XR16M654 Datasheet, PDF (24/58 Pages) Exar Corporation – 1.62V TO 3.63V QUAD UART WITH 64-BYTE FIFO

XR16M654/654D
1.62V TO 3.63V QUAD UART WITH 64-BYTE FIFO
REV. 1.0.0
2.17 Sleep Mode with Auto Wake-Up
The M654 supports low voltage system designs, hence, a sleep mode is included to reduce its power
consumption when the chip is not actively used.
All of these conditions must be satisfied for the M654 to enter sleep mode:
■ no interrupts pending for all four channels of the M654 (ISR bit-0 = 1)
■ sleep mode of all channels are enabled (IER bit-4 = 1)
■ modem inputs are not toggling (MSR bits 0-3 = 0)
■ RX input pins are idling HIGH
The M654 stops its crystal oscillator to conserve power in the sleep mode. User can check the XTAL2 pin for
no clock output as an indication that the device has entered the sleep mode.
The M654 resumes normal operation by any of the following:
■ a receive data start bit transition (HIGH to LOW)
■ a data byte is loaded to the transmitter, THR or FIFO
■ a change of logic state on any of the modem or general purpose serial inputs: CTS#, DSR#, CD#, RI#
If the M654 is awakened by any one of the above conditions, it will return to the sleep mode automatically after
all interrupting conditions have been serviced and cleared. If the M654 is awakened by the modem inputs, a
read to the MSR is required to reset the modem inputs. In any case, the sleep mode will not be entered while
an interrupt is pending from any channel. The M654 will stay in the sleep mode of operation until it is disabled
by setting IER bit-4 to a logic 0.
If the address lines, data bus lines, IOW#, IOR#, CSA#, CSB#, CSC#, CSD# and modem input lines remain
steady when the M654 is in sleep mode, the maximum current will be in the microamp range as specified in the
DC Electrical Characteristics on page 42. If the input lines are floating or are toggling while the M654 is in
sleep mode, the current can be up to 100 times more. If any of those signals are toggling or floating, then an
external buffer would be required to keep the address, data and control lines steady to achieve the low current.
A word of caution: owing to the starting up delay of the crystal oscillator after waking up from sleep mode, the
first few receive characters may be lost. Also, make sure the RX A-D pins are idling HIGH or “marking”
condition during sleep mode. This may not occur when the external interface transceivers (RS-232, RS-485 or
another type) are also put to sleep mode and cannot maintain the “marking” condition. To avoid this, the
system design engineer can use a 47k ohm pull-up resistor on each of the RX A-D inputs.
2.18 Internal Loopback
The M654 UART provides an internal loopback capability for system diagnostic purposes. The internal
loopback mode is enabled by setting MCR register bit-4 to logic 1. All regular UART functions operate normally.
Figure 14 shows how the modem port signals are re-configured. Transmit data from the transmit shift register
output is internally routed to the receive shift register input allowing the system to receive the same data that it
was sending. The TX pin is held HIGH or mark condition while RTS# and DTR# are de-asserted, and CTS#,
DSR# CD# and RI# inputs are ignored. Caution: the RX input must be held HIGH during loopback test else
upon exiting the loopback test the UART may detect and report a false “break” signal.
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