Philips SCN2681AE1A44, SCN2681AE1F40, SCN2681AE1N24, SCN2681AE1N28, SCN2681AE1N40 Datasheet

INTEGRATED CIRCUITS

SCN2681

Dual asynchronous receiver/transmitter (DUART)

Product specification

1998 Sep 04

Supersedes data of 1995 May 01

IC19 Data Handbook

m n r

Philips Semiconductors	Product specification
Dual asynchronous receiver/transmitter (DUART)	SCN2681

DESCRIPTION

The Philips Semiconductors SCN2681 Dual Universal Asynchronous Receiver/Transmitter (DUART) is a single-chip MOS-LSI communications device that provides two independent full-duplex asynchronous receiver/transmitter channels in a single package. It interfaces directly with microprocessors and may be used in a polled or interrupt driven system.

The operating mode and data format of each channel can be programmed independently. Additionally, each receiver and transmitter can select its operating speed as one of eighteen fixed baud rates, a 16X clock derived from a programmable counter/timer, or an external 1X or 16X clock. The baud rate generator and counter/timer can operate directly from a crystal or from external clock inputs. The ability to independently program the operating speed of the receiver and transmitter make the DUART particularly attractive for dual-speed channel applications such as clustered terminal systems.

Each receiver is quadruply buffered to minimize the potential of receiver over-run or to reduce interrupt overhead in interrupt driven systems. In addition, a flow control capability is provided to disable a remote DUART transmitter when the buffer of the receiving device is full.

Also provided on the SCN2681 are a multipurpose 7-bit input port and a multipurpose 8-bit output port. These can be used as general purpose I/O ports or can be assigned specific functions (such as clock inputs or status/interrupt outputs) under program control.

The SCN2681 is available in three package versions: 40-pin and 28±pin, both 0.6º wide DIPs; a compact 24-pin 0.4º wide DIP; and a 44-pin PLCC.

FEATURES

•Dual full-duplex asynchronous receiver/transmitter

•Quadruple buffered receiver data registers

•Programmable data format

±5 to 8 data bits plus parity

±Odd, even, no parity or force parity

±1, 1.5 or 2 stop bits programmable in 1/16-bit increments

•Programmable baud rate for each receiver and transmitter selectable from:

± 22 fixed rates: 50 to 115.2k baud

•16-bit programmable Counter/Timer

±Non-standard rates to 115.2Kb

±One user-defined rate derived from programmable timer/counter

±External 1X or 16X clock

•Parity, framing, and overrun error detection

•False start bit detection

•Line break detection and generation

•Programmable channel mode

±Normal (full-duplex)

±Automatic echo

±Local loopback

±Remote loopback

•Multi-function programmable 16-bit counter/timer

•Multi-function 7-bit input port

±Can serve as clock or control inputs

±Change of state detection on four inputs

±100kΩ typical pull-up resistor

•Multi-function 8-bit output port

±Individual bit set/reset capability

±Outputs can be programmed to be status/interrupt signals

•Versatile interrupt system

±Single interrupt output with eight maskable interrupting conditions

±Output port can be configured to provide a total of up to six separate wire-ORable interrupt outputs

•Maximum data transfer: 1X ± 1MB/sec, 16X ± 125kB/sec

•Automatic wake-up mode for multidrop applications

•Start-end break interrupt/status

•Detects break which originates in the middle of a character

•On-chip crystal oscillator

•Single +5V power supply

•Commercial and industrial temperature ranges available

•DIP and PLCC packages

ORDERING INFORMATION

			ORDER CODE
	DESCRIPTION	Commercial			Industrial
		VCC = +5V +5%, TA = 0°C to +70°C		VCC = +5V +10%, TA = -40°C to +85°C
		Plastic DIP	Plastic LCC	Plastic DIP		Plastic LCC
	24-Pin1	SCN2681AC1N24	Not available	SCN2681AE1N24		Not available
	28-Pin2	SCN2681AC1N28	Not available	SCN2681AE1N28		Not available
	40-Pin2	SCN2681AC1N40	Not available	SCN2681AE1N40		Not available
	44-Pin	Not available	SCN2681AC1A44	Not available		SCN2681AE1A44

NOTES:

