SCN2681AC1N28 NXP Semiconductors, SCN2681AC1N28 Datasheet - Page 10

• Manufacturer Part Number: SCN2681AC1N28
• Manufacturer: NXP Semiconductors

Specifications of SCN2681AC1N28

• Number Of Channels: 2
• Transmitter And Receiver Fifo Counter: No
• Operating Supply Voltage (typ): 5V
• Package Type: PDIP
• Operating Supply Voltage (max): 5.25V
• Operating Supply Voltage (min): 4.75V
• Mounting: Through Hole
• Operating Temperature (min): 0C
• Operating Temperature (max): 70C
• Operating Temperature Classification: Commercial
• Lead Free Status / Rohs Status: Compliant

Philips Semiconductors
the FIFO and re-align the FIFO read/write pointers. This has the
appearance of “clearing or flushing” the receiver FIFO. In fact, the
FIFO is NEVER cleared! The data in the FIFO remains valid until
overwritten by another received character. Because of this,
erroneous reading or extra reads of the receiver FIFO will miss-align
the FIFO pointers and result in the reading of previously read data.
A receiver reset will re-align the pointers.
Multidrop Mode
The DUART is equipped with a wake up mode for multidrop
applications. This mode is selected by programming bits MR1A[4:3]
or MR1B[4:3] to ‘11’ for Channels A and B, respectively. In this mode
of operation, a ‘master’ station transmits an address character
followed by data characters for the addressed ‘slave’ station. The
slave stations, with receivers that are normally disabled, examine
the received data stream and ‘wake up’ the CPU (by setting RxRDY)
only upon receipt of an address character. The CPU compares the
received address to its station address and enables the receiver if it
wishes to receive the subsequent data characters. Upon receipt of
another address character, the CPU may disable the receiver to
initiate the process again.
A transmitted character consists of a start bit, the programmed
number of data bits, and Address/Data (A/D) bit, and the
programmed number of stop bits. The polarity of the transmitted A/D
bit is selected by the CPU by programming bit MR1A[2]/MR1B[2].
MR1A[2]/MR1B[2] = 0 transmits a zero in the A/D bit position, which
identifies the corresponding data bits as data while
MR1A[2]/MR1B[2] = 1 transmits a one in the A/D bit position, which
identifies the corresponding data bits as an address. The CPU
should program the mode register prior to loading the corresponding
data bits into the THR.
In this mode, the receiver continuously looks at the received data
stream, whether it is enabled or disabled. If disabled, it sets the
RxRDY status bit and loads the character into the RHR FIFO if the
received A/D bit is a one (address tag), but discards the received
character if the received A/D bit is a zero (data tag). If enabled, all
Table 1. SCN2681 Register Addressing
* See Table 5 for BRG Test frequencies in this data sheet, and “Extended baud rates for SCN2681, SCN68681, SCC2691, SCC2692,
SCC68681 and SCC2698B” in application notes elsewhere in this publication.
2004 Mar 18
Dual asynchronous receiver/transmitter (DUART)
A3
0
0
0
0
0
0
0
0
1
1
1
1
1
1
1
1
A2
0
0
0
0
1
1
1
1
0
0
0
0
1
1
1
1
A1
0
0
1
1
0
0
1
1
0
0
1
1
0
0
1
1
A0
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
Mode Register A (MR1A, MR2A)
Status Register A (SRA)
BRG Test *
Rx Holding Register A (RHRA)
Input Port Change Register (IPCR)
Interrupt Status Register (ISR)
Counter/Timer Upper Value (CTU)
Counter/Timer Lower Value (CTL)
Mode Register B (MR1B, MR2B)
Status Register B (SRB)
1X/16X Test
Rx Holding Register B (RHRB)
*Reserved*
Input Ports IP0 to IP6
Start Counter Command
Stop Counter Command
READ (RDN = 0)
10
received characters are transferred to the CPU via the RHR. In
either case, the data bits are loaded into the data FIFO while the
A/D bit is loaded into the status FIFO position normally used for
parity error (SRA[5] or SRB[5]). Framing error, overrun error, and
break detect operate normally whether or not the receive is enabled.
PROGRAMMING
The operation of the DUART is programmed by writing control words
into the appropriate registers. Operational feedback is provided via
status registers which can be read by the CPU. The addressing of
the registers is described in Table 1.
The contents of certain control registers are initialized to zero on
RESET. Care should be exercised if the contents of a register are
changed during operation, since certain changes may cause
operational problems.
For example, changing the number of bits per character while the
transmitter is active may cause the transmission of an incorrect
character. In general, the contents of the MR, the CSR, and the
OPCR should only be changed while the receiver(s) and
transmitter(s) are not enabled, and certain changes to the ACR
should only be made while the C/T is stopped.
Mode registers 1 and 2 of each channel are accessed via
independent auxiliary pointers. The pointer is set to MR1x by
RESET or by issuing a ‘reset pointer’ command via the
corresponding command register. Any read or write of the mode
register while the pointer is at MR1x, switches the pointer to MR2x.
The pointer then remains at MR2x, so that subsequent accesses are
always to MR2x unless the pointer is reset to MR1x as described
above.
Mode, command, clock select, and status registers are duplicated
for each channel to provide total independent operation and control.
Refer to Table 2 for register bit descriptions. The reserved registers
at addresses H‘02’ and H‘OA’ should never be read during normal
operation since they are reserved for internal diagnostics.
Mode Register A (MR1A, MR2A)
Clock Select Register A (CSRA)
Command Register A (CRA)
Tx Holding Register A (THRA)
Aux. Control Register (ACR)
Interrupt Mask Register (IMR)
C/T Upper Preset Value (CRUR)
C/T Lower Preset Value (CTLR)
Mode Register B (MR1B, MR2B)
Clock Select Register B (CSRB)
Command Register B (CRB)
Tx Holding Register B (THRB)
*Reserved*
Output Port Conf. Register (OPCR)
Set Output Port Bits Command
Reset Output Port Bits Command
WRITE (WRN = 0)
SCN2681
Product data