Quality RTOS & Embedded Software

 Real time embedded FreeRTOS RSS feed 
Real time embedded FreeRTOS mailing list 
Quick Start Supported MCUs PDF Books Trace Tools Ecosystem TCP & FAT Training




Loading

Timebase timer problem in the STR75x port?

Posted by Ricky on April 15, 2008
I have an STR75x application (IAR compiler) in a syringe pump that uses a FreeRTOS that I've customized in order to use interrupts more efficiently. I just downloaded version 5.0.0 and am merging in my changes. I've come to a point where the timebase timer is set up and I don't think I understand how the auto reload value is computed in the prvSetupTimerInterrupt function in port.c.

The 60 MHz processor clock is fed through a divide-by-20 prescaler, giving the timebase timer an input frequency of 3 MHz. My application needs a 1 millisecond tick (configTICK_RATE_HZ = 1000) so it has an auto reload value of (configCPU_CLOCK_HZ / portPRESCALE) / configTICK_RATE_HZ, or (6 MHz / 20) / 1000 = 3000 counts. 3 MHz / 3000 counts = 1 mS between interrupts. I've done timing runs on the pump to insure the flow is accurate and my clock, which runs off the timer hook, runs very accurately with this setting (I've compared it with an external clock).

The FreeRTOS version 5 timebase setup still feeds a 60 MHz processor clock through a divide-by-20 prescaler so the input frequency into the timebase timer is still 3 MHz. But the auto reload value in version 5 is computed as ((configCPU_CLOCK_HZ / (portPRESCALE + 1)) / configTICK_RATE_HZ) + 1 which is (6 MHz / 21) / 1000 + 1, or 2858 counts, rounded. This means the clock is running at an actual tick rate of 1050 Hz instead of the expected 1000 so the clock would be running too fast, adding one second every 20 seconds. Is this correct or am I missing something here?

I'm going to leave my 3000 count setup alone since it's working fine.

RE: Timebase timer problem in the STR75x port?

Posted by Dave on April 15, 2008
I cannot see anything different with the port layer code in the STR750. Are you saying the code that sets up the time has changed, or that the same code is giving different timing results?

RE: Timebase timer problem in the STR75x port

Posted by Richard Damon on April 15, 2008
While I am not familiar with that particular processor, but from what that formula looks like, it wants portPRESCALE to not be the scale factor but the value of the prescaling port, which is typically 1 less than the divide ratio. A divide by 20 would count from 19 down to 0 and restart.

You would need to look at how the value of portPRESCALE is used elsewhere to confirm.

RE: Timebase timer problem in the STR75x port?

Posted by Ricky on April 16, 2008
I found it! In port.c in the prvSetupTimerInterrupt function, the code currently reads:

TB_InitStructure.TB_Prescaler = portPRESCALE;
TB_InitStructure.TB_AutoReload = ( ( configCPU_CLOCK_HZ / ( portPRESCALE + 1 ) ) / configTICK_RATE_HZ ) + 1;

where configCPU_CLOCK_HZ = 60,000,000, portPRESCALE = 20, and configTICK_RATE_HZ = 1000.

The correct code should be:

TB_InitStructure.TB_Prescaler = portPRESCALE - 1;
TB_InitStructure.TB_AutoReload = ( configCPU_CLOCK_HZ / portPRESCALE ) / configTICK_RATE_HZ;

The Prescaler register expects the prescaler value to be zero-based so a prescaler of 20 needs to be written as 19. The current method in FreeRTOS is essentially correct when using a prescaler of 21 but a slight inaccuracy is introduced because 60 MHz / 21 is a fractional number that gets rounded to an integer. This rounding, plus the addition of 1 at the end, causes an inaccuracy of .03% (one second in 55 minutes), not enough to worry about for the vast majority of applications out there, but it's too much for mine so I use the corrected version.

My first post assumed an error of one second every 20 seconds but that was because I forgot that my prescaler was set to a different value.

RE: Timebase timer problem in the STR75x port?

Posted by Richard on April 16, 2008
Thanks for the feedback - I have updated the files in SVN.

Regards.

RE: Timebase timer problem in the STR75x port?

Posted by Ricky on October 14, 2009
I just downloaded version 5.4.2 and I see that the prescaler code has been updated but the auto reload value hasn't. I'm still off by around 5% in the timing so I'm going to modify Port.c in my project to give me the correct timing. I'll be using ( configCPU_CLOCK_HZ / portPRESCALE ) / configTICK_RATE_HZ.

Was there some reason it wasn't changed in the new version? And will it be changed in version 6? Thanks!

RE: Timebase timer problem in the STR75x port?

Posted by Richard on October 15, 2009
Simply an error. I will correct for V6.

Regards.


[ Back to the top ]    [ About FreeRTOS ]    [ Sitemap ]    [ ]




Copyright (C) 2004-2010 Richard Barry. Copyright (C) 2010-2016 Real Time Engineers Ltd.
Any and all data, files, source code, html content and documentation included in the FreeRTOSTM distribution or available on this site are the exclusive property of Real Time Engineers Ltd.. See the files license.txt (included in the distribution) and this copyright notice for more information. FreeRTOSTM and FreeRTOS.orgTM are trade marks of Real Time Engineers Ltd.

Latest News:

FreeRTOS V9.0.0 is now available for download.


Free TCP/IP and file system demos for the RTOS


Sponsored Links

⇓ Now With No Code Size Limit! ⇓
⇑ Free Download Without Registering ⇑


FreeRTOS Partners

ARM Connected RTOS partner for all ARM microcontroller cores

Renesas Electronics Gold Alliance RTOS Partner.jpg

Microchip Premier RTOS Partner

RTOS partner of NXP for all NXP ARM microcontrollers

Atmel RTOS partner supporting ARM Cortex-M3 and AVR32 microcontrollers

STMicro RTOS partner supporting ARM7, ARM Cortex-M3, ARM Cortex-M4 and ARM Cortex-M0

Xilinx Microblaze and Zynq partner

Silicon Labs low power RTOS partner

Altera RTOS partner for Nios II and Cortex-A9 SoC

Freescale Alliance RTOS Member supporting ARM and ColdFire microcontrollers

Infineon ARM Cortex-M microcontrollers

Texas Instruments MCU Developer Network RTOS partner for ARM and MSP430 microcontrollers

Cypress RTOS partner supporting ARM Cortex-M3

Fujitsu RTOS partner supporting ARM Cortex-M3 and FM3

Microsemi (previously Actel) RTOS partner supporting ARM Cortex-M3

Atollic Partner

IAR Partner

Keil ARM Partner

Embedded Artists