the CRC values to correctly reflect the newly modified data.
To prevent the output EDH chip from indicating erroneous
CRC errors on each field, the GS9021 has two special
modes of operation, CRC_MODE and FLAG_MAP MODE.
packet. Finally the S[1:0] pins only change value after
receipt of an EDH packet and are thus static between
packets. During this inter-packet time, the S[1:0] pins
display a value of "01" and the FL[4:0] pins display the ANC
EDH flags from the preceding EDH packet.
3.10.1 CRC_MODE
For reliable data output on the FLAG PORT, switching the
FLAG_MAP pin when an EDH packet is exiting the device is
not advised. Also, if the EDH core is bypassed by asserting
the BYPASS_EDH pin HIGH, the flag port will always display
zeros. This is because the incoming flags (which will be
decoded and written to the HOSTIF table) will not be up-
dated to reflect the condition of the input data, and as a
result no outgoing flags will be generated (the FLAG PORT
only displays the outgoing EDH flags).
In CRC_MODE, the CRC values in the EDH packet are
updated by the chip but the error flags are preserved and
unaltered, unless they are overwritten via the HOSTIF or the
FLAG PORT. This mode should be used by the output EDH
chip to prevent the newly processed data from creating
misleading EDH errors due to CRC mismatches. The
CRC_MODE pin takes precedence over the R/T pin in the
GS9021 with respect to the handling of the EDH flags. Thus
when CRC_MODE is HIGH, no flags are set or altered
(unless overwritten by the flags or HOSTIF port) regardless
of the state of the R/T pin. See Table 2 for the effect of the
different settings of R/T and CRC_MODE. The device is
placed in CRC_MODE by asserting the CRC_MODE pin
HIGH.
FLAG_MAP mode can be used to write EDH flags to any
chip, the most common use being applicable when the
processing circuitry following the EDH chip corrupts the
EDH packet. In this case, the FLAG_MAP mode can be
used to route the EDH flags from an input EDH chip around
the processing core and write them to an output EDH chip.
In this configuration, the input IC is in FLAG_MAP mode. It
receives the EDH packet, does normal EDH processing and
transfers the new EDH flags to the output IC. The output IC,
which is not in FLAG_MAP mode but is in write mode
(FLAG_MAP and F_R/W stay LOW) receives these flags as
they are written to it by the EDH chip. The output EDH chip
then updates the EDH packet with the new CRC values and
inserts the preserved EDH flags that have been transferred
from the input IC. A diagram of this can be found in Figure
6b.
CRC_MODE is applicable when the processing circuitry
does not corrupt the EDH packet, as illustrated in Figure 6a.
In this configuration, the input EDH chip operates in normal
mode while the output EDH chip is in CRC_MODE. In this
scenario, the input IC receives the EDH packet and does
normal EDH processing. The output IC updates the EDH
packet with new CRC values but passes the EDH flags
through unaltered. Because of this, erroneous EDH flag
handling by the second GS9021 is not performed.
3.10.2 FLAG_MAP Mode
Because the flags are output as soon as they are decoded,
the maximum processing latency supported between the
two EDH chips is the number of clock cycles in the shortest
field of the standard minus 15 clock cycles.
In FLAG_MAP mode, the FLAG PORT is used to read EDH
flags from the GS9021 and write them to another EDH chip.
To enable FLAG_MAP mode, the FLAG_MAP mode pin and
the F_R/W pin must be asserted HIGH (set F_R/W at least
one cycle ahead of FLAG_MAP). After a delay of tFEN, the
FL[4:0] and S[1:0] pins of the FLAG PORT become outputs
and can be connected to the chip which you wish the
GS9021 to write the FLAG data to. In this mode the GS9021
automatically increments the value of S[1:0] and
subsequently displays the appropriate flags on the FL[4:0]
port, synchronous to the rising edge of PCLKOUT. This is
illustrated in Figure 5d.
For example, D1 has one field of 262 x 1716 = 449592
clock cycles, and one field of 263 x 1716 = 451308 clock
cycles. Thus the maximum latency for D1 is 449592 - 15 =
449577 clock cycles.
Any additional latency requires that the flags be delayed
before they can be piped to the output chip. Since writing to
the flag port takes precedence over the HOSTIF writing, if
any of the flags need to be forced at the output EDH chip,
external logic in the routing path must be added.
Alternately, the HOSTIF of the EDH chip can be used to
perform any additional flag masking.
Figure 5d displays three properties of the FLAG PORT in
FLAG_MAP mode.
First, each data is present on the FLAG PORT for two clock
cycles to eliminate any setup time violations that might
occur due to clock data skew between chips placed far
apart. However, the designer must still ensure that the hold
time is satisfied. Second, the S[1:0] pins never cycle to the
value of "11" in FLAG_MAP mode since the values
contained in the FL[4:0] register when S[1:0] ="11" are not
considered EDH flags. Also, the chip cycles S[1:0] in the
sequence "01", "00", "10" since this is the order in which the
flags are stored and subsequently decoded from the EDH
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GENNUM [ GENNUM CORPORATION ]