1.400mil-wide Dual In-Line Package

2.600mil-wide Dual In-Line Package

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853±1077 19970

Philips Semiconductors	Product specification
Dual asynchronous receiver/transmitter (DUART)	SCN2681

PIN CONFIGURATIONS

	A0	1		40	VCC
	IP3				IP4
		2		39
	A1				IP5
		3		38
						A0					VCC
	IP1				IP6			1		28
		4		37
						A1					IP2		1	24
								2		27		A1			A0
	A2	5	36		IP2
						A2					CEN
								3		26		A2	2	23	VCC
	A3	6	35		CEN	A3					RESET
								4		25		A3	3	22	CEN
	IP0	7	34		RESET	WRN					X2
								5		24		WRN	4	21	RESET
	WRN	8	33		X2	RDN					X1/CLK
								6		23		RDN	5	20	X1/CLK
	RDN	9	32		X1/CLK
						RXDB					RXDA
								7		22
	RXDB	10	31		RXDA				DIP			RXDB	6	19	RXDA
														DIP
			DIP
	TXDB	11	30		TXDA	TXDB		8		21	TXDA	TXDB	7	18	TXDA
						OP1					OP0
								9		20
	OP1	12	29		OP0							D1	8	17	D0
						D1					D0
								10		19
	OP3	13	28		OP2	D3					D2	D3	9	16	D2
								11		18
	OP5	14	27		OP4	D5					D4	D5	10	15	D4
								12		17
	OP7	15	26		OP6	D7					D6	D7	11	14	D6
								13		16
	D1	16	25		D0							GND	12	13	INTRN
						GND					INTRN
								14		15
	D3	17		24	D2
					D4
	D5	18		23
	D7				D6
		19		22
					INTRN
	GND	20		21

											PIN/FUNCTION		PIN/FUNCTION
INDEX											1	NC	23	NC
CORNER											2	A0	24	INTRN
			6		1		40				3	IP3	25	D6
											4	A1	26	D4
	7								39		5	IP1	27	D2
											6	A2	28	D0
											7	A3	29	OP6
											8	IP0	30	OP4
				PLCC							9	WRN	31	OP2
											10	RDN	32	OP0
											11	RXDB	33	TXDA
											12	NC	34	NC
											13	TXDB	35	RXDA
	17									29	14	OP1	36	X1/CLK
											15	OP3	37	X2
			18				28				16	OP5	38	RESET
				TOP VIEW							17	OP7	39	CEN
											18	D1	40	IP2
											19	D3	41	IP6
											20	D5	42	IP5
											21	D7	43	IP4
											22	GND	44	VCC

SD00084

Figure 1. Pin Configurations

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Philips Semiconductors	Product specification
Dual asynchronous receiver/transmitter (DUART)	SCN2681

