begin with Slow i2c 100kHz mode about ssd1306ascii HOT 8 OPEN

I will add a call to set the I2C speed. This is the function from the Wire library.

void twi_setFrequency(uint32_t frequency)
{
  TWBR = ((F_CPU / frequency) - 16) / 2;
  
  /* twi bit rate formula from atmega128 manual pg 204
  SCL Frequency = CPU Clock Frequency / (16 + (2 * TWBR))
  note: TWBR should be 10 or higher for master mode
  It is 72 for a 16mhz Wiring board with 100kHz TWI */
}

The comment implies this works from about 40 kHz to 444,000 kHz with FCPU = 16 MHz.

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The above function works on any AVR board without including Wire.

To set 100 kHz on a 16 MHz AVR board you can just add a line like this.

TWBR = 72;

I have added a setClock() call to the AvrI2c so you will be able do the following in the next version of SSD1306AsciiAvrI2c.

oled.setI2cClock(100000L);

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I just do :
Wire.setClock(2500000L);
to change the i2c speed to my favorite speed (Yes I need High speed)

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KK but I'm not using Wire, just SSD1306Ascii and SSD1306AsciiAvrI2c

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@greiman
will
oled.setI2cClock(100000L);
affects communication with my other I2c devices too or just the SSD1306 ?
I want to use different speeds for my various slower devices. Today I speed up the clock when talking to the SSD1306 and reduce the speed just after to allow my other devices to work

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deladriere
Changing the I2C frequency on an AVR board is global. There is a single TWBR baud rate register. You must set the I2C frequency in your program before using a device.

MasterBlatter
I published an update that allows setting the I2C frequency with AvrI2c.

There are two methods.
You can change the default frequency by editing SSD1306Ascii.h.

/** AvrI2c uses 400 kHz fast mode if AVRI2C_FASTMODE is nonzero else 100 kHz. */
#define AVRI2C_FASTMODE 1

You can call oled.setI2cClock(frequency) in your sketch.

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@greiman
Thanks for the update
My board is a SAMD21 clone and I cannot have a global change of speed because my other device is slower than the display.
So I guess I'll stay with Wire.setClock(high-speed); before driving any SSD1306 command and Wire.setClock(low-speed); just after them

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deladriere

I guess this is a universal problem for Wire on all boards.

Too bad the Wire library is not like SPI. The SPI beginTransaction() protocol insures each device uses the proper clock.

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