XR88C681CP/40 Exar Corporation, XR88C681CP/40 Datasheet - Page 57

• Manufacturer Part Number: XR88C681CP/40
• Description: Dual Channel UART
• Manufacturer: Exar Corporation

Specifications of XR88C681CP/40

• Features: *
• Number Of Channels: 2, DUART
• Fifo's: 1 Byte, 3 Byte
• Voltage - Supply: 5V
• With Parallel Port: Yes
• With Cmos: Yes
• Mounting Type: Through Hole
• Package / Case: 40-DIP (0.600", 15.24mm)
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

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,
Actual Baud Rate =
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.
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.
characteristic
communication.
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.
Rev. 2.11
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:
Command Register A, CRA
Command Register A, CRA
Command Register B, CRB
Command Register B, CRB
Register
of
Although the receiver and remote
[Table 6 Baud Rate Value] · fo
asynchronous
Table 19. Command Register Controls Over the Extend Bit
3.6864 MHz
A “16X clock”
serial
This is a
data
Contents
0A
0B
08
09
57
16
16
16
16
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).
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.
Transmitter
TX Clock
Figure 29. Example of a Serial Data
TX
Clear Rx BRG Select Extend Bit (X = 0)
Clear Tx BRG Select Extend Bit (X = 1)
Transmission System
Set Rx BRG Select Extend Bit (X = 1)
Set Tx BRG Select Extend Bit (X = 1)
Resulting Action
XR88C681
RX Clock
Receiver
RX