PIN DESCRIPTION

	SYMBOL	APPLICABLE			TYPE	NAME AND FUNCTION
		40/44	28	24
	D0±D7	X	X	X	I/O	Data Bus: Bidirectional 3-State data bus used to transfer commands, data and status be-
						tween the DUART and the CPU. D0 is the least significant bit.
	CEN	X	X	X	I	Chip Enable: Active-Low input signal. When Low, data transfers between the CPU and the
						DUART are enabled on D0-D7 as controlled by the WRN, RDN and A0-A3 inputs. When
						High, places the D0-D7 lines in the 3-State condition.
	WRN	X	X	X	I	Write Strobe: When Low and CEN is also Low, the contents of the data bus is loaded into
						the addressed register. The transfer occurs on the rising edge of the signal.
	RDN	X	X	X	I	Read Strobe: When Low and CEN is also Low, causes the contents of the addressed regis-
						ter to be presented on the data bus. The read cycle begins on the falling edge of RDN.
	A0±A3	X	X	X	I	Address Inputs: Select the DUART internal registers and ports for read/write operations.
	RESET	X	X	X	I	Reset: A High level clears internal registers (SRA, SRB, IMR, ISR, OPR, OPCR), puts
						OP0±OP7 in the High state, stops the counter/timer, and puts Channels A and B in the inac-
						tive state, with the TxDA and TxDB outputs in the mark (High) state. Clears Test modes, sets
						MR pointer to MR1.
	INTRN	X	X	X	O	Interrupt Request: Active-Low, open-drain, output which signals the CPU that one or more
						of the eight maskable interrupting conditions are true.
	X1/CLK	X	X	X	I	Crystal 1: Crystal connection or an external clock input. A crystal of a clock the appropriate
						frequency (nominally 3.6864 MHz) must be supplied at all times. For crystal connections see
						Figure 7, Clock Timing.
	X2	X	X		I	Crystal 2: Crystal connection. See Figure 7. If a crystal is not used it is best to keep this pin
						not connected although it is permissible to ground it.
	RxDA	X	X	X	I	Channel A Receiver Serial Data Input: The least significant bit is received first. ªMarkº is
						High, ªspaceº is Low.
	RxDB	X	X	X	I	Channel B Receive Serial Data Input: The least significant bit is received first. ªMarkº is
						High, ªspaceº is Low.
	TxDA	X	X	X	O	Channel A Transmitter Serial Data Output: The least significant bit is transmitted first.
						This output is held in the ªmarkº condition when the transmitter is disabled, idle or when oper-
						ating in local loopback mode. ªMarkº is High, ªspaceº is Low.
	TxDB	X	X	X	O	Channel B Transmitter Serial Data Output: The least significant bit is transmitted first.
						This output is held in the ªmarkº condition when the transmitter is disabled, idle or when oper-
						ating in local loopback mode. ªMarkº is High, ªspaceº is Low.
	OP0	X	X		O	Output 0: General purpose output or Channel A request to send (RTSAN, active-Low). Can
						be deactivated automatically on receive or transmit.
	OP1	X	X		O	Output 1: General purpose output or Channel B request to send (RTSBN, active-Low). Can
						be deactivated automatically on receive or transmit.
	OP2	X			O	Output 2: General purpose output or Channel A transmitter 1X or 16X clock output, or Chan-
						nel A receiver 1X clock output.
	OP3	X			O	Output 3: General purpose output or open-drain, active-Low counter/timer output or Channel
						B transmitter 1X clock output, or Channel B receiver 1X clock output.
	OP4	X			O	Output 4: General purpose output or Channel A open-drain, active-Low, RxRDYA/FFULLA
						output.
	OP5	X			O	Output 5: General purpose output or Channel B open-drain, active-Low, RxRDYB/FFULLB
						output.
	OP6	X			O	Output 6: General purpose output or Channel A open-drain, active-Low, TxRDYA output.
	OP7	X			O	Output 7: General purpose output or Channel B open-drain, active-Low, TxRDYB output.
	IP0	X			I	Input 0: General purpose input or Channel A clear to send active-Low input (CTSAN). Pin
						has an internal VCC pull-up device supplying 1 to 4 A of current.
	IP1	X			I	Input 1: General purpose input or Channel B clear to send active-Low input (CTSBN). Pin
						has an internal VCC pull-up device supplying 1 to 4 A of current.
	IP2	X	X		I	Input 2: General purpose input or counter/timer external clock input. Pin has an internal VCC
						pull-up device supplying 1 to 4 A of current.
	IP3	X			I	Input 3: General purpose input or Channel A transmitter external clock input (TxCA). When
						the external clock is used by the transmitter, the transmitted data is clocked on the falling
						edge of the clock. Pin has an internal VCC pull-up device supplying 1 to 4 A of current.
	IP4	X			I	Input 4: General purpose input or Channel A receiver external clock input (RxCA). When the
						external clock is used by the receiver, the received data is sampled on the rising edge of the
						clock. Pin has an internal VCC pull-up device supplying 1 to 4 A of current.

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Philips Semiconductors	Product specification
Dual asynchronous receiver/transmitter (DUART)	SCN2681

PIN DESCRIPTION (Continued)

	SYMBOL	APPLICABLE			TYPE	NAME AND FUNCTION
		40/44	28	24
	IP5	X			I	Input 5: General purpose input or Channel B transmitter external clock input (TxCB). When
						the external clock is used by the transmitter, the transmitted data is clocked on the falling
						edge of the clock. Pin has an internal VCC pull-up device supplying 1 to 4 A of current.
	IP6	X			I	Input 6: General purpose input or Channel B receiver external clock input (RxCB). When the
						external clock is used by the receiver, the received data is sampled on the rising edge of the
						clock. Pin has an internal VCC pull-up device supplying 1 to 4 A of current.
	VCC	X	X		I	Power Supply: +5V supply input.
	GND	X	X		I	Ground

