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Update us_ticker.c 

Used precomputed variables to replace runtime mult and div in us_ticker_read().
pull/905/head
Wim 2015-02-19 15:09:18 +01:00
parent 7bd12f26f7
commit 9241e5eaba
1 changed files with 21 additions and 5 deletions
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26
libraries/mbed/targets/hal/TARGET_NXP/TARGET_LPC81X/us_ticker.c
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@ -20,7 +20,8 @@
//New, using MRT instead of SCT, needed to free up SCT for PWM
//Ported from LPC824 libs
static int us_ticker_inited = 0;
static int ticker_expired = 0;
unsigned int ticker_fullcount_us;
unsigned long int ticker_expired_count_us = 0;
int MRT_Clock_MHz;
#define US_TICKER_TIMER_IRQn MRT_IRQn
@ -31,9 +32,11 @@ void us_ticker_init(void) {
return;
us_ticker_inited = 1;
// Calculate MRT clock value (MRT has no prescaler)
MRT_Clock_MHz = (SystemCoreClock / 1000000);
// Calculate fullcounter value in us (MRT has 31 bits and clock is 30 MHz)
ticker_fullcount_us = 0x80000000UL/MRT_Clock_MHz;
// Enable the MRT clock
LPC_SYSCON->SYSAHBCLKCTRL |= (1 << 10);
@ -64,8 +67,18 @@ uint32_t us_ticker_read() {
// Generate ticker value
// MRT source clock is SystemCoreClock (30MHz) and MRT is a 31-bit countdown timer
// Calculate expected value using number of expired times to mimic a 32bit timer @ 1 MHz
return (0x7FFFFFFFUL - LPC_MRT->TIMER0)/MRT_Clock_MHz + (ticker_expired * (0x80000000UL/MRT_Clock_MHz));
// Calculate expected value using current count and number of expired times to mimic a 32bit timer @ 1 MHz
//
// ticker_expired_count_us
// The variable ticker_expired_count_us keeps track of the number of 31bits overflows (counted by TIMER0) and
// corrects that back to us counts.
//
// (0x7FFFFFFFUL - LPC_MRT->TIMER0)/MRT_Clock_MHz
// The counter is a 31bit downcounter from 7FFFFFFF so correct to actual count-up value and correct
// for 30 counts per us.
//
// Added up these 2 parts result in current us time returned as 32 bits.
return (0x7FFFFFFFUL - LPC_MRT->TIMER0)/MRT_Clock_MHz + ticker_expired_count_us;
}
//TIMER1 is used for Timestamped interrupts (Ticker(), Timeout())
@ -100,6 +113,9 @@ void us_ticker_clear_interrupt() {
//Timer0 for us counter (31 bits downcounter @ SystemCoreClock)
if (LPC_MRT->STAT0 & 1) {
LPC_MRT->STAT0 = 1;
ticker_expired++;
// ticker_expired_count_us = (ticker_expired * 0x80000000UL) / MRT_Clock_MHz
// The variable ticker_expired_count_us keeps track of the number of 31bits overflows (counted by TIMER0) and
// the multiplication/division corrects that back to us counts.
ticker_expired_count_us += ticker_fullcount_us;
}
}