RTC

This module contains functions for working with the real time clock. More...

Files
file	stm32f103cb_rtc.h
	Header file for stm32f103cb real time clock (RTC)

Functions
mcu_error	RTC_setup (uint32_t resolution)
	Initialize the Real time clock. More...
mcu_error	RTC_calibrate (uint32_t resolution)
	Calibrate the RTC to a given resolution in milliseconds. More...
mcu_error	RTC_setPrescaler (uint32_t resolution)
	Set the raw prescaler value of the RTC. More...
uint8_t	RTC_isAlarmEnabled (void)
	Determine if the alarm interrupt is enabled. More...
uint8_t	RTC_isOverflowEnabled (void)
	Determine if the overflow interrupt is enabled. More...
uint8_t	RTC_isResolutionEnabled (void)
	Determine if the resolution (second) interrupt is enabled. More...
uint32_t	RTC_getCount (void)
	Get the current count value. More...
void	RTC_setCount (uint32_t count)
	Set the current count value. More...
void	RTC_enableResolutionISR (void(*res_isr)(void))
	Enables the resolution ISR. More...
void	RTC_enableAlarmISR (uint32_t count, void(*al_isr)(void))
	Enables the alarm ISR. More...
void	RTC_enableOverflowISR (void(*ov_isr)(void))
	Enables the overflow ISR. More...
void	RTC_disableResolutionISR (void)
	disables the resolution ISR.
void	RTC_disableAlarmISR (void)
	disables the alarm ISR.
void	RTC_disableOverflowISR (void)
	disables the overflow ISR.
void	RTC_clockEnable (void)
	Enable the RTC clock. More...

Detailed Description

This module contains functions for working with the real time clock.

The real time clock works with the internal LSI oscillator, this runs at approximately 40kHz. This clock functions seperate from the main clock and can be used in standby mode to wake the core from sleep state. This is acheived by setting the alarm to the desired wakeup time.

Both the counter and alarm registers are 32 bits wide, for a 1 second tick this is approximately 136 years before an overflow event occurs! At 1ms it is 7 weeks. The prescaler is 20 bits wide, giving a meer 3600 years as a maximum value.

The accuracy of the RTC increases with increasing resolution approximated as

• 1ms tick (minimum) ~0.5ms accuracy
• 10ms tick ~0.05ms accuracy
• 100ms tick ~0.005ms accuracy

and so on. These values assume RTC_calibrate() was used to determine the preload value.

There are three interrupts which can be generated with the RTC

• resolution, occuring with the value set (ms) in RTC_calibrate() or RTC_setup()
• alarm, occuring when the counter value reaches a set value. This is also used to wake the core from standby mode.
• overflow, occurs when the counter overflows.

Author

Stuart Ianna

Version

0.1

Date

June 2018

Copyright

GNU GPLv3

Bug:

None

Todo:

Warning

RTC_configure() requires a one-shot usage of systick. This means that this function should be called before any other systick usage.

Compilers

• arm-none-eabi-gcc (15:4.9.3+svn231177-1) 4.9.3 20150529 (prerelease)

Working example.

#include <stm32f103cb_core.h>

void alarm(void);
void resolution(void);
void overflow(void);

int main(void){

    //Coms for terminal info
    USB_coms(TIMER_2);

    //The value passed here is meaningless as RTC_calibrate is called next.
    RTC_setup(500);

    //This uses systick!
    //This takes 500ms to complete! 
    //(smaller resolution, shorter time, less accurate)
    RTC_calibrate(500);
    //Systick could be used here again if needed
   
    //Good to reset the count after so everything starts at 0
    RTC_setCount(0);

    //Call alarm after 2 resultion counts (1 second)
    RTC_enableAlarmISR(2,&alarm);

    //Call resolution every period. (500ms)
    RTC_enableResolutionISR(&resolution);

    //Call this after a couple of years.
    RTC_enableOverflowISR(&overflow);

    //Done
    while(1);
    return 0;
}

void alarm(void){

    printl("Alarm");

    //The clock continues to count.
    //Set the alarm again after 2 counts (1 second)
    RTC_enableAlarmISR(RTC_getCount() + 2,&alarm);
}

void resolution(void){

    printl("Resolution");
} 

void overflow(void){

    printl("Overflow");

}

Function Documentation

mcu_error RTC_calibrate

(

uint32_t

resolution

)

Calibrate the RTC to a given resolution in milliseconds.

