how to communicate with a new external peripheral

sorry, my english is poor.

i want to connect with Nios II system(SOPC Builder) and an altsynram(User Logic)

now, i want to write a data (16 signed bits) to altsynram(data bits=16, addr bits=8)

but the altsynram's CLK is connect to external oscillator (12.288MHz), i can't write data to altsynram properly.(my Nios II system's CLK is 50MHz) Orz

example:

IOWR(RAM_BASE, 0x00, 0x1234);

IOWR(RAM_BASE, 0x01, 0x5678);

x=IORD(RAM_BASE, 0x00);

y=IORD(RAM_BASE, 0x01);

correct answer should be x=0x1234, y=0x5678, but sometimes x=0x0000, y=0x5678 or x=0x1234, y=0x1234 or etc.

my Nios II system and external oscillator are asynchronous, how to improve it??

thx <(_ _)>