ABSOLUTE MAXIMUM RATINGS1

SYMBOL	PARAMETER	RATING	UNIT
T	Operating ambient temperature range2	See Note 4	°C
A
TSTG	Storage temperature range	-65 to +150	°C
	All voltages with respect to ground3	-0.5 to +6.0	V

NOTES:

1.Stresses above those listed under Absolute Maximum Ratings may cause permanent damage to the device. This is a stress rating only and functional operation of the device at these or any other condition above those indicated in the operation section of this specification is not implied.

2.For operating at elevated temperatures, the device must be derated based on +150oC maximum junction temperature.

3.This product includes circuitry specifically designed for the protection of its internal devices from damaging effects of excessive static charge. Nonetheless, it is suggested that conventional precautions be taken to avoid applying any voltages larger than the rated maxima.

4.Parameters are valid over specified temperature range. See Ordering information table for applicable operating temperature range and VCC supply range.

DC ELECTRICAL CHARACTERISTICS1, 2, 3 TA = -40°C to +85°C, VCC = +5.0V 10%

	SYMBOL	PARAMETER	TEST CONDITIONS			LIMITS		UNIT
					Min	Typ	Max
	VIL	Input low voltage			2		0.8	V
	V	Input high voltage (except X1/CLK)5						V
	IH	Input high voltage (except X1/CLK)4			2.5			V
	V
	IH	Input high voltage (X1/CLK)			4			V
	VIH		IOL = 2.4mA
	VOL	Output low voltage					0.4	V
	V	Output high voltage (except o.d. outputs)5	IOH = -400μA		2.4			V
	OH	Output high voltage (except o.d. outputs)4	I	= -400μA	2.9			V
	V
	OH		OH
	IIL	Input leakage current	VIN = 0 to VCC		-10		10	μA
	ILL	Data bus 3-stage leakage current	VO = 0.4 to VCC		-10	-2	10	μA
	IX1L	X1/CLK low input current	VIN = 0, X2 grounded		-4	-1.5	0	mA
			VIN = 0, X2 floated		-3	0.2	0	mA
	IX1H	X1/CLK high input current	VIN = VCC, X2 grounded		-1	3.5	1	mA
			VIN = VCC, X2 floated		0	-30	10	mA
	IX2L	X2 low input current	VIN = 0, X1/CLK floated		-100	+30	0	μA
	IX2H		VIN = VCC, X1/CLK floated		0		100	μA
	IOC	X2 high input current	VO = 0.4 to VCC		-10		10	μA
	IOCC	Open-collector output leakage current	0°C to +70°C version				150	mA
		Power supply current	-40°C to +85°C version				175	mA

NOTES:

1.Parameters are valid over specified temperature range. See Ordering information table for applicable operating temperature range and VCC supply range.

2.All voltage measurements are referenced to ground (GND). For testing, all inputs except X1/CLK swing between 0.4V and 2.4V with a transition time of 20ns maximum. For X1/CLK this swing is between 0.4V and 4.4V. All time measurements are referenced at input voltages of 0.8V and 2.0V as appropriate.

3.Typical values are at +25°C, typical supply voltages, and typical processing parameters.

4.TA < 0°C

5.TA > 0°C

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Philips Semiconductors	Product specification
Dual asynchronous receiver/transmitter (DUART)	SCN2681

