MAX3140EEI Datasheet MAX3140EEI+T, MAX3140CEI+T, MAX3140EEI | P25-P27

MAX3140

SPI/MICROWIRE-Compatible UART with Integrated

True Fail-Safe RS-485/RS-422 Transceivers

______________________________________________________________________________________ 25

Receiver Input Filtering

The receivers of the MAX3140, when operating in

115kbps or 500kbps mode, incorporate input filtering in

addition to input hysteresis. This filtering enhances

noise immunity with differential signals that have very

slow rise and fall times. Receiver propagation delay

increases by 20% due to this filtering.

Phase Control Circuitry

Occasionally, twisted-pair lines are connected back-

ward from normal orientation. The MAX3140 has two

pins that invert the phase of the driver and the receiver

to correct for this problem. For normal operation, drive

TXP and RXP low, connect them to ground, or leave

them unconnected (internal pull-down). To invert the

driver phase, drive TXP high or connect it to V

. To

invert the receiver phase, drive RXP high or connect it

to V

. Note that the receiver threshold is positive

when RXP is high.

Applications Information

Crystals, Oscillators, and

Ceramic Resonators

The MAX3140 includes an oscillator circuit derived

from an external crystal for baud-rate generation. For

standard baud rates, use a 1.8432MHz or 3.6864MHz

crystal. The 1.8432MHz crystal results in lower operat-

ing current; however, the 3.6864MHz crystal may be

more readily available in surface-mount packages.

Ceramic resonators are low-cost alternatives to crystals

and operate similarly, though the Q and accuracy are

lower. Some ceramic resonators are available with inte-

gral load capacitors, which can further reduce cost.

The trade-off between crystals and ceramic resonators

is in initial frequency accuracy and temperature drift.

Keep the total error in the baud-rate generator below

1% for reliable operation with other systems. This is

accomplished easily with a crystal, and in most cases

is achieved with ceramic resonators. Table 8 lists differ-

ent types of crystals and resonators and their suppliers.

The MAX3140’s oscillator supports parallel-resonant

mode crystals and ceramic resonators, or can be driven

from an external clock source. Internally, the oscillator

consists of an inverting amplifier with its input (X1) tied

to its output (X2) by a bias network that self-biases the

inverter at approximately V

/2. The external feedback

circuit, usually a crystal from X2 to X1, provides 180° of

phase shift, causing the circuit to oscillate. As shown in

the standard application circuit, the crystal or resonator

is connected between X1 and X2, with the load capaci-

tance for the crystal being the series combination of C1

and C2. For example, for a 1.8432MHz crystal with a

specified load capacitance of 11pF, use 22pF capaci-

tors on either side of the crystal to ground. Series-res-

onant mode crystals have a slight frequency error,

typically oscillating 0.03% higher than specified series-

resonant frequency when operated in parallel mode.

Note: It is very important to keep crystal, resonator,

and load-capacitor leads and traces as short and

direct as possible. Make the X1 and X2 trace lengths

and ground tracks short, with no intervening traces.

This helps minimize parasitic capacitance and noise

pickup in the oscillator, and reduces EMI. Minimize

capacitive loading on X2 to minimize supply current.

The MAX3140’s X1 input can be driven directly by an

external CMOS clock source. The trip level is approxi-

mately equal to V

/2. Make no connection to X2 in this

mode. If a TTL or non-CMOS clock source is used, AC-

couple with a 10nF capacitor to X1. A 2V peak-to-peak

swing on the input is required for reliable operation.

Table 8. Component and Supplier List

Murata North America

ECS International, Inc.

SUPPLIER

CSA1.84MG

ECS-18-13-1

PART

NUMBER

(800) 831-9172

(913) 782-7787

PHONE

NUMBER

DESCRIPTION

1.8432

Through-Hole

Ceramic Resonator

1.8432

Through-Hole Crystal

(HC-49/U)

FREQUENCY

(MHz)

TYPICAL

C1, C2 (pF)

ECS International, Inc.

ECS-36-20-5P

ECS-36-18-4

(913) 782-7787

3.6864SMT Crystal

3.6864

Through-Hole Crystal

(HC-49/US)

AVX/Kyocera PBRC-3.68B (803) 448-94113.6864

SMT Ceramic

Resonator

None

(integral)

MAX3140

SPI/MICROWIRE-Compatible UART with Integrated

True Fail-Safe RS-485/RS-422 Transceivers

26 ______________________________________________________________________________________

START

STOP

START

STOP

NORMAL

UART FRAME

DATA BITS

01 1 11 1000 0

NORMAL UART

11 11 1000 0

IrDA

Figure 16. IrDA Timing

100kHz/div0 1MHz

20dB/div

MAX3140 FIG17

Figure 17. Driver Output Waveform and FFT Plot of MAX3140

with SRL = GND, Transmitting at 20kHz

9-Bit Networks

The MAX3140 supports a common multidrop communi-

cation technique referred to as 9-bit mode. In this

mode, the parity bit is set to indicate a message that

contains a header with a destination address. Set the

MAX3140’s parity mask to generate interrupts for this

condition. Operating a network in this mode reduces

the processing overhead of all nodes by enabling the

slave controllers to ignore most message traffic. This

relieves the remote processor to handle more useful

tasks.

