DAN-143 Datasheet, PDF (2/3 Pages) Exar Corporation – EXAR’S XR17C154 COMPARED

DATA COMMUNICATIONS APPLICATION NOTE
DAN143
ensure that the first character is not accidentally interpreted as a software flow control character if it was not
meant to be. The Automatic 2 Character Flow Control provides a much more reliable mechanism. The
OX16PCI954 only has the Automatic 1 character Xon/Xoff Software Flow Control.
• The XR17C154 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 (programmed into 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 completely reach the farthest station on a long and heavily loaded cable network
before switching off the line driver. This turn-around delay prevents any data loss and inproves polling time.
The OX16PCI954 does not have the Turn-Around Delay feature.
• The XR17C154 has 16 selectable levels of RTS Hysteresis ranging from ±4 to ±52. 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 OX16PCI954 has a similar feature. For the
OX16PCI954, 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 OX16PCI954 can be programmed to operate in a wake-up mode for Multidrop applications. This feature
is not available in the XR17C154.
• The OX16PCI954 can disable and enable the TX or RX output. This feature is not available in the
XR17C154.
• The XR17C154 has a BRG prescaler of 1 or 4. The OX16PCI954 has a Baud Rate Generator Prescaler of 1
to 31.875.
• The XR17C154 has a Data Sampling Rate of 8X or 16X. The OX16PCI954 has a Data Sampling Rate of 4X
to 16X.
• The XR17C154 has TX and RX FIFOs of 64 bytes deep and the OX16PCI954 has TX and RX FIFOs of 128
bytes deep.
1.3 REPLACING THE OX16PCI954 WITH THE XR17C154
Replacing the OX16PCI954 with the XR17C154 will require a new design since the XR17C154 comes in a
smaller 144-pin TQFP package. The XR17C158 is also available in the 144-pin TQFP package and is pin-to-
pin and software compatible with the XR17C154.
The software will need to be updated to take advantage of the enhanced features of the XR17C154 that are
not available in or different from the OX16PCI954.
In a nutshell, the XR17C154 has a much simpler internal register set than the OX16PCI954 for faster through-
put and easier software development, and has more enhanced features for higher throughput and perfor-
mance. The XR17C154 is also migratable to the XR17C158 in the same package.
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