AC CHARACTERISTICS TA = -40°C to +85°C1, VCC = +5.0V 10%2, 3, 4, 5

SYMBOL	PARAMETER		LIMITS		UNIT
		Min	Typ	Max
Reset Timing (Figure 3)
tRES	RESET pulse width	200			ns
Bus Timing	(Figure 4)6
tAS	A0-A3 setup time to RDN, WRN Low	10			ns
tAH	A0-A3 hold time from RDN, WRN Low	100			ns
tCS	CEN setup time to RDN, WRN Low	0			ns
tCH	CEN hold time from RDN, WRN High	0			ns
tRW	WRN, RDN pulse width	225			ns
tDD	Data valid after RDN Low			175	ns
tDF	Data bus floating after RDN High			100	ns
tDS	Data setup time before WRN High	100			ns
tDH	Data hold time after WRN High	20			ns
t	High time between READs and/or WRITE7, 8	200			ns
RWD
Port Timing	(Figure 5)6
tPS	Port input setup time before RDN Low	0			ns
tPH	Port input hold time after RDN High	0			ns
tPD	Port output valid after WRN High			400	ns
Interrupt	Timing (Figure 6)
tIR	INTRN (or OP3-OP7 when used as interrupts) negated from:
	Read RHR (RxRDY/FFULL interrupt)			300	ns
	Write THR (TxRDY interrupt)			300	ns
	Reset command (delta break interrupt)			300	ns
	Stop C/T command (counter interrupt)			300	ns
	Read IPCR (input port change interrupt)			300	ns
	Write IMR (clear of interrupt mask bit)			300	ns
Clock Timing (Figure 7)10
tCLK	X1/CLK High or Low time	100			ns
fCLK	X1/CLK frequency	2.0	3.6864	4.0	MHz
tCTC	CTCLK (IP2) High or Low time	100			ns
fCTC	CTCLK (IP2) frequency	0		4.0	MHz
tRX9	RxC High or Low time	220			ns
fRX9	RxC frequency (16X)	0		2.0	MHz
	(1X)	0		1.0	MHz
tTX9	TxC High or Low time	220			ns
fTX9	TxC frequency (16X)	0		2.0	MHz
	(1X)	0		1.0	MHz
Transmitter	Timing (Figure 8)
tTXD9	TxD output delay from TxC external clock input on IP pin			350	ns
tTCS9	Output delay from TxC low at OP pin to TxD data output	0		150	ns
Receiver	Timing (Figure 10)
tRXS9	RxD data setup time before RxC high at external clock input on IP pin	240			ns
tRXH9	RxD data hold time after RxC high at external clock input on IP pin	200			ns

NOTES:

1.For operating at elevated temperatures, the device must be derated based on +150°C maximum junction temperature.

2.Parameters are valid over specified temperature range.

3.All voltage measurements are referenced to ground (GND). For testing, all inputs except X1/CLK swing between 0.4V and 2.4V with a transition time of < 20ns. For X1/CLK this swing is between 0.4V and 4.4V. All time measurements are referenced at input voltages of 0.8V and 2.0V as appropriate.

4.Typical values are at +25°C, typical supply voltages, and typical processing parameters.

5.Test condition for outputs: CL = 150pF, except interrupt outputs. Test condition for interrupt outputs: CL = 50pF, RL = 2.7kΩ to VCC.

6.Timing is illustrated and referenced to the WRN and RDN inputs. The device may also be operated with CEN as the `strobing' input. In this case, all timing specifications apply referenced to the falling and rising edges of CEN, CEN and RDN (also CEN and WRN) are ANDed internally. As a consequence, the signal asserted last initiates the cycle and the signal negated first terminates the cycle.

7.If CEN is used as the `strobing' input, the parameter defines the minimum High times between one CEN and the next. The RDN signal must be negated for tRWD to guarantee that any status register changes are valid.

8.Consecutive write operations to the same command register require at least three edges of the X1 clock between writes.

9.This parameter is not applicable to the 28-pin device.

10.Operation to 0MHz is assured by design. However, operation at low frequencies is not tested and has not been characterized.

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Philips Semiconductors	Product specification
Dual asynchronous receiver/transmitter (DUART)	SCN2681

BLOCK DIAGRAM

	8				CHANNEL A
D0±D7	BUS BUFFER				TRANSMIT
							TxDA
					HOLDING REG
					TRANSMIT
					SHIFT REGISTER
RDN	OPERATION CONTROL
WRN	ADDRESS				RECEIVE
					HOLDING REG (3)
CEN	DECODE						RxDA
	4				RECEIVE
A0±A3
	R/W CONTROL				SHIFT REGISTER
RESET
					MRA1, 2
					CRA
					SRA
	INTERRUPT CONTROL
INTRN	IMR				CHANNEL B		TxDB
	ISR				(AS ABOVE)		RxDB
				DATABUS	INPUT PORT
	TIMING	CONTROL	TIMING	INTERNAL	CHANGE OF
					STATE	7
					DETECTORS (4)		IP0-IP6
	BAUD RATE
	GENERATOR				IPCR
					ACR
	CLOCK
	SELECTORS
	COUNTER/				OUTPUT PORT
	TIMER
					FUNCTION	8	OP0-OP7
X1/CLK					SELECT LOGIC
	XTAL OSC
X2					OPCR
					OPR
	CSRA
	CSRB
	ACR
	CTUR
	CTLR						VCC
							GND
							SD00085

