
LPC_PWM1->IR = ISR_table[i].PWM_mask; // clear the interrupt flag bits for expected interrupts
*ISR_table[i].clr_register = ISR_table[i].write_mask; // set channel to inactive
}
}
LPC_PWM1->IR = 0x70F; // guarantees all interrupt flags are cleared which, if there is an unexpected
// PWM interrupt, will keep the ISR from hanging which will crash the controller
return;
}
#endif
/////////////////////////////////////////////////////////////////
///////////////// HARDWARE FIRMWARE INTERACTION ////////////////
/////////////////////////////////////////////////////////////////
/**
* Almost all changes to the hardware registers must be coordinated with the Match Register 0 (MR0)
* interrupt. The only exception is detaching pins. It doesn't matter when they go
* tristate.
*
* The LPC1768_PWM_init routine kicks off the MR0 interrupt. This interrupt is never disabled or
* delayed.
*
* The PWM_table_swap flag is set when the firmware has swapped in an updated table. It is
* cleared by the ISR during the MR0 interrupt as it completes the swap and accompanying updates.
* It serves two purposes:
* 1) Tells the ISR that the tables have been swapped
* 2) Keeps the firmware from starting a new update until the previous one has been completed.
*
* The PWM_MR0_wait flag is set when the firmware is ready to swap in an updated table and cleared by
* the ISR during the MR0 interrupt. It is used to avoid delaying the MR0 interrupt when swapping in
* an updated table. This avoids glitches in pulse width and/or repetition rate.
*
* The sequence of events during a write to a PWM channel is:
* 1) Waits until PWM_table_swap flag is false before starting
* 2) Copies the active table into the work table
* 3) Updates the work table
* NOTES - MR1-MR6 are updated at this time. The updates aren't put into use until the first
* MR0 after the LER register has been written. The LER register is written during the
* table swap process.
* - The MCR mask is created at this time. It is not used until the ISR writes the MCR
* during the MR0 interrupt in the table swap process.
* 4) Sets the PWM_MR0_wait flag
* 5) ISR clears the PWM_MR0_wait flag during the next MR0 interrupt
* 6) Once the PWM_MR0_wait flag is cleared then the firmware:
* disables the ISR interrupt
* swaps the pointers to the tables
* writes to the LER register
* sets the PWM_table_swap flag active
* re-enables the ISR
* 7) On the next interrupt the ISR changes its pointer to the work table which is now the old,
* unmodified, active table.
* 8) On the next MR0 interrupt the ISR:
* switches over to the active table
* clears the PWM_table_swap and PWM_MR0_wait flags
* updates the MCR register with the possibly new interrupt sources/assignments
* writes to the PCR register to enable the direct control of the Servo 0, 1 & 3 pins by the PWM1 module
* sets the PINSEL3 register to function/mode 0x2 for the Servo 0, 1 & 3 pins
* NOTE - PCR must be set before PINSEL
* sets the pins controlled by the ISR to their active states
*/
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