XR16M2551 Datasheet, PDF (21/51 Pages) Exar Corporation – HIGH PERFORMANCE LOW VOLTAGE DUART WITH 16-BYTE FIFO AND POWERSAVE FEATURE

XR16M2551
REV. 1.0.2
HIGH PERFORMANCE LOW VOLTAGE DUART WITH 16-BYTE FIFO AND POWERSAVE FEATURE
2.18.1 Sleep Mode
All of these conditions must be satisfied for the M2551 to enter sleep mode:
■ no interrupts pending for both channels of the M2551 (ISR bit-0 = 1)
■ sleep mode of both channels are enabled (IER bit-4 = 1)
■ modem inputs are not toggling (MSR bits 0-3 = 0)
■ RX input pins are idling HIGH
The M2551 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 M2551 resumes normal operation by any of the following when PowerSave mode is disabled:
■ 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 sleep mode is enabled and the M2551 is awakened by one of the conditions described above, an
interrupt is issued by the M2551 to signal to the CPU that it is awake. The lower nibble of the interrupt source
register (ISR) will read a value of 0x1 for this interrupt and reading the ISR clears this interrupt. Since the same
value (0x1) is also used to indicate no pending interrupt, users should exercise caution while using the sleep
mode. The M2551 will return to the sleep mode automatically after all interrupting conditions have been
serviced and cleared. If the M2551 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 channel
A or B. The M2551 will stay in the sleep mode of operation until it is disabled by setting IER bit-4 to LOW.
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. The number of characters lost during the restart also depends on your
operating data rate. More characters are lost when operating at higher data rate. Also, it is important to keep
RX A/B inputs idling HIGH or “marking” condition during sleep mode to avoid receiving a “break” condition
upon the restart. This may occur when the external interface transceivers (RS-232, RS-422 or another type)
are also put to sleep mode and cannot maintain the “marking” condition. To avoid this, the designer can use a
47k-100k ohm pull-up resistor on the RXA and RXB pins.
2.18.2 PowerSave Feature
If the address lines, data bus lines, IOW#, IOR#, CSA#, CSB#, and modem input lines remain steady when the
M2551 is in sleep mode, the maximum current will be in the microamp range as specified in the DC Electrical
Characteristics on page 38. If the input lines are floating or are toggling while the M2551 is in sleep mode, the
current can be up to 100 times more. If not using the PowerSave feature, then an external buffer would be
required to keep the address and data bus lines from toggling or floating to achieve the low current. But if the
PowerSave feature is enabled (pin 12 connected to VCC), this will eliminate the need for an external buffer by
internally isolating the address, data and control signals (see Figure 1 on page 1) from other bus activities that
could cause wasteful power drain. The M2551 enters PowerSave mode when the PwrSave pin is connected
to VCC and the M2551 is in sleep mode (see Sleep Mode section above).
Since PowerSave mode isolates the address, data and control signals, the device will wake-up by:
■ a receive data start bit transition (HIGH to LOW)
■ a change of logic state on any of the modem or general purpose serial inputs: CTS#, DSR#, CD#, RI#
The M2551 will return to the PowerSave mode automatically after a read to the MSR (to reset the modem
inputs) and all interrupting conditions have been serviced and cleared. The M2551 will stay in the PowerSave
mode of operation until it is disabled by setting IER bit-4 to LOW and/or the PwrSave pin is connected to GND.
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