Figure 2. Block Diagram

BLOCK DIAGRAM

Operation Control

The SCN2681 DUART consists of the following eight major sections: data bus buffer, operation control, interrupt control, timing, communications Channels A and B, input port and output port. Refer to the block diagram.

Data Bus Buffer

The data bus buffer provides the interface between the external and internal data buses. It is controlled by the operation control block to allow read and write operations to take place between the controlling CPU and the DUART.

The operation control logic receives operation commands from the CPU and generates appropriate signals to internal sections to control device operation. It contains address decoding and read and write circuits to permit communications with the microprocessor via the data bus buffer.

Interrupt Control

A single active-Low interrupt output (INTRN) is provided which is activated upon the occurrence of any of eight internal events. Associated with the interrupt system are the Interrupt Mask Register

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Philips Semiconductors	Product specification
Dual asynchronous receiver/transmitter (DUART)	SCN2681

(IMR) and the Interrupt Status Register (ISR). The IMR may be programmed to select only certain conditions to cause INTRN to be asserted. The ISR can be read by the CPU to determine all currently active interrupting conditions.

Outputs OP3-OP7 can be programmed to provide discrete interrupt outputs for the transmitter, receivers, and counter/timer.

Timing Circuits

The timing block consists of a crystal oscillator, a baud rate generator, a programmable 16-bit counter/timer, and four clock selectors. The crystal oscillator operates directly from a 3.6864MHz crystal connected across the X1/CLK and X2 inputs. If an external clock of the appropriate frequency is available, it may be connected to X1/CLK. The clock serves as the basic timing reference for the Baud Rate Generator (BRG), the counter/timer, and other internal circuits. A clock signal within the limits specified in the specifications section of this data sheet must always be supplied to the DUART.

If an external clock is used instead of a crystal, both X1 and X2 should use a configuration similar to the one in Figure 7.

The baud rate generator operates from the oscillator or external clock input and is capable of generating 18 commonly used data communications baud rates ranging from 50 to 38.4k baud. The clock outputs from the BRG are at 16X the actual baud rate. The counter/timer can be used as a timer to produce a 16X clock for any other baud rate by counting down the crystal clock or an external clock. The four clock selectors allow the independent selection, for each receiver and transmitter, of any of these baud rates or external timing signal.

Counter/Timer (C/T)

The counter timer is a 16 bit programmable divider that operates one of three modes: Counter, Timer or Time Out mode. In all three modes it uses the 16-bit value loaded to the CTUR and CTLR registers. (Counter timer upper and lower preset registers).

•In the timer mode it generates a square wave.

•In the counter mode it generates a time delay.

•In the time out mode it monitors the receiver data flow and signals data flow has paused. In the time out mode the receiver controls the starting/stopping of the C/T.

The counter operates as a down counter and sets its output bit in the ISR (Interrupt Status Register) each time it passes through 0. The output of the counter/timer may be seen on one of the OP pins or as an Rx or Tx clock.

The Timer/Counter is controlled with six (6) ªcommandsº; Start C/T, Stop C/T, write C/T, preset registers, read C/T value, set or reset time out mode.

Please see the detail of the commands under the Counter/Timer register descriptions.

Communications Channels A and B

Each communications channel of the SCN2681 comprises a full-duplex asynchronous receiver/transmitter (UART). The operating frequency for each receiver and transmitter can be selected independently from the baud rate generator, the counter timer, or from an external input.

The transmitter accepts parallel data from the CPU, converts it to a serial bit stream, inserts the appropriate start, stop, and optional parity bits and outputs a composite serial stream of data on the TxD

output pin. The receiver accepts serial data on the RxD pin, converts this serial input to parallel format, checks for start bit, stop bit, parity bit (if any), or break condition and sends an assembled character to the CPU.

