I2C speed not working...

Using Arduino Programming Questions
#1

I followed the instructions per my I2C DAC's datasheet to change it to fast mode, and upped the I2C frequency by adding this line to my code:

#define TWI_FREQ 1000000L

(1Mhz).

However, when timing data transfers with the micros() function, it still takes 8 microseconds, regardless of the frequency.

Thanks! baum

#2

First, count the number of zeros. The value you specified does not match your comment.

Second, the micros() function increments in 4 microsecond increments. It makes a lousy timer for things that happen as rapidly as I2C.

#3

I count 6 zeros (1,000,000), but thanks for the help with micros()! is there any other way of timing? Can I count the clk cycles a certain instruction takes?

baum

#4

Can I count the clk cycles a certain instruction takes?

Nearly all AVR machine instructions take a single 62.5nsec clock cycle when running the chip at 16Mhz.

Lefty

#5

Alright, but that doesn’t solve the problem… I can’t change the speed!

#6

I thought the datasheet limits the I2C frequency to 400kHz?

#7

I thought the datasheet limits the I2C frequency to 400kHz?

Yup.

#8

My DAC supports "High-speed" mode, max. freq. 3.4Mhz. (http://www.sparkfun.com/datasheets/BreakoutBoards/MCP4725.pdf)

What datasheet are you talking about?

baum

#9

The DAC may support higher speeds but the clock signal is generated from the master (Arduino) which only supports speeds up to 400kHz.

#10

Oh... where does it say this?

#11

page 215

#12

OK. I'll keep it at 400khz, which will take 40us to transfer 2 bytes, max update frequency is 25khz... do you think this is fast enough for 22khz wav, or should I use a parallel DAC?

baum

#13

I'll keep it at 400khz, which will take 40us to transfer 2 bytes, max update frequency is 25khz.

I make it more like 20kHz. Don't forget start and stop bits.

#14

The DAC allows be to continuously clock out two bytes of data, no start and stop bits needed. Though I did forget about the Ack bit between each byte... I'll go with the parallel DAC, then.

Thanks for the help! baum

#15