How to calculate and set the SPI speed ?

[jean_prieur]

Posted on December 19, 2013 at 15:03

Hello everybody,

I communicate with a popular screen, the DOGM128 (ST7565RLCD Controller) with my STM32F4 uC. Sometimes, when I power on my board, the screen have troubles to be initialized (it remains white, whatever I do). I think it's due to the SPI speed, that I can't estimate. The max SPI clock speed of the screen controller is 20MHz. This is my SPI screen init :

RCC_APB2PeriphClockCmd(RCC_APB2Periph_SYSCFG, ENABLE);
GPIO_InitTypeDef GPIO_InitStructure;
// CLOCK and MOSI Pins declaration
GPIO_InitStructure.GPIO_Pin = PIN_SCL|PIN_SI;
GPIO_InitStructure.GPIO_Mode = GPIO_Mode_AF;
GPIO_InitStructure.GPIO_OType = GPIO_OType_PP;
GPIO_InitStructure.GPIO_Speed = GPIO_Speed_100MHz;
GPIO_InitStructure.GPIO_PuPd = GPIO_PuPd_DOWN;
GPIO_Init(GPIOB, &GPIO_InitStructure);
// CS pin
GPIO_InitStructure.GPIO_Pin = PIN_CS|PIN_A0;
GPIO_InitStructure.GPIO_Mode = GPIO_Mode_OUT;
GPIO_InitStructure.GPIO_OType = GPIO_OType_PP;
GPIO_InitStructure.GPIO_Speed = GPIO_Speed_100MHz;
GPIO_InitStructure.GPIO_PuPd = GPIO_PuPd_NOPULL;
GPIO_Init(GPIOB, &GPIO_InitStructure);

// SI & SCL = Alternate Functions
GPIO_PinAFConfig(GPIOB, GPIO_PinSource10, GPIO_AF_SPI2);
GPIO_PinAFConfig(GPIOB, GPIO_PinSource15, GPIO_AF_SPI2);
GPIO_PinAFConfig(GPIOB, GPIO_PinSource13, GPIO_AF_SPI2); 

SPI_InitTypeDef SPI_InitStruct;
SPI_I2S_DeInit(SPI2);
SPI_InitStruct.SPI_Direction = SPI_Direction_2Lines_FullDuplex; 
SPI_InitStruct.SPI_Mode = SPI_Mode_Master;
SPI_InitStruct.SPI_DataSize = SPI_DataSize_8b;
SPI_InitStruct.SPI_CPOL = SPI_CPOL_Low; 
SPI_InitStruct.SPI_CPHA = SPI_CPHA_1Edge; 
SPI_InitStruct.SPI_NSS = SPI_NSS_Soft;
SPI_InitStruct.SPI_BaudRatePrescaler = SPI_BaudRatePrescaler_2;
SPI_InitStruct.SPI_FirstBit = SPI_FirstBit_MSB;
SPI_InitStruct.SPI_CRCPolynomial = 7;
SPI_Init(SPI2, &SPI_InitStruct);
SPI_Cmd(SPI2, ENABLE);

With SPI_BaudRatePrescaler_2 : The screen works almost everytime

With SPI_BaudRatePrescaler_4 :

The screen works half the time

With SPI_BaudRatePrescaler_8 :

The screen never works

So how can I increase the SPI speed, in order to try to solve my problem ?