The input port pulse detection circuitry uses a 38.4kHz sampling clock derived from one of the baud rate generator taps. This results in a sampling period of slightly more than 25μs (this assumes that the clock input is 3.6864MHz). The detection circuitry, in order to guarantee that a true change in level has occurred, requires two successive samples at the new logic level be observed. As a consequence, the minimum duration of the signal change is 25μs if the transition occurs ªcoincident with the first sample pulseº. The 50μs time refers to the situation in which the change-of-state is ªjust missedº and the first change-of-state is not detected until 25μs later.

Input Port

The inputs to this unlatched 7-bit port can be read by the CPU by performing a read operation at address D16. A High input results in a logic 1 while a Low input results in a logic 0. D7 will always read as a logic 1. The pins of this port can also serve as auxiliary inputs to certain portions of the DUART logic.

Four change-of-state detectors are provided which are associated with inputs IP3, IP2, IP1 and IP0. A High-to-Low or Low-to-High transition of these inputs lasting longer than 25 ± 50μs, will set the corresponding bit in the input port change register. The bits are cleared when the register is read by the CPU. Any change-of-state can also be programmed to generate an interrupt to the CPU.

All the IP pins have a small pull-up device that will source 1 to 4 A of current from VCC. These pins do not require pull-up devices or VCC connections if they are not used.

Output Port

The output port pins may be controlled by the OPR, OPCR, MR and CR registers. Via appropriate programming they may be just another parallel port to external circuits, or they may represent many internal conditions of the UART. When this 8-bit port is used as a general purpose output port, the output port pins drive a state which is the complement of the Output Port Register (OPR). OPR(n) = 1 results in OP(n) = Low and vice versa. Bits of the OPR can be individually set and reset. A bit is set by performing a write operation at address E16 with the accompanying data specifying the bits to be set (1 = set, 0 = no change).

Likewise, a bit is reset by a write at address F16 with the accompanying data specifying the bits to be reset (1 = reset, 0 = no change).

Outputs can be also individually assigned specific functions by appropriate programming of the Channel A mode registers (MR1A, MR2A), the Channel B mode registers (MR1B, MR2B), and the Output Port Configuration Register (OPCR).

Please note that these pins drive both high and low. HOWEVER when they are programmed to represent interrupt type functions (such as receiver ready, transmitter ready or counter/timer ready) they will be switched to an open drain configuration in which case an external pull-up device would be required.

TRANSMITTER OPERATION

The SCN2681 is conditioned to transmit data when the transmitter is enabled through the command register. The SCN2681 indicates to the CPU that it is ready to accept a character by setting the TxRDY bit in the status register. This condition can be programmed to

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Philips Semiconductors	Product specification
Dual asynchronous receiver/transmitter (DUART)	SCN2681

generate an interrupt request at OP6 or OP7 and INTRN. When a character is loaded into the Transmit Holding Register (THR), the above conditions are negated. Data is transferred from the holding register to transmit shift register when it is idle or has completed transmission of the previous character. The TxRDY conditions are then asserted again which means one full character time of buffering is provided. Characters cannot be loaded into the THR while the transmitter is disabled.

The transmitter converts the parallel data from the CPU to a serial bit stream on the TxD output pin. It automatically sends a start bit followed by the programmed number of data bits, an optional parity bit, and the programmed number of stop bits. The least significant bit is sent first. Following the transmission of the stop bits, if a new character is not available in the THR, the TxD output remains High and the TxEMT bit in the Status Register (SR) will be set to 1. Transmission resumes and the TxEMT bit is cleared when the CPU loads a new character into the THR.

If the transmitter is disabled, it continues operating until the character currently being transmitted is completely sent out. The transmitter can be forced to send a continuous Low condition by issuing a send break command.

The transmitter can be reset through a software command. If it is reset, operation ceases immediately and the transmitter must be enabled through the command register before resuming operation. If CTS operation is enable, the CTSN input must be Low in order for the character to be transmitted. If it goes High in the middle of a transmission, the character in the shift register is transmitted and TxDA then remains in the marking state until CTSN goes Low. The transmitter can also control the deactivation of the RTSN output. If programmed, the RTSN output will be reset one bit time after the character in the transmit shift register and transmit holding register (if any) are completely transmitted, if the transmitter has been disabled.

