ds1307 i2c not working when crystal is > 20mhz

[raman00084]

i am facing a problem in ds1307 i am using this as a ram to store values. when i increase my crystal clock speed to >20mhz i cannot read are write in ds1307 . i am using ccs complier and pic 18f4520 ic, other i2c devices like pcf8574 port expander is working fine if the clock is >20mhz but ds1307 is not working kindly help


regards
kalyan

 

[andre_luis]

Working with access to serial devices is sometimes necessary to insert delays to avoid that. By the way, had you checked on DS1307 information concerning to the best choice of I2C pullup resistors ?

 

[paulfjujo]

hello,

What is your I2C bus speed 100Khz or 400Khz ?
I2C software or I2C Hardware ?
and SSPAD register value ?


DS1307 allready tested OK at I2C bus speed 400Khz ( and FOSC 16MHz) ..not checked at FOSC=20MHz

 

[raman00084]

bus speed 400khz. i2c hardware

 

[paulfjujo]

Can you test to switch I2C speed to 100Khz when adressing device DS1307 ?
I2C speed must be independant of FOSC MCU ..
Pull up Resistance on I2C bus ? 2,2K or less...