ACS104A Просмотр технического описания (PDF) - Semtech Corporation

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ACS104A

Fiber Modem

Semtech Corporation 

Advanced Communications

programmed using pins HD(1:2) on the TQFP44 package, in

accordance with the Table 2.

HD2 HD1

Handshake

Bandwidth

Skew

w.r.t. RxD

0

0*

0

1

1

0

1

1

600 Hz

10

kHz

5

kHz

2.5

kHz

10 ms.

1 - 2 data bits

1 - 2 data bits

1 - 2 data bits

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. For

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.

Super-Compress mode

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.

Locking

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. Adjustments to machine cycles are made

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.

Diagnostic and Locking Modes

The diagnostic and operational modes, shown in Table 3, are

selected using the DM pins. DM3 is held high internally on the

PLCC28 package.

DM3 DM2 DM1 Mode

Lock

0

0

0

Full-duplex

0

0

1

Full-duplex

0

1

0

Full-duplex

1

0

1

Local loopback

1

1

0

Remote loopback

1

1

1

Full-duplex

Drift

Active

Memory

Random

Random

Random

Table 3. Diagnostic and operational modes

ACS104A Data Sheet

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. The data is also sent to the far-end modem and

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. Data also

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

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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. Therefore, if the

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. This mode synchronises the

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. 5 seconds. Active lock will succeed even when

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

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