© 2005 Fairchild Semiconductor Corporation www.fairchildsemi.com
FIN24AC Rev. 1.0.3 9
FIN24AC 22-Bit Bi-Directional Serializer/Deserializer
Embedded Word Clock Operation
The FIN24AC sends and receives serial data source
synchronously with a bit clock. The bit clock has been
modified to create a word boundary at the end of each
data word. The word boundary has been implemented
by skipping a low clock pulse. This appears in the serial
clock stream as 3 consecutive bit times where signal
CKSO remains HIGH.
To implement this sort of scheme, two extra data bits are
required. During the word boundary phase, the data tog-
gles either HIGH-then-LOW or LOW-then-HIGH depen-
dent upon the last bit of the actual data word. Table 2
provides some examples of the actual data word and the
data word with the word boundary bits added. Note that
a 24-bit word is extended to 26-bits during serial trans-
mission. Bit 25 and Bit 26 are defined with-respect-to Bit
24. Bit 25 is always the inverse of Bit 24, and Bit 26 is
always the same as Bit 24. This ensures that a “0” →
“1” and a “1” → “0” transition always occurs during the
embedded word phase where CKSO is HIGH.
The serializer generates the word boundary data bits
and the boundary clock condition and embeds them into
the serial data stream. The deserializer looks for the end
of the word boundary condition to capture and transfer
the data to the parallel port. The deserializer only uses
the embedded word boundary information to find and
capture the data. These boundary bits are stripped prior
to the word being sent out the parallel port.
LVCMOS Data I/O
The LVCMOS input buffers have a nominal threshold
value equal to half V
DDP
. The input buffers are only oper-
ational when the device is operating as a serializer.
When the device is operating as a deserializer, the inputs
are gated off to conserve power.
The LVCMOS 3-STATE output buffers are rated for a
source/sink current of 2mA at 1.8V. The outputs are
active when the DIRI signal is asserted LOW. When the
DIRI signal is asserted HIGH, the bi-directional LVCMOS
I/Os are in a HIGH-Z state. Under purely capacitive load
conditions, the output swings between GND and V
DDP
.
Unused LVCMOS input buffers must be tied off to either a
valid logic LOW or a valid logic HIGH level to prevent
static current draw due to a floating input. Unused LVC-
MOS output should be left floating. Unused bi-directional
pins should be connected to GND through a high-value
resistor. If a FIN24AC device is configured as an unidi-
rectional serializer, unused data I/O can be treated as
unused inputs. If the FIN24AC is hardwired as a deseri-
alizer, unused data I/O can be treated as unused out-
puts.
Figure 9. LVCMOS I/O
Differential I/O Circuitry
The FIN24AC employs FSC proprietary CTL I/O technol-
ogy. CTL is a low-power, low-EMI, differential swing I/O
technology. The CTL output driver generates a constant
output source and sink current. The CTL input receiver
senses the current difference and direction from the out-
put buffer to which it is connected. This differs from
LVDS, which uses a constant current source output, but
a voltage sense receiver. Like LVDS, an input source ter-
mination resistor is required to properly terminate the
transmission line. The FIN24AC device incorporates an
internal termination resistor on the CKSI receiver and a
gated internal termination resistor on the DS input
receiver. The gated termination resistor ensures proper
termination regardless of direction of data flow. The rela-
tively greater sensitivity of the current sense receiver of
CTL allows it to work at much lower current drive and a
much lower voltage.
During power-down mode, the differential inputs are dis-
abled and powered down and the differential outputs are
placed in a HIGH-Z state. CTL inputs have an inherent
fail-safe capability that supports floating inputs. When
the CKSI input pair of the serializer is unused, it can reli-
ably be left floating. Alternately both of the inputs can be
connected to ground. CTL inputs should never be con-
nected to V
DD
. When the CKSO output of the deserial-
izer is unused, it should be allowed to float.
From
Deserializer
To
Serializer
From
Control
DP[n]
Table 2. Word Boundary Data Bits
24-Bit Data Words 24-Bit Data Word with Word Boundary
Hex Binary Hex Binary
3FFFFFh 0011 1111 1111 1111 1111 1111b 1FFFFFFh 01 1111 1111 1111 1111 1111 1111b
155555h 0101 0101 0101 0101 01010 0101b 1155555h 01 0101 0101 0101 0101 0101 0101b
xxxxxxh 0xxx xxxx xxxx xxxx xxxx xxxxb 1xxxxxxh 01 0xxx xxxx xxxx xxxx xxxx xxxxb
