Philips Semiconductors
Product specification
SCN2681
Dual asynchronous receiver/transmitter (DUART)
1998 Sep 04
24
Table 5.
Baud Rates Extended
Normal BRG
BRG Test
CSR[7:4]
ACR[7] = 0
ACR[7] = 1
ACR[7] = 0
ACR[7] = 1
0000
50
75
4,800
7,200
0001
110
880
0010
134.5
1,076
0011
200
150
19.2K
14.4K
0100
300
28.8K
0101
600
57.6K
0110
1,200
115.2K
0111
1,050
2,000
1,050
2,000
1000
2,400
57.6K
1001
4,800
1010
7,200
1,800
57.6K
14.4K
1011
9,600
1100
38.4K
19.2K
38.4K
19.2K
1101
Timer
1110
I/O2 – 16X
1111
I/O2 – 1X
NOTE: Each read on address H‘2’ will toggle the baud rate test mode. When in the BRG test mode, the baud rates change as shown to the left.
This change affects all receivers and transmitters on the DUART. See
“Extended baud rates for SCN2681, SCN68681, SCC2691, SCC2692,
SCC68681 and SCC2698B” in application notes elsewhere in this publication
The test mode at address H‘A’ changes all transmitters and receivers to the 1x mode and connects the output ports to some internal nodes.
A condition that occurs infrequently has been observed where the receiver will ignore all data. It is caused by a corruption of the start bit
generally due to noise. When this occurs the receiver will appear to be asleep or locked up. The receiver must be reset for the UART to
continue to function properly.
Reset in the Normal Mode (Receiver Enabled)
Recovery can be accomplished easily by issuing a receiver software reset followed by a receiver enable. All receiver data, status and
programming will be preserved and available before reset. The reset will NOT affect the programming.
Reset in the Wake-Up Mode (MR1[4:3] = 11)
Recovery can also be accomplished easily by first exiting the wake-up mode (MR1[4:3] = 00 or 01 or 10), then issuing a receiver software
reset followed by a wake-up re-entry (MR1[4:3] = 11). All receiver data, status and programming will be preserved and available before
reset. The reset will NOT affect the programming.
The receiver has a digital filter designed to reject “noisy” data transitions and the receiver state machine was designed to reject noisy start
bits or noise that might be considered a start bit. In spite of these precautions, corruption of the start bit can occur in 15ns window
approximately 100ns prior to the rising edge of the data clock. The probability of this occurring is less than 10–5 at 9600 baud.
A corrupted start bit may have some deleterious effects in ASYNC operation if it occurs within a normal data block. The receiver will tend
to align its data clock to the next ‘0’ bit in the data stream, thus potentially corrupting the remainder of the data block. A good design
practice, in environments where start bit corruption is possible, is to monitor data quality (framing error, parity error, break change and
received break) and “data stopped” time out periods. Time out periods can be enabled using the counter/timer in the SCC2691, SCC2692,
SCC2698B and SC68692 products. This monitoring can indicate a potential start bit corruption problem.
SD00097