Receiver

The SCN2681 is conditioned to receive data when enabled through the command register. The receiver looks for a High-to-Low (mark-to-space) transition of the start bit on the RxD input pin. If a transition is detected, the state of the RxD pin is sampled each 16X clock for 7 1/2 clocks (16X clock mode) or at the next rising edge of the bit time clock (1X clock mode). If RxD is sampled High, the start bit is invalid and the search for a valid start bit begins again. If RxD is still Low, a valid start bit is assumed and the receiver continues to sample the input at one bit time intervals at the theoretical center of the bit, until the proper number of data bits and parity bit (if any) have been assembled, and one stop bit has been detected. The least significant bit is received first. The data is then transferred to the Receive Holding Register (RHR) and the RxRDY bit in the SR is set to a 1. This condition can be programmed to generate an interrupt at OP4 or OP5 and INTRN. If the character length is less than eight bits, the most significant unused bits in the RHR are set to zero.

After the stop bit is detected, the receiver will immediately look for the next start bit. However, if a non-zero character was received without a stop bit (framing error) and RxD remains Low for one half of the bit period after the stop bit was sampled, then the receiver operates as if a new start bit transition had been detected at that point (one-half bit time after the stop bit was sampled).

The parity error, framing error, overrun error and received break state (if any) are strobed into the SR at the received character boundary, before the RxRDY status bit is set. If a break condition is

detected (RxD is Low for the entire character including the stop bit), a character consisting of all zeros will be loaded into the RHR and the received break bit in the SR is set to 1. The RxD input must return to high for two (2) clock edges of the X1 crystal clock for the receiver to recognize the end of the break condition and begin the search for a start bit. This will usually require a high time of one

X1 clock period or 3 X1 edges since the clock of the controller is not synchronous to the X1 clock.

Receiver FIFO

The RHR consists of a First-In-First-Out (FIFO) stack with a capacity of three characters. Data is loaded from the receive shift register into the topmost empty position of the FIFO. The RxRDY bit in the status register is set whenever one or more characters are available to be read, and a FFULL status bit is set if all three stack positions are filled with data. Either of these bits can be selected to cause an interrupt. A read of the RHR outputs the data at the top of the FIFO. After the read cycle, the data FIFO and its associated status bits (see below) are `popped' thus emptying a FIFO position for new data.

Receiver Status Bits

In addition to the data word, three status bits (parity error, framing error, and received break) are also appended to each data character in the FIFO (overrun is not). Status can be provided in two ways, as programmed by the error mode control bit in the mode register. In the `character' mode, status is provided on a character-by-character basis; the status applies only to the character at the top of the FIFO. In the `block' mode, the status provided in the SR for these three bits is the logical-OR of the status for all characters coming to the top of the FIFO since the last `reset error' command was issued. In either mode reading the SR does not affect the FIFO. The FIFO is `popped' only when the RHR is read. Therefore the status register should be read prior to reading the FIFO.

If the FIFO is full when a new character is received, that character is held in the receive shift register until a FIFO position is available. If an additional character is received while this state exits, the contents of the FIFO are not affected; the character previously in the shift register is lost and the overrun error status bit (SR[4] will be set-upon receipt of the start bit of the new (overrunning) character).

The receiver can control the deactivation of RTS. If programmed to operate in this mode, the RTSN output will be negated when a valid start bit was received and the FIFO is full. When a FIFO position becomes available, the RTSN output will be re-asserted automatically. This feature can be used to prevent an overrun, in the receiver, by connecting the RTSN output to the CTSN input of the transmitting device.

Receiver Reset and Disable

Receiver disable stops the receiver immediately ± data being assembled if the receiver shift register is lost. Data and status in the FIFO is preserved and may be read. A re-enable of the receiver after a disable will cause the receiver to begin assembling characters at the next start bit detected.

A receiver reset will discard the present shift register data, reset the receiver ready bit (RxRDY), clear the status of the byte at the top of the FIFO and re-align the FIFO read/write pointers. This has the appearance of ªclearing or flushingº 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

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