how to calculate i2c timing for stm32l0

Question

I got a stm32l0 series procesor and i dont know how to calculate i2c timing. I want to work with Bosh bme680/bme280 sensor. I find only Timming configuration tool for stm32F0/f3. Some1 know how to calculate it?

Answer 1

TLDR : RM0377 Reference manual "Ultra-low-power STM32L0x1 advanced Arm®-based 32-bit MCUs", page 614 contains examples.

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Note: I haven't actually worked with I2C yet. All the following is based on documentation.

1. UM1749 User Manual "Description of STM32L0 HAL and Low Layer drivers" (p. 233):

   uint32_t I2C_InitTypeDef::Timing Specifies the I2C_TIMINGR_register value. This parameter calculated by referring to I2C initialization section in Reference manual

2. RM0377 Reference manual "Ultra-low-power STM32L0x1 advanced Arm®-based 32-bit MCUs" (p.641):

   Timing register (I2C_TIMINGR)

   (...)

   PRESC[3:0] : Timing prescaler

   This field is used to prescale I2CCLK in order to generate the clock period t_PRESC used for data setup and hold counters (refer to I2C timings on page 587 ) and for SCL high and low level counters (refer to I2C master initialization on page 602 ).

   t_PRESC = (PRESC+1) x t_I2CCLK

   SCLDEL[3:0] : Data setup time This field is used to generate a delay t_SCLDEL between SDA edge and SCL rising edge. In master mode and in slave mode with NOSTRETCH = 0, the SCL line is stretched low during t_SCLDEL.

   t_SCLDEL = (SCLDEL+1) x t_PRESC

   Note: t_SCLDEL is used to generate t_SU:DAT timing.

   SDADEL[3:0] : Data hold time This field is used to generate the delay t_SDADEL between SCL falling edge and SDA edge. In master mode and in slave mode with NOSTRETCH = 0, the SCL line is stretched low during t_SDADEL.

   t_SDADEL = SDADEL x t_PRESC

   Note: SDADEL is used to generate t_HD:DAT timing.

   SCLH[7:0] : SCL high period (master mode) This field is used to generate the SCL high period in master mode.

   t_SCLH = (SCLH+1) x t_PRESC

   Note: SCLH is also used to generate t_SU:STO and t_HD:STA timing.

   SCLL[7:0] : SCL low period (master mode) This field is used to generate the SCL low period in master mode.

   t_SCLL = (SCLL+1) x t_PRESC

   Note: SCLL is also used to generate t_BUF and t_SU:STA timings.

   More info about prescalers can be found in paragraphs about timers (p. 433).

3. RM0377 (p. 581):

   The interface is connected to the I2C bus by a data pin (SDA) and by a clock pin (SCL). It can be connected with a standard (up to 100 kHz), Fast-mode (up to 400 kHz) or Fast-mode Plus (up to 1MHz) I2C bus.

   So in order to communicate the I2C clock needs to be have appropriate frequency.

4. RM0377 (p. 583):

   This independent clock source can be selected from the following three clock sources:

   • PCLK1: APB1 clock (default value)
   • HSI16: internal 16 MHz RC oscillator
   • SYSCLK: system clock Refer to Section 7: Reset and clock control (RCC) for more details.

5. RM0377 (p. 604):

6. RM0377 (p. 602):

   t_SCL = t_SYNC1 + t_SYNC2 + {[(SCLH+1) + (SCLL+1)] x (PRESC+1) x t_I2CCLK}

   The duration of t_SYNC1 depends on these parameters:

   • SCL falling slope
   • When enabled, input delay induced by the analog filter.
   • When enabled, input delay induced by the digital filter: DNF x t_I2CCLK
   • Delay due to SCL synchronization with I2CCLK clock (2 to 3 I2CCLK periods)

   The duration of t_SYNC2 depends on these parameters:

   • SCL rising slope
   • When enabled, input delay induced by the analog filter.
   • When enabled, input delay induced by the digital filter: DNF x t_I2CCLK
   • Delay due to SCL synchronization with I2CCLK clock (2 to 3 I2CCLK periods)

7. BME280 Datasheet (p. 30):

   All modes (standard, fast, high speed) are supported.

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Gathering it all up : Basing on the I2C clock frequency, and the speed you want to use, you need to choose such values of PRESC , SCLDEL , SDADEL , SCLH , SCLL that you comply to the I2C-SMBUS specification clock timings using formula t_SCL = t_SYNC1 + t_SYNC2 + {[(SCLH+1) + (SCLL+1)] x (PRESC+1) x t_I2CCLK}

There are also some I2C_TIMINGR register configuration examples in RM0377 at page 614.