ACS104A Semtech Corporation, ACS104A Datasheet - Page 3

• Manufacturer Part Number: ACS104A
• Description: Acs104a Fiber Modem
• Manufacturer: Semtech Corporation

programmed using pins HD(1:2) on the TQFP44 package, in
accordance with the Table 2.
Table 2. Handshake signal bandwidth allocation
* When HD2 = HD1 = 0 super-compress mode is selected. See
section headed Super-Compress mode.
Handshake data rates which exceed the allocated bandwidth will
be delayed, and consequently result in additional skew between
handshake signals and data.
The HD pins enable the user to allocate a maximum bandwidth to
the handshake signals and thus limit the power consumption of the
device. The power consumption is, however, dependent on the
actual bandwidth used and not the bandwidth selected.
example; if the handshake signals were toggled at 1kHz the power
consumption would be the same for an allocated bandwidth of
2.5kHz as it would for an allocation of 10kHz. See section headed
Current and Power Consumption for more details.
This mode is selected when HD2 = HD1 = 0. Super-compress
mode performs a second stage of data compression, thus further
reducing the power consumption of the modem. Normally, data is
compressed in a manner which is independent of the data type. In
super-compress mode, an additional stage of compression further
reduces the data by a factor of 1 to 3 depending on the data itself.
Example: The super-compress stage will compress DC data by an
additional Compression Factor (CF) of 3, whilst data close to the
maximum frequency will not be compressed beyond the standard
CF of 1.
Super-compress mode provides benefits where the user is
interested in low average power consumption (e.g. battery life)
rather than peak power. If the intended system is idle for most of
the time with periodic bursts of activity, the additional data
compression afforded will approach a CF of 3.
To achieve low power consumption the ACS104A is active for a
small percentage of the frame (machine-cycle) known as the
'transmit' window and the 'receive' window, collectively these
windows are known as the 'active time'. Outside the 'active time'
the device is largely dormant accept for the maintenance of the
oscillator and basic 'house-keeping' functions.
Communicating modems attain a stable state known as 'locked',
where the 'transmit' window of one modem coincides with the
'receive' window of the other, allowing for the delay through the
optical link.
automatically during operation, to compensate for differences in
XTAL frequencies which cause loss of synchronisation.
The ACS104A locking algorithm is statistical, and consequently the
locking time will differ on each attempt to lock.
The diagnostic and operational modes, shown in Table 3, are
selected using the DM pins. DM3 is held high internally on the
PLCC28 package.
Table 3. Diagnostic and operational modes
Super-Compress mode
Locking
Diagnostic and Locking Modes
DM3
HD2
0
0
1
1
0
0
0
1
1
1
Advanced Communications
DM2
HD1
0*
1
0
1
0
0
1
0
1
1
Adjustments to machine cycles are made
DM1
0
1
0
1
0
1
600
10
5
2.5
Handshake
Bandwidth
Mode
Full-duplex
Full-duplex
Full-duplex
Local loopback
Remote loopback
Full-duplex
Hz
kHz
kHz
kHz
10 ms.
1 - 2 data bits
1 - 2 data bits
1 - 2 data bits
w.r.t. RxD
Skew
Lock
Memory
Random
Random
Random
Drift
Active
For
3
Local Loopback
In local loopback mode TxD data is looped back inside the near-end
modem and appears at its own RxD output. RTS, DTR and RII are
also looped back appearing at their own CTS, DSR and RIO outputs
respectively.
synchronisation between the modems is maintained.
In local loopback mode 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 ACS104A and does not test the integrity of
the link itself, i.e. local loopback operates independently of
synchronisation with a second modem.
Remote Loopback
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 RxD of the initiating modem. RTS,
DTR and RII follows the same path, returning data back to CTS,
DSR and RIO respectively of the initiating modem.
appears at the far-end modem outputs RxD, CTS, DSR and RIO. In
the process both modems are exercised completely, as well as the
LED/PINs 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 initiating modem (far-end), requests to
initiate remote loopback will be ignored.
Drift lock
Communicating modems attain a stable state where 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 would
otherwise cause loss of synchronisation.
Using drift lock, synchronisation described above depends on a
difference in the XTAL frequencies at each end of the link, and the
greater the difference the faster the locking.
difference between XTAL frequencies is very small (a few ppm),
automatic locking may take tens of seconds or even minutes.
Drift lock will not operate if the two communicating devices are driven by
a clock derived from a single source (i.e. tolerance of 0ppm).
Active Lock Mode
Active lock mode may be used to accelerate synchronisation of a
pair of communicating modems.
modems in less than 3 seconds by adjusting the machine cycles of
the modems. Active lock reduces the machine cycle of the device
by 0.5 % ensuring rapid lock. After synchronisation the machine
cycle reverts automatically to normal.
Only one device may be configured in active lock mode at any one
time. Active lock mode is usually invoked temporarily on power-up.
This can be achieved on the ACS104A by connecting DM1 to an RC
arrangement, i.e. with the capacitor to 5V and the resistor to GND, to
create a 5V à 0V ramp on power-up. The RC time constant should
be Ca.
communicating devices are driven from clocks derived from a single
source (0ppm).
Random Lock
This mode achieves moderate locking times (typically 5 seconds,
worst case 10 seconds) with the advantage that the ACS104A’s are
configured as peers. Communicating modems may be permanently
configured in this mode by hard wiring the DM pins.
Random lock will succeed even when communicating devices are
driven from clocks derived from a single source (0ppm). Random
lock mode is compatible with drift lock and active lock.
Memory Lock
Following the assertion of a reset (PORB = 0) communicating
devices will initiate an arbitration process where within 10 seconds
5 seconds.
The data is also sent to the far-end modem and
Active lock will succeed even when
ACS104A Data Sheet
This mode synchronises the
Therefore, if the
Data also
2