© 2006 Fairchild Semiconductor Corporation

www.fairchildsemi.com

FIN224AC Rev.1.1.2

7

When operating in mode 3, the effective serial speed is divided by two. This

mode has been implemented to accommodate cases where the reference clock

frequency is high compared to the actual strobe frequency. The actual strobe

frequency must be less than or equal to 50% of the CKREF frequency for this

mode to work properly. This mode, in all other ways, operates the same as

described in the section where CKREF does not equal STROBE.

Figure 7. CKREF > 2x STROBE Frequency; Mode 3 Operation (S1 = S2 = 1)

A third method of serialization can be acheived by providing a free-running bit

clock on the CKSI signal. This mode is enabled by grounding the CKREF signal

and driving the DIRI signal HIGH. At power-up, the device is configured to accept

a serialization clock from CKSI. If a CKREF is received, this device enables the

CKREF serialization mode. The device remains in this mode even if CKREF is

stopped. To re-enable this mode, the device must be powered down and then

powered back up with a “logic 0” on CKREF.

Figure 8. Serializer Timing Diagram Using Provided Bit Clock (No CKREF)

No Data

WORD n-1

WORD n

DP[1:24]

WORD n-1

WORD n

WORD n+1

No Data

CKREF

DSO

b1 b2 b3 b4 b5 b6 b7

b22 b23 b24 b25 b26

b1 b2 b3

CKS0

STROBE

WORD n-1

WORD n-1

WORD n

No Data

No Data

DP[1:24]

DSO

CKS0

CKSI

STROBE

b1 b2 b3

b1 b2 b3

b4 b5 b6 b7

b22 b23 b24 b25 b26

WORD n+1

WORD n

Serializer Operation: (Figure 7),

MODE 3 (S1 = S2 = 1), DIRI = 1,

CKREF Divide by 2 mode

Serializer Operation: (Figure 8),

DIRI = 1, No CKREF