CAN bus timings

adamajames · ‎2010-02-04

Posted on February 04, 2010 at 15:28

CAN bus timings

adamajames · ‎2011-05-17

Posted on May 17, 2011 at 13:39

NOTE: Someone in a previous post did mention an excel spreadsheet, but attachments don't appear to be working yet with this new site.

adamajames · ‎2011-05-17

Posted on May 17, 2011 at 13:39

I've now tried with my settings below on a CAN bus car and I'm getting a bus error every 10 messages. which is completly unacceptable. Can anyone offer some advice?

adamajames · ‎2011-05-17

Posted on May 17, 2011 at 13:39

Still suffering from this problem. Can anyone offer some advice.

cschumann · ‎2011-05-17

Posted on May 17, 2011 at 13:39

I guess you are suffering from bit timing errors. With a APB of 36 MHZ you are out of the specs for the CAN controller which needs 8 MHz clock (or a multiple of that). Try using 32 MHz APB clock (= SystemClock 64MHz) and you will be fine.

Carsten

armmcu2 · ‎2011-05-17

Posted on May 17, 2011 at 13:39

Hi Carsten,

Actually, you are mentioning an info which is strange/new for me

Could you please clarify which ST document (section) you refer to?

armmcu.

adamajames · ‎2011-05-17

Posted on May 17, 2011 at 13:39

Excellent - you've given me some things to look into - I'll give them a go and post my results (hopefully later today).

PB

smart2 · ‎2011-05-17

Posted on May 17, 2011 at 13:39

try these combination with APB1 18 Mhz

SJW_value=CAN_SJW_1tq;

BS1_value= CAN_BS1_9tq;

BS2_value=CAN_BS2_2tq;

Prescaler_value=3;

adamajames · ‎2011-05-17

Posted on May 17, 2011 at 13:39

Hi,

I was unable to try your suggestion (Avr.smart) as I am using an external 8Mhz clock. However. I did change the PLL multiplier to make the clock 64Mhz as suggested by Carsten so this makes my APB1 (CAN peripheral) 32Mhz.

My clock function looks like this:

RCC_HSEConfig(RCC_HSE_ON);

HSEStartUpStatus = RCC_WaitForHSEStartUp();

if(HSEStartUpStatus == SUCCESS)

{

/* Enable Prefetch Buffer */

FLASH_PrefetchBufferCmd(FLASH_PrefetchBuffer_Enable);

/* Flash 2 wait state */

FLASH_SetLatency(FLASH_Latency_2);

/* HCLK = SYSCLK */

RCC_HCLKConfig(RCC_SYSCLK_Div1);

/* PCLK2 = HCLK */

RCC_PCLK2Config(RCC_HCLK_Div1);

/* PCLK1 = HCLK/2 */

RCC_PCLK1Config(RCC_HCLK_Div2);

/* PLLCLK = 8MHz * 8 = 64 MHz */

// RCC_PLLConfig(RCC_PLLSource_HSE_Div1, RCC_PLLMul_9);

RCC_PLLConfig(RCC_PLLSource_HSE_Div1, RCC_PLLMul_8);

/* Enable PLL */

RCC_PLLCmd(ENABLE);

/* Wait till PLL is ready */

while(RCC_GetFlagStatus(RCC_FLAG_PLLRDY) == RESET){}

/* Select PLL as system clock source */

RCC_SYSCLKConfig(RCC_SYSCLKSource_PLLCLK);

/* Wait till PLL is used as system clock source */

while(RCC_GetSYSCLKSource() != 0x08){}

}

I'm thrn trying the follow CAN set-up;

CAN_InitStructure.CAN_SJW=CAN_SJW_3tq;

CAN_InitStructure.CAN_BS1=CAN_BS1_10tq;

CAN_InitStructure.CAN_BS2=CAN_BS2_5tq;

CAN_InitStructure.CAN_Prescaler=4;

CAN_Init(&CAN_InitStructure);

and I see a slight improvemt - sometimes 15000 messages before an error, but then sometimes under 400

This setting

CAN_InitStructure.CAN_SJW=CAN_SJW_3tq;

CAN_InitStructure.CAN_BS1=CAN_BS1_12tq;

CAN_InitStructure.CAN_BS2=CAN_BS2_3tq;

CAN_InitStructure.CAN_Prescaler=4;

CAN_Init(&CAN_InitStructure);

is a little less reliable

This setting:

CAN_InitStructure.CAN_SJW=CAN_SJW_3tq;

CAN_InitStructure.CAN_BS1=CAN_BS1_8tq;

CAN_InitStructure.CAN_BS2=CAN_BS2_7tq;

CAN_InitStructure.CAN_Prescaler=4;

falls over after 9000 loops then 11000 loops

This setting

CAN_InitStructure.CAN_SJW=CAN_SJW_3tq;

CAN_InitStructure.CAN_BS1=CAN_BS1_7tq;

CAN_InitStructure.CAN_BS2=CAN_BS2_8tq;

CAN_InitStructure.CAN_Prescaler=4;

CAN_Init(&CAN_InitStructure);

falls over after 3000 loops then 25000 loops

Anyone?

adamajames · ‎2011-05-17

Posted on May 17, 2011 at 13:39

OK - The problem I had was I was re-initializing the CAN chip for each request. This causes data to be transmitted and if other modules are on the bus they will see this as an error and report framing errors. I simply removed the initialization and now have perfect comms with all of the settings I tried even with the board running at 72MHz (36MHz for the CAN chip).