ACS405CS Semtech Corporation, ACS405CS Datasheet - Page 11

• Manufacturer Part Number: ACS405CS
• Description: Acapella Optical Modem ic
• Manufacturer: Semtech Corporation

(DPLL) system makes periodic adjustments to the
RCLK clock to ensure that the average frequency is
exactly the same as the far-end TCLK clock. In
summary, each TCLK is an independent master clock
and each RCLK a slave of the far-end TCLK clock.
The relationship between TmCLK and RmCLK are
treated similarly.
In slave mode, the TCLK and RCLK clock is derived
from the TCLK clock of the far-end modem, such
that their average frequencies are identical. Clearly,
it is essential that only one modem within a
communicating pair is configured in slave mode.
The CKC pin should be forced to GND, so that
TCLK is always configured as an output. Since only
one device in the modem pair may be configured in
slave mode, the mode also selects active lock. See
section headed, Locking Modes .
The relationship between TmCLK and RmCLK are
treated similarly. The CKM pin should be forced to
GND, so that TmCLK is always configured as an
output.
In master mode, the local RCLK clock is internally
generated from the local TCLK clock.
TCLK clock may be internally or externally
generated. Master mode is only valid if the far-end
device is configured in slave mode or if the far-end
TCLK clock is derived from the far-end RCLK clock.
Only one modem within a communicating pair may
be configured as a master.
The relationship between TmCLK and RmCLK are
treated similarly.
In local loopback mode, TPN and TmD data is looped
back inside the near-end modem and is output at its
own RPN and RmD outputs.
Data received from the far-end device is ignored,
except to maintain lock. If concurrent requests occur
for local and remote loopback, local loopback is
selected.
The local loopback diagnostic mode is used to test
data flow up to, and back from, the local ACS405CS
and does not test the integrity of the link itself.
Therefore, local loopback operates independently
of synchronisation with a second modem (i.e. DCD
may be High or Low).
In remote loopback mode, the near-end modem
sends a request to the far-end modem to loopback
its received data, thus returning the data so that it
appears at the RPN and RmD of the initiating
modem.
Full-Duplex Slave
Full-Duplex Master
Local Loopback
Remote Loopback
The local
11
Both modems are exercised completely, as well as
the Lasers/LEDs and the fiber optic link. The remote
loopback test is normally used to check the integrity
of the entire link from the near-end (initiating
modem).
Whilst a device is responding to a request for
remote loopback from the far-end, requests from
the near-end to initiate remote loopback will be
ignored.
Communicating modems attain a stable state when
the “transmit window” of one modem coincides with
the “receive window” of the other, allowing for delay
through the optical link. Adjustments to machine
cycles are made automatically during operation, to
compensate for differences in XTAL frequencies
which
synchronisation.
When both modems are configured in drift lock,
synchronisation described above depends on a
difference in the XTAL or system clock frequencies
at each end of the link, and the greater the
difference the faster the locking. Therefore, if the
difference between XTAL frequencies is very small
(a few ppm), automatic locking may take tens of
seconds or even minutes. For this reason, normally
only one modem in the communication pair will be
configured in drift lock mode.
Drift lock will not succeed if the two modems are
driven by an external XTAL clock derived from a
single source (i.e. tolerance of 0 ppm).
Active lock mode may be used to accelerate
synchronisation of a pair of communicating modems
so that they achieve lock in less than 1 second.
Active lock reduces the machine cycle of the device
by 0.5 % ensuring that the receive window moves
swiftly through the transmit window of the opposing
modem. To effect active lock, one modem should
be permanently configured in drift lock and the other
in active lock. If this is not possible because the
system mandates that all modems are peers (
configured identically ), then the same effect may
be realised by temporarily invoking lock for a short
time after power-up. This is achieved as follows:
connect pins DM1, DM2 and DM3 together attaching
the node to an RC arrangement, with the capacitor
to VDD and the resistor to GND, to create a 5 V to
should be Ca. 5 seconds.
Active lock will succeed even when communicating
devices are driven from clocks derived from a single
source (0 ppm).
Drift Lock
Active Lock Mode
0 V ramp on power-up. The RC time-constant
would
otherwise
cause
ACS405CS Issue 4.3 June 1999.
loss
of