FIN24C_06 Datasheet, PDF (7/25 Pages) Fairchild Semiconductor – uSerDes™Low-Voltage 24-Bit Bi-Directional Serializer/Deserializer

Serializer Operation Mode
The serializer configuration is described in the following sections. The basic serialization circuitry works essentially the
same in these modes, but the actual data and clock streams differ depending on if CKREF is the same as the
STROBE signal or not. When CKREF equals STROBE, the CKREF and STROBE signals are hardwired together as
one signal. When CKREF does not equal STROBE, each signal is distinct and CKREF must be running at a frequency
high enough to avoid any loss of data condition. CKREF must never be a lower frequency than STROBE.
Serializer Operation: (Figure 4)
DIRI = 1,
CKREF = STROBE
The Phase-Locked Loop (PLL) must receive a stable CKREF signal to achieve
lock prior to any valid data being sent. The CKREF signal can be used as the data
STROBE signal, provided that data can be ignored during the PLL lock phase.
Once the PLL is stable and locked, the device can begin to capture and serialize
data. Data is captured on the rising edge of the STROBE signal and then serial-
ized. The serialized data stream is synchronized and sent source synchronously
with a bit clock with an embedded word boundary. Serialized data is sent at 26
times the CKREF clock rate. Two additional data bits are sent that define the word
boundary. When in this mode, the internal deserializer circuitry is disabled; includ-
ing the serial clock, serial data input buffers, the bidirectional parallel outputs, and
the CKP word clock. The CKP word clock is driven HIGH.
DPI[1:24] WORD n-1
CKREF
DSO b24 b25 b26 b1 b2 b3 b4
CKS0
WORD n
WORD n+1
b22 b23 b24 b25 b26 b1 b2 b3 b4 b5
WORD n-2
WORD n-1
WORD n
Figure 4. Serializer Timing Diagram (CKREF equals STROBE)
Serializer Operation: (Figure 5),
DIRI = 1,
CKREF does not = STROBE
If the same signal is not used for CKREF and STROBE, the CKREF signal must
be run at a higher frequency than the STROBE rate to serialize the data correctly.
The actual serial transfer rate remains at 26 times the CKREF frequency. A data
bit value of zero is sent when no valid data is present in the serial bit stream. The
operation of the serializer otherwise remains the same.
The exact frequency that the reference clock needs is dependent upon the stabil-
ity of the CKREF and STROBE signal. If the source of the CKREF signal imple-
ments spread spectrum technology, the maximum frequency of this spread
spectrum clock should be used in calculating the ratio of STROBE frequency to
the CKREF frequency. Similarly if the STROBE signal has significant cycle-to-
cycle variation, the maximum cycle-to-cycle time needs to be factored into the
selection of the CKREF frequency.
CKREF
DP[1:24] WORD n-1
STROBE
WORD n
DSO
CKS0
b1 b2 b3 b4 b5 b6 b7
b22 b23 b24 b25 b26
WORD n+1
b1 b2 b3
No Data
WORD n-1
No Data
WORD n
Figure 5. Serializer Timing Diagram (CKREF does not equal STROBE)
©2005 Fairchild Semiconductor Corporation
FIN24C Rev. 1.0.2
7
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