Hi,
Maybe, I'm trying to explain something redundant...
Here I tried to explain
(1) Terminologies around the Timer
(2) A simple sample using Timer (TCPWM)
(3) A simple sample using SysTick
(1) Terminologies around the Timer
First of all , IMHO, the word "Period" in timer/pwm is misleading.
It actually is the Maximum value of the counter.
So I use "Max_Value " instead of "Period" to avoid confusion, here.
As the counter starts from 0 to the Max_Value , the real "Period" becomes Max_Value + 1.
And the real time it takes is the period x clock's period.
And clock's period = 1/(clock's frequency).
So the interrupt period of the timer will be
Timer's Period = (Max_Value+1) x clock's period = (Max_Value+1) / (clock's frequency)
But usually the clock could be very fast, so using the Prescaler, clock's frequency can be divided
or clock's period can be multiplied.
Timer's Period = (Max_Value+1) / (clock's frequency / prescaler) = (Max_Value+1) x prescaler / (clock's frequency)
About the clock's fraction setting, please refer to the document below
Setting the Clock Frequency as a Fractional Value of Source Clock in PSoC 4/PSoC 6 MCU - KBA229104
(2) A simple sample using Timer (TCPWM)
To test this I wrote a sample project for CY8C4025AZI-S413
test_timer_201006
Schematic
Note: To make it easer to see the result, I changed the clock frequency to 10kHz. (10,000 Hz)
clock configuration
Pins
Note: Since I don't have a board with this MCU, I could not physically test it.
Please specify appropriate pin number for LED.
(It worked with CY8CKIT-044 (CY8C4247AZI-M485))
main.c
======================
#include "project.h"
CY_ISR(pit_isr)
{
LED_Write(!LED_Read()) ; /* toggle LED */
Timer_ClearInterrupt(Timer_INTR_MASK_TC) ;
}
/*
* Timer Period = Timer Period / Clock Frequency
*/
int main(void)
{
uint16_t period = 9999 ; /* make period to 10,000 */
CyGlobalIntEnable; /* Enable global interrupts. */
pit_int_ClearPending() ;
pit_int_StartEx(pit_isr) ;
Timer_Init() ;
Timer_WritePeriod(period) ;
Timer_WriteCounter(0) ;
Timer_Enable() ;
for(;;)
{
/* Place your application code here. */
}
}
======================
(3) A simple sample using SysTick
After writing above, I have another suggestion.
If all you need is a pit (periodical interrupt time) of ms order,
cortex-M has SysTick Timer in the core.
Which does not consume the timer/pwm component.
Following is a sample of using SysTick
sys_tick_test_201006
Schematic
Pins
Note: Since I don't have a board with this MCU, I could not physically test it.
Please specify appropriate pin number for LED.
(It worked with CY8CKIT-044 (CY8C4247AZI-M485))
main.c
======================
#include "project.h"
volatile uint32_t led_tick_count = 0 ;
CY_ISR(SysTick_ISR)
{
led_tick_count++ ;
if ((led_tick_count % 1000) == 0) {
led_tick_count = 0 ;
LED_Write(!LED_Read()) ;
}
}
int find_empty_slot(void)
{
int result = -1 ;
uint32_t i ;
for (i = 0 ; i < CY_SYS_SYST_NUM_OF_CALLBACKS ; i++ ) {
if (CySysTickGetCallback(i) == NULL) {
result = i ;
break ;
}
}
return(result) ;
}
int main(void)
{
int sys_tick_slot = 0 ;
CyGlobalIntEnable; /* Enable global interrupts. */
sys_tick_slot = find_empty_slot() ;
if (sys_tick_slot < 0) {
while(1) { } /* halting here */
} else {
CySysTickStart() ;
CySysTickSetCallback(sys_tick_slot, SysTick_ISR) ;
}
CySysTickStart();
for(;;)
{
}
}
======================
moto
Hi,
If all you need is 1sec pit, I would recommend to use SysTick.
About the 10min vs 9.58min, you (we) must aware of the accuracy of the clock.
(You can access this window by selecting Clocks in the Design Wide Resources, then double clicking a clock shown in the table.)
As you can see, the Accuracy of IMO is +/- 2%. So 2sec in 10mins (600sec) seems to be doing OK (within the spec))
You could add an external crystal with higher accuracy, but unless you can provide a method to re-adjust the timer
it will not be possible to keep "exact" time for very long time (I mean months, years, centuries...)
For 2 sec interval, you can
(1) For TCPWM sample
> uint16_t period = 9999 ; /* make period to 10,000 */
< uint16_t period = 1999 ; /* make period to 20,000 */
(2) For SysTick sample
> if ((led_tick_count % 1000) == 0) {
< if ((led_tick_count % 2000) == 0) {
moto
P.S. Oops, I misunderstood your last line, but I hope that you understand the idea 😉
Hello BhSa,
As correctly pointed out by Moto, the IMO clock being used in the application has an accuracy of +/2 % thus you are not seeing the exact 10 mins.
In order to increase the accuracy we will suggest you the following:
1. Use an EXTCLK or ECO as the the source
2. In case you are using PSoC 4 MCU with provision to connect WCO, You can enable the WCO (under low frequency clocks tab), and the enable Trim with WCO (Under High frequency clocks tab)
Best Regards
Ekta
BhSa,
i tried to enable the Extclk and disable the IMO....but i am little bit confused about how to connect the external crystal in actual hardware....i found only one pin in cydwr pin section page....
The part you've chosen (CY8C4025AZI-S413) does not have the ability to connect a XTAL directly to it. There is NO ECO (External Crystal Oscillator) option.
You can insert an Externally generated high frequency clock (ExtClk) into P0[6]. This comes from another device like a Clock Oscillator (eq. Frequency Control CTS Corporation ) where the crystal and oscillation circuit are packaged together. For most of these device you just need to supply power and if it has it an enable signal.
This part does support a WCO (Watch Crystal Oscillator). You would place the watch crystal (32.768Khz) on pins P0[4] and P0[5] along with the required caps on each leg to GND. The cap values depend on the crystal manufacturer recommended value. Usually it ranges from 10pF to 25pF.
In general, it is quite common to use a WCO with very good PPM accuracy for long timing such as the 10 minutes you are targeting.
The watch crystal is usually cheaper than a high frequency crystal or clock oscillator.
Additionally using the WCO, you can also use the RTC component on the PSoC4 which doesn't use additional TCPWM or UDB resources. With the RTC, you can set it to provide an interrupt at long time intervals (down to one second) and it can be still operational in lower power modes where as the TCPWMs and UDBs cannot operate beyond active and 'low power' modes.
Len
