S P I/Mic ro w ire -Co m p a t ib le
UART in QS OP -1 6
Shutd own c le a rs the re c e ive 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.
Table 7. Baud-Rate Selection Table*
BAUD
RATE
BAUD
RATE
BAUD
B3 B2 B1 B0
DIVISION
RATIO
(f
OSC
=
(f
OSC
=
1.8432MHz) 3.6864MHz)
0
0
0
0
0
0
0
0
1
1
1
1
1
1
1
1
0
0
0
0
1
1
1
1
0
0
0
0
1
1
1
1
0
0
1
1
0
0
1
1
0
0
1
1
0
0
1
1
0**
1
1
2
115.2k**
57.6k
28.8k
14.4k
7200
3600
1800
900
230.4k**
115.2k
57.6k
28.8k
14.4k
7200
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.
0
4
MAX310
1
8
0
16
32
64
128
3
1
0
3600
1
1800
__________Ap p lic a t io n s In fo rm a t io n
0
38.4k
19.2k
9600
4800
2400
1200
600
76.8k
38.4k
19.2k
9600
4800
2400
1200
600
1
6
Drivin g Op t o -Co u p le rs
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.
0
12
24
48
96
192
384
1
0
1
0
1
300
*Standard baud rates shown in bold
**Default baud rate
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.
Clo c k -Os c illa t o r Ba u d Ra t e s
Bits B0–B3 of the write-configuration register determine
the baud rate. Table 7 shows baud-rate divisors for given
inp ut c od e s , a s we ll a s the g ive n b a ud ra te for
1.8432MHz and 3.6864MHz crystals. Note that the baud
rate = crystal frequency / 16x division ratio.
Crys t a l-Os c illa t o r Op e ra t io n —
X1 , X2 Co n n e c t io n
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.
S h u t d o w n Mo d e
In shutdown, the oscillator turns off to reduce power
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.
dissipation (I
< 10µA). The MAX3100 enters shut-
CC
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.
12 ______________________________________________________________________________________
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
