Clock-Oscillator Baud Rates

Bits B0–B3 of the write-configuration register determine

the baud rate. Table 7 shows baud-rate divisors for given

input codes, as well as the given baud rate for

1.8432MHz and 3.6864MHz crystals. Note that the baud

rate = crystal frequency / 16x division ratio.

Shutdown Mode

In shutdown, the oscillator turns off to reduce power

down in one of two ways: by a software command

(SHDNi bit = 1) or by a hardware command (SHDN =

logic low). The hardware shutdown is effective immedi-

ately and will immediately terminate any transmission in

progress. The software shutdown, requested by setting

SHDNi bit = 1, is entered upon completing the trans-

mission of the data in both the transmit register and the

transmit-buffer register. The SHDNo bit is set when the

MAX3100 enters shutdown (either hardware or soft-

ware). The microcontroller (µC) can monitor the SHDNo

bit to determine when all data has been transmitted,

and shut down any external circuitry (such as RS-232

transceivers) at that time.

Shutdown clears the receive FIFO, R, A, RA/FE,

D0r–D7r, Pr, and Pt registers and sets the T bit high.

Configuration bits (RM, TM, PM, RAM, IR, ST, PE, L,

B0-3, and RTS) can be modified when SHDNo = 1 and

CTS can also be read. Even though RA is reset upon

entering shutdown, it will go high when any transitions

are detected on the RX pin. This allows the UART to

monitor activity on the receiver when in shutdown.

The command to power up (SHDNi = 0) turns on the

oscillator when CS goes high if SHDN pin = logic high,

with a start-up time of about 25ms. This is done through

a write configuration, which clears all registers but RTS

and CTS. Since the crystal oscillator typically requires

25ms to start, the first received characters will be gar-

bled, and a framing error may occur.

__________Applications Information

Driving Opto-Couplers

Figure 7 shows the MAX3100 in an isolated serial inter-

face. The MAX3100 Schmitt-trigger inputs are driven

directly by opto-coupler outputs. Isolated power is pro-

vided by the MAX845 transformer driver and linear reg-

ulator shown. A significant feature of this application is

that the opto-coupler’s skew does not affect the asyn-

chronous serial output’s timing. Only the set-up and

hold times of the SPI interface need to be met.

Figure 8 shows a bidirectional opto-isolated interface

using only two opto-isolators. Over 81% power savings

is realized using IrDA mode due to its 3/16-wide baud

periods.

Crystal-Oscillator Operation—

X1, X2 Connection

The MAX3100 includes a crystal oscillator for baud-rate

generation. For standard baud rates, use a 1.8432MHz

or 3.6864MHz crystal. The 1.8432MHz crystal results in

lower operating current; however, the 3.6864MHz crys-

tal may be more readily available in surface mount.

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 tradeoff between crystals and ceramic resonators

is in initial frequency accuracy and temperature drift.

The total error in the baud-rate generator should be

kept below 1% for reliable operation with other sys-

tems. This is accomplished easily with a crystal, and in

most cases can be achieved with ceramic resonators.

Table 8 lists the different types of crystals and res-

onators and their suppliers.

SPI/Microwire-Compatible

UART in QSOP-16

12

______________________________________________________________________________________

Table 7. Baud-Rate Selection Table*

*Standard baud rates shown in bold

**Default baud rate

115.2k

230.4k**

BAUD

RATE

3.6864MHz)

BAUD

B3

B2 B1 B0

2

0

0

0

1

1

0

0

0

0**

DIVISION

RATIO

57.6k

115.2k**

BAUD

RATE

1.8432MHz)

28.8k

57.6k

8

0

0

1

1

4

0

0

1

0

14.4k

28.8k

7200

14.4k

1800

3600

128

0

1

1

1

64

0

1

1

0

900

1800

32

0

1

0

1

16

0

1

0

0

3600

7200

38.4k

76.8k

9600

19.2k

24

1

0

1

1

12

1

0

1

0

4800

9600

2400

4800

600

1200

384

1

1

1

1

192

1

1

1

0

300

600

96

1

1

0

1

48

1

1

0

0

1200

2400

6

1

0

0

1

3

1

0

0

0

19.2k

38.4k