DAN-144 Datasheet, PDF (2/3 Pages) Exar Corporation – EXAR’S XR16L784 COMPARED

DATA COMMUNICATIONS APPLICATION NOTE
DAN144
preted as a software flow control character if it was not meant to be. More importantly, it will allow the soft-
ware routine to be able to use the entire character set including the Xon and Xoff characters as part of the
data stream since they will not necessarily be interpreted as software flow control characters unless they are
received one after another. The OX16C954 only has the Automatic 1 character Xon/Xoff Software Flow
Control.
• The XR16L784 has an Automatic RS485 Half-Duplex Control with Turn-Around Delay. RTS# or DTR# out-
put is a logic one while transmitting and becomes a logic zero after a specified delay indicated in MSR bits 7-
4 following the last stop bit of the last character that has been transmitted. This helps in turning around the
transceiver to receive the remote stations’s response. The delay optimizes the time needed for the last
transmission to reach the farthest station on a long cable network before switching off the line driver. This
delay prevents undesirable line signal disturbance that causes signal degradation. The OX16C954 does not
have the Turn-Around Delay feature.
• The XR16L784 has 16 selectable levels of RTS Hysteresis ranging from ±4 to ±52 when using programma-
ble trigger levels (Table-D). For example if the RX Trigger Level was programmed for 32 bytes and the RTS
Hysteresis was selected at ±20, the RTS# pin will not be forced to a logic 1 (RTS off) until the receive FIFO
reaches 52 bytes. The RTS# pin will return to a logic 0 (RTS on) after the RX FIFO is unloaded to 12 bytes.
The OX16C954 has a similar feature. For the OX16C954, the software driver has to manually select the
upper level to halt transmission and the lower level to resume transmission independent of the RX Trigger
Level. It is also up to the software driver to ensure that the upper level is greater than the lower level since
the device does not perform that check.
• The OX16C954 can be programmed to operate in a wake-up mode for Multidrop applications. This feature is
not available in the XR16L784.
• The OX16C954 can disable and enable the TX or RX output. This feature is not available in the XR16L784.
• The XR16L784 has a BRG prescaler of 1 or 4. The OX16C954 has a Baud Rate Generator Prescaler of 1 to
31.875.
• The XR16L784 has a Data Sampling Rate of 8X or 16X. The OX16C954 has a Data Sampling Rate of 4X to
16X.
• The XR16L784 has TX and RX FIFOs of 64 bytes deep and the OX16C954 has TX and RX FIFOs of 128
bytes deep.
1.3 REPLACING THE OX16C954 WITH THE XR16L784
The XR16L784 is a much simpler device to design than the OX16C954 because the XR16L784 has a single
chip select input and interrupt output. Since it is able to do simultaneous writes to all the channels, the initial-
ization of the XR16L784 is much easier and quicker than the OX16C954. With a Global Interrupt Source Reg-
ister, data throughput is increased and multiple channels can be serviced per interrupt, reducing CPU band-
width requirements. In addition to that, the XR16L784 has many enhanced features for increased performance
that are not available in the OX16C954 as mentioned above in the Firmware Differences section.
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