In 9-bit mode, the MAX3140 is set up with eight bits

plus parity. The parity bit in all normal messages is

clear, but is set in an address-type message. The

MAX3140’s parity-interrupt mask generates an interrupt

on high parity when enabled. When the master sends

an address message with the parity bit set, all

MAX3140 nodes issue an interrupt. All nodes then

retrieve the received byte to compare to their assigned

address. Once addressed, the node continues to

process each received byte. If the node was not

addressed, it ignores all message traffic until a new

address is sent out by the master.

The parity/9th-bit interrupt is controlled only by the data

in the receive register and is not affected by data in the

FIFO, so the most effective use of the parity/9th-bit

interrupt is with FIFO disabled. With the FIFO disabled,

received nonaddress words are ignored and not even

read from the UART.

SIR IrDA Mode

The MAX3140’s IrDA mode communicates with other

IrDA SIR-compatible devices, or reduces power con-

sumption in opto-isolated applications.

In IrDA mode, a bit period is shortened to 3/16 of a

baud period (1.61µs at 115,200 baud) (Figure 16). A

data zero is transmitted as a pulse of light (TX = logic

low, RX = logic high).

In receive mode, the RX signal’s sampling is done

halfway into the transmission of a high level. The sam-

pling is done once, instead of three times, as in normal

mode. The MAX3140 ignores pulses shorter than

approximately 1/16 of the baud period. The IrDA device

that is communicating with the MAX3140 must transmit

pulses at 3/16 of the baud period. For compatibility with

other IrDA devices, set the format to 8-bit data, one

stop, no parity.

256 RS-485 Transceivers on the Bus

The standard RS-485 receiver input impedance is 12kΩ

(one unit load), and the standard driver can drive up to

32 unit loads. The MAX3140 has a 1/8-unit-load receiver

input impedance (96kΩ), allowing up to 256 trans-

ceivers to be connected in parallel on one communica-

tion line. Any combination of these devices and/or other

RS-485 transceivers with a total of 32 unit loads or less

can be connected to the line.

Reduced EMI and Reflections for the

RS-485/RS-422 Driver

The MAX3140 with SRL = V

or unconnected, is slew-

rate limited, minimizing EMI and reducing reflections

caused by improperly terminated cables. Figure 17

shows the driver output waveform and its Fourier analy-

MAX3140

SPI/MICROWIRE-Compatible UART with Integrated

True Fail-Safe RS-485/RS-422 Transceivers

______________________________________________________________________________________ 27

sis of a 20kHz signal transmitted with SRL = GND. High-

frequency harmonic components with large amplitudes

are evident. Figure 18 shows the same signal for SRL =

, transmitting under the same conditions. Figure

18’s high-frequency harmonic components are much

lower in amplitude, compared with Figure 17’s, and the

potential for EMI is significantly reduced. Figure 19

shows the same signal for SRL = unconnected, trans-

mitting under the same conditions. In general, a trans-

mitter’s rise time relates directly to the length of an

unterminated stub, which can be driven with only minor

waveform reflections, The following equation expresses

this relationship conservatively:

Length = t

RISE

/ (10 · 1.5ns/ft)

where t

RISE

is the transmitter’s rise time.

For example, consider a rise time of 1320ns. This

results in excellent waveforms with a stub length up to

90 feet. A system can work well with longer unterminat-

ed stubs, even with severe reflections, if the waveform

settles out before the UART samples them.

RS-485/RS-422 Transceiver

Low-Power Shutdown Mode

Low-power shutdown mode is initiated by bringing both

RE high and DE low. RE and DE may be driven simulta-

neously; the MAX3140 is guaranteed not to enter shut-

down if RE is high and DE is low for less than 50ns. If

the inputs are in this state for at least 600ns, the device

is guaranteed to enter shutdown.

Enable times t

and t

in the

Switching Char-

acteristics

tables assume the device was not in a low-

power shutdown state. Enable times t

ZH(SHDN)

and

ZL(SHDN)

assume the device was shut down. It takes

drivers and receivers longer to become enabled from

low-power shutdown mode (t

ZH(SHDN)

, t

ZH(SHDN)

) than

from driver/receiver-disable mode (t

, t

Driver Output Protection

Two mechanisms prevent excessive output current and

power dissipation caused by faults or by bus con-

tention. The first, a foldback current limit on the output

stage, provides immediate protection against short cir-

cuits over the whole common-mode voltage range (see

Typical Operating Characteristics

). The second, a ther-

mal shutdown circuit, forces the driver outputs into a

high-impedance state if the die temperature becomes

excessive.

Line Length vs. Data Rate

The RS-485/RS-422 standard covers line lengths up to

4000 feet. For line lengths greater than 4000 feet, use

the repeater application shown in Figure 20.

Figures 21, 22, and 23 show the system differential volt-

age for the parts driving 4000 feet of 26AWG twisted-

pair wire into 120Ω loads.

100kHz/divO 1MHz

20dB/div

MAX3140 FIG18

Figure 18. Driver Output Waveform and FFT Plot of MAX3140

with SRL = V

, Transmitting a 20kHz Signal

100kHz/divO 1MHz

20dB/div

MAX3140 FIG19

Figure 19. Driver Output Waveform and FFT Plot of MAX3140

with SRL = Unconnected, Transmitting a 20kHz Signal

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