The MAX3100 X1 input can be driven directly by an
external CMOS clock source. The trip level is approxi-
mately equal to V
CC
/ 2. No connection should be
made to X2 in this mode. If a TTL or non-CMOS clock
source is used, AC couple with a 10nF capacitor to X1.
The peak-to-peak swing on the input should be at least
2V for reliable operation.
9-Bit Networks
The MAX3100 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. The MAX3100 parity
mask can be set to generate interrupts for this condition.
Operating a network in this mode reduces the process-
ing overhead of all nodes by enabling the slave con-
trollers to ignore most message traffic. This can relieve
the remote processor to handle more useful tasks.
In 9-bit mode, the MAX3100 is set up with 8 bits plus
parity. The parity bit in all normal messages is clear, but
is set in an address-type message. The MAX3100 pari-
ty-interrupt mask is enabled to generate an interrupt on
high parity. When the master sends an address mes-
sage with the parity bit set, all MAX3100 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 can be ignored and not
even read from the UART.
SIR IrDA Mode
The MAX3100’s IrDA mode can be used to communicate
with other IrDA SIR-compatible devices, or to reduce
power consumption in opto-isolated applications.
In IrDA mode, a bit period is shortened to 3/16 of a
baud period (1.6µs at 115,200 baud) (Figure 9). A data
zero is transmitted as a pulse of light (TX pin = logic
low, RX pin = 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 MAX3100 ignores pulses shorter than
approximately 1/16 of the baud period. The IrDA device
that is communicating with the MAX3100 must be set to
transmit pulses at 3/16 of the baud period. For compati-
bility with other IrDA devices, set the format to 8-bit
data, one stop, no parity.
IrDA Module
The MAX3100 was optimized for direct optocoupler
drive, whereas IrDA modules contain inverting buffers.
Invert the RX and TX outputs as shown in Figure 10.
8051 Example: IrDA to RS-232 Converter
Figure 10 shows the MAX3100 with an 8051 µC. This
circuit receives IrDA data and outputs standard RS-232
data. Although the 8051 contains an internal UART, it
does not support IrDA or high-speed communications.
The MAX3100 can easily interface to the 8051 to sup-
port these high-performance communications modes.
The 8051 does not have an SPI interface, so communi-
cation with the MAX3100 is accomplished with port
pins and a short software routine (Figure 12a).
The software routine polls the IRQ output to see if data
is available from the MAX3100 UART. It then shifts the
data out, using the 8051 port pins, and transmits it out
the RS-232 side through the MAX3221 driver. The 8051
simultaneously monitors its internal UART for incoming
communications from the RS-232 side, and transmits
this data out the IrDA side through the MAX3100. The
low-level routine (UTLK) is the core routine that sends
and receives data over the port pins to simulate an SPI
port on the 8051. This technique is useful for any 8051-
based MAX3100 port-pin-interfaced application.
MAX3100
SPI/Microwire-Compatible
UART in QSOP-16
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