[francescatodiego]

Posted on December 28, 2013 at 09:56

OK the Main clock is configured for 168MHz

APB1 and APB2 can be changed with RCC_CFGR fields PPRE1 and PPRE2 File system_stm32f4xx.c SetSysclock procedure contain this code

RCC->CFGR |= RCC_CFGR_HPRE_DIV1;
#if defined (STM32F40_41xxx) || defined (STM32F427_437xx) || defined (STM32F429_439xx)
/* PCLK2 = HCLK / 2*/
RCC->CFGR |= RCC_CFGR_PPRE2_DIV2;
/* PCLK1 = HCLK / 4*/
RCC->CFGR |= RCC_CFGR_PPRE1_DIV4;
#endif /* STM32F40_41xxx || STM32F427_437x || STM32F429_439xx */
#if defined (STM32F401xx)
/* PCLK2 = HCLK / 2*/
RCC->CFGR |= RCC_CFGR_PPRE2_DIV1;
/* PCLK1 = HCLK / 4*/
RCC->CFGR |= RCC_CFGR_PPRE1_DIV2;
#endif /* STM32F401xx */

I don't know

what device

you

are using

for

the

F

407

that I use

: AHB max 168MHz --> HPRE = 0 (RCC_CFGR_HPRE_DIV1) use PPL frequency output = 168MHz APB1 max 42MHz --> PPRE1 = 101 (RCC_CFGR_PPRE2_DIV4) datasheet max frequency APB2 max 84MHz --> PPRE2 = 100 (RCC_CFGR_PPRE2_DIV2) datasheet max freqeuncy SPI2 prescaler divide APB1 clock

For your problem

try to

fix

the clock line

with

a

resistor

pullup

/

dn

(sorry my mistake use a pullup on CS line

but you probably

already do

)

at startup

until the

micro

does not configure

the

SPI

line

the clock line is is held to fixed value

[jean]

Posted on February 12, 2014 at 17:10

Sorry for the delay I was not here during a long long time....

''For your problem

 

try to

 

fix

 the clock line 

with

 

a

 

resistor

 

pullup

 

/

 

dn

 (sorry my mistake use a pullup on CS line 

but you probably

 

already do

)'' > My CS line is pulled with the integrated pullup. But it change nothing to my problem...

''at startup

 

until the

 

micro

 

does not configure

 

the

 

SPI

 

line

 the clock line is is held to fixed value''> I don't understand... Should I fix myself the clock line to a fixed value at the startup ? For example to the high level ?Thanks !

[francescatodiego]

Posted on February 13, 2014 at 08:15

I reread the thread

if you're sure that the CS is always high during display startup the problem isn't a spurious switching of the SPI at startup Another assumption :you're too fast and you start the configuration of the display before it has finished the powerup procedure. Adding a delay before init LCD may solve. I have a doubt: The first post contain code init for SPI2 but after you say that you have added: RCC_APB1PeriphClockCmd (RCC_APB1Periph_SPI2, ENABLE), without this call the SPI2 can't work Are you sure use SPI2 ? You can check the SPI lines with oscilloscope ?

[jean_prieur]

Posted on March 12, 2014 at 12:09

hello,

I think I tried everything to solve my problem but the init error still occurs. I try to init my screen after a long delay from the SPI2 init (yes I'm sure to use the SP2 :) ) but no change.My board is USB powered, with a mechanical switch to ON/OFF the 5V power. When I plug the USB connector with the switch ON the problem never happend. But when I use the ON/OFF switch, sometime (like 1 time on 20) there is a perpetual black screen. Strange isn't it ?

[Tesla DeLorean]

Posted on March 12, 2014 at 15:25

Strange isn't it ?

Stranger perhaps that your description of the problem is still anecdotal, rather than having some clear analysis supported by timings observed on a scope or logic analyser, and contrasting the behaviour between working and failing conditions.

[francescatodiego]

Posted on March 12, 2014 at 16:21

EADOG LCD controller ST7565R Ver 1.5 page 40:

When the power is turned on, the IC internal state becomes unstable,  and  it  is  necessary  to  initialize  it  using  the  /RES terminal.  After  the  initialization,  each  input  terminal  should be controlled normally. The problem can be solved only using RST pin (I have a lot of board with same display that working without problem)

[jean_prieur]

Posted on March 12, 2014 at 18:15

Yes I think it's the only solution... it's a pity that the screen chip don't manage that, PCB designers have to put another track just for reset the screen at startup...