XR88C681_06 Datasheet, PDF (57/101 Pages) Exar Corporation – CMOS Dual Channel UART (DUART)

XR88C681
Register
Command Register A, CRA
Command Register A, CRA
Command Register B, CRB
Command Register B, CRB
Contents
08₁₆
09₁₆
0A₁₆
0B₁₆
Resulting Action
Set Rx BRG Select Extend Bit (X = 1)
Clear Rx BRG Select Extend Bit (X = 0)
Set Tx BRG Select Extend Bit (X = 1)
Clear Tx BRG Select Extend Bit (X = 1)
Table 19. Command Register Controls Over the Extend Bit
Note: If the user programs either nibble of the Clock Select Register (CSRn[7:4] or CSRn[3:0]) with values ranging from 016 to C16,
then the user is using the BRG as a source for timing. However, these standard bit rates (presented in Table 18) apply only if
the X1/CLK pin is driven with a 3.6864 MHz signal. If a signal with a different frequency (fo) is applied to the X1/CLK pin, then
the DUART channel is running at the following baud rate:
Actual Baud Rate =
[Table 6 Baud Rate Value] · fo
3.6864 MHz
provided that fo is between 2.0 MHz and 4.0 MHz.
Additionally, as in the case for standard baud rates, the actual frequency of the clock signal will be 16 times these values.
1X vs 16X Clock Signals
The terms “1X Clock” and “16X Clock” have been applied
throughout this text. Therefore, it is important to discuss
their meaning and significance. A “16X clock”
over-samples the received serial data by a factor of 16.
Whereas a “1X clock” only samples the signals once per
bit period. From this one should correctly conclude that
greater accuracy (lower bit error rates) are achieved via
the use of the 16X clock in lieu of the 1X clock. The
following paragraphs will clarify the reasons.
A receiver in one of the DUART channels is clocked by a
local timing source (from the Timing Control Block). If this
receiver is active and is receiving data from a remote
serial transmitter, that transmitter is also clocked by its
own local timing source. Hence, there is no guarantee
that the clock frequency for the receiver is exactly the
same as that for the remote transmitter. This is a
characteristic of asynchronous serial data
communication. Although the receiver and remote
transmitter have been programmed to receive and
transmit data at exactly the same baud rate, sufficient
differences in the frequencies of the two clock sources
(local receiver and remote transmitter) can contribute to
bit errors in the receiving process, as presented in the
following discussion.
Suppose that we have a serial data transmission system
as depicted in Figure 29. This system consists of a
remote transmitter (TX), and a local receiver (RX).
Transmitter
TX
Receiver
RX
TX Clock
RX Clock
Figure 29. Example of a Serial Data
Transmission System
Let us further assume that the Receiver is clocked by a
source that is slightly faster than that of the Transmitter,
and that the Receiver is only sampling the serial data once
per bit period. Figure 30 presents the results of this
phenomenon.
Rev. 2.11
57