PIC18F4525 to DAC (PCF8591P) I2CWrite too fast?

[mister_e]

Do you have any Pull-up resistors on SDA & SCL ~4.7K?

What are your configuration fuses settings?

In your code.. what you call 'DACAddressByte' have to be %10010000 instead, and DACControlByte can't be a constant. must be a Byte variable.

Finnally, it's possible that you have a SCL frequency a bit higher than 100kHZ... but 1MHz it's impossible... unless i really missed something one thing is sure, depending of the signal you measure, if it's noise.. frequency measure could be anything that's for sure.

Anyways. Your current device support MAX 100KHz. But @20Mhz, I2CWRITE will produce a higher frequency, you need to add the following line to your code

DEFINE I2C_SLOW 1

Read I2CWRITE section in your manual for more details.