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ACS104A-PL Datasheet(PDF) 3 Page - Semtech Corporation |
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ACS104A-PL Datasheet(HTML) 3 Page - Semtech Corporation |
3 / 12 page 3 2 Advanced Communications ACS104A Data Sheet programmed using pins HD(1:2) on the TQFP44 package, in accordance with the Table 2. HD2 HD1 Handshake Skew Bandwidth w.r.t. RxD 0 0* 600 Hz 10 ms. 0 1 10 kHz 1 - 2 data bits 1 0 5 kHz 1 - 2 data bits 1 1 2.5 kHz 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 Drift 0 0 1 Full-duplex Active 0 1 0 Full-duplex Memory 1 0 1 Local loopback Random 1 1 0 Remote loopback Random 1 1 1 Full-duplex Random Table 3. Diagnostic and operational modes 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 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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