GS9021-CFU データシートの表示（PDF） - Gennum -> Semtech
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GS9021-CFU Datasheet PDF : 26 Pages
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3.3 CRC Calculation And Updating
PIN
LOGIC OPR
HOST BIT
INCOMING FF CRC
OUTGOING FF CRC
INCOMING AP CRC
OUTGOING AP CRC
Since the device has the potential of modifying the full-field
and active picture data with features like ITU-R-601 clipping
and TRS insertion, the full field and active picture CRC
values must be calculated for both the incoming and
outgoing data streams. The calculated CRC values based
on the incoming data stream are used for comparison with
the embedded CRC values. However, the calculated CRC
values based on the outgoing data stream are the ones
inserted into the data stream. As a result, the CRC values in
the outgoing data stream correctly reflect the contents of
the outgoing data stream.
The INCOMING FF and AP CRC values for the Full Field
(FF) and Active Picture (AP) regions can be read from the
HOSTIF read table. Similarly, the OUTGOING (calculated)
FF and AP CRC values for the Full Field and Active Picture
regions can be read from the HOSTIF read table.
3.4 Validity Bit
PIN
LOGIC OPR
HOST BIT
FFV
APV
The VALIDITY (V) bits (as per SMPTE 165) present in the
incoming EDH packet are used to indicate whether the CRC
values are valid or invalid. If the V bit is HIGH, the CRC
value is considered valid. In this case, the incoming CRC
value is compared with the calculated CRC value to identify
errors. If the V bit is LOW, the incoming CRC is invalid and a
CRC comparison is not performed. If the device receives an
EDH packet with the V bit set LOW it behaves as follows:
1. EDH = 0 (Not asserted for an invalid CRC)
2. EDA = EDAin "OR" EDHin (EDA calculated as usual)
3. A new calculated CRC value replaces the invalid one in
the output EDH packet
4. The V bit will be set HIGH in the output EDH packet
5. Depending on whether one or both of FFV or APV is low,
the Unknown Error Status (UES) flag corresponding to
either FF or AP or both, is set HIGH in the output data.
(No CRC check could be performed, so the data may or
may not contain errors).
The incoming V bits for the Full Field and Active Picture
regions are available in the HOSTIF read table as FFV and
APV, respectively. Outgoing full field (FFV) and active
picture (APV) validity bits are set HIGH unless explicitly
over-written through the HOSTIF write table or the flag port.
3.5 Ancillary Checksum Verification
PIN
LOGIC OPR
HOST BIT
ANC_CHKSM
EDH_CHKSM
For each received ANC packet in the incoming data, the
device compares the calculated checksum value to the
embedded checksum for that ANC packet. If the
checksum values do not match for any ANC packets within
a field, an error is reported via the ANC EDH flag in the EDH
packet. In addition, if the ANC_CHKSM input pin or
HOSTIF write table bit is asserted HIGH, the ancillary
checksum correction block is enabled and the checksum in
the ANC packet is replaced with the calculated one. This
update is required to prevent the ANC data error from being
flagged at every downstream EDH chip.
When implementing applications which use the EDH core
(ie. BYPASS_EDH set LOW), the ANC_CHKSM function will
indicate a downstream FF/AP EDH error when an illegal/
non-allowed (3FCH-3FFH) ANC_CHKSM input value is
detected. As such, these values should not be present in
the incoming data and the corresponding FF/AP EDH errors
should not occur. However, if the user wishes to disable the
ANC_CHKSM function, it can be deactivated by setting
both the ANC_CHSKM pin and the ANC_CHKSM host
interface bit LOW.
If the chip is receiving ANC EDH flag information through
the flag port or the HOSTIF, then the ANC EDH flag
generated by the ancillary checksum verification block will
be overwritten. However, the additional FF/AP EDH flag will
still appear at the next downstream chip if an illegal
checksum of 3FCH-3FFH was detected and the
ANC_CHKSM function was enabled.
If a checksum error is detected in the EDH packet itself, an
additional separate error flag, EDH_CHKSM is set HIGH in
the HOSTIF read table.
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