I have looked at DS1306 datasheet and found the Figure "TYPICAL OPERATING CIRCUIT" at page 1. It shows that SDI and SDO may be tied together.

I think that is only for 3wire communication, i cannot get thru with spi mode and data lines connected.

You see, 3-wire has some "uncompatibles" with SPI. For example, it sends LSB first so you will need re-arrange bits.

This is not problem for 8252, AFAIK there is a DORD bit in spi configuration register which control data order.

I`ve try this configurations:

(this is what you suggested)
SERMODE=GND (3wire)
CPOL=0
CPHA=1
when i connect data lines i recive 0x00

SERMODE=VCC (SPI)
CPOL=0/1
CPHA=1
this is working only with separate data lines

and for last i`ve tried this one:

SERMODE=GND
CPOL=0
CPHA=1
DORD=1

this is actualy working but only with tri-state buffer
between master and slave`s. Now i can read temp and time from both chips connected. Dont know i`m able to write i will check it later.

I still dont know where the problem resides... Any ideas ?

btw: ds1306 indeed seems like both CPOL values

Thanks for help
Lucas Bajger