This function uses SYSTICK to calculate the required RTC prescaler. SYSTICK must not be used for any other function while this is being called. After complete SYSTICK can be utalised again.

This function will take as long as the resultion to complete!

Parameters

resolution

The required resolution of the RTC in milliseconds.

Warning

Read description.

Returns

Error number as defined in mcu_error, E_RTC_NOERROR if ok or E_RTC_PRESCALER if an error with prescaler value has occured.

Definition at line 64 of file stm32f103cb_rtc.c.

void RTC_clockEnable

(

void

)

Enable the RTC clock.

This can be used to renable the clock after system has been in stop mode.

Definition at line 96 of file stm32f103cb_rtc.c.

void RTC_enableAlarmISR	(	uint32_t	count,
		void(*)(void)	al_isr
	)

Enables the alarm ISR.

This interrupt occurs when the counter value matches the alarm value.

Parameters

count	The value at which the count register triggers the interrupt.
al_isr	The function to call on interrupt.

Definition at line 133 of file stm32f103cb_rtc.c.

void RTC_enableOverflowISR

(

void(*)(void)

ov_isr

)

Enables the overflow ISR.

This interrupt occurs when the counter register overflows (never)

Parameters

ov_isr

The function to call on interrupt.

Definition at line 155 of file stm32f103cb_rtc.c.

void RTC_enableResolutionISR

(

void(*)(void)

res_isr

)

Enables the resolution ISR.

This interrupt occurs each resoultion time value.

Parameters

res_isr

The function to call on interrupt.

Definition at line 111 of file stm32f103cb_rtc.c.

uint32_t RTC_getCount

(

void

)

Get the current count value.

Returns

The current value (Of units respect to resolution)

Definition at line 101 of file stm32f103cb_rtc.c.

uint8_t RTC_isAlarmEnabled

(

void

)

Determine if the alarm interrupt is enabled.

Returns

• 0 if the alarm is disabled
• Non-zero if the alarm is enabled

Definition at line 147 of file stm32f103cb_rtc.c.

uint8_t RTC_isOverflowEnabled

(

void

)

Determine if the overflow interrupt is enabled.

Returns

• 0 if the overflow is disabled
• Non-zero if the overflow is enabled

Definition at line 168 of file stm32f103cb_rtc.c.

uint8_t RTC_isResolutionEnabled

(

void

)

Determine if the resolution (second) interrupt is enabled.

Returns

• 0 if the second is disabled
• Non-zero if the second is enabled

Definition at line 124 of file stm32f103cb_rtc.c.

void RTC_setCount

(

uint32_t

count

)

Set the current count value.

Parameters

count

The current value to set the count to

Definition at line 90 of file stm32f103cb_rtc.c.

mcu_error RTC_setPrescaler

(

uint32_t

resolution

)

Set the raw prescaler value of the RTC.

This value determines the period of the count incrementation. Usage of RTC_calabrate() superseeds the need for this function.

Parameters

resolution

The value written the the RTC prescaler register.

Returns

Error number as defined in mcu_error, E_RTC_NOERROR if ok or E_RTC_PRESCALER if an error with prescaler value has occured.

Definition at line 49 of file stm32f103cb_rtc.c.

mcu_error RTC_setup

(

uint32_t

resolution

)

Initialize the Real time clock.

This must be called before any other feature is used.

The resolution here is loaded directly to the prescalar register. The final period of the clock can be estimated using

period (ms) = (resolution/4)

This doesn't produce a very accurate result, and if possible RTC_calibrate should be called following this funcition. This automatically sets the period to the desired value.

Parameters

resolution

The value written to the prescaler register

Returns

Error number as defined in mcu_error, E_RTC_NOERROR if ok or E_RTC_PRESCALER if an error with prescaler value has occured.

Definition at line 33 of file stm32f103cb_rtc.c.