ST16C654D Exar Corporation, ST16C654D Datasheet - Page 20

• Manufacturer Part Number: ST16C654D
• Description: St16c654d -quad Uart With 64-byte Fifo And Infrared Irda Encoder/decoder
• Manufacturer: Exar Corporation

ST16C654/654D
2.97V TO 5.5V QUAD UART WITH 64-BYTE FIFO
The 654 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 654 to enter sleep mode:
The 654 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 654 resumes normal operation by any of the following:
If the 654 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 2750 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 654 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 654 is in sleep mode, the maximum current will be in the microamp range as specified in the
DC Electrical Characteristics on
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 at logic 1 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.
The 654 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 13
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 at logic 1 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 to a logic 1 during loopback
test else upon exiting the loopback test the UART may detect and report a false “break” signal.
2.16
2.17
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no interrupts pending for all four channels of the 654 (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 at a logic 1
a receive data start bit transition (logic 1 to 0)
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#
Sleep Mode with Auto Wake-Up
Internal Loopback
shows how the modem port signals are re-configured. Transmit data from the transmit shift register
page
37. If the input lines are floating or are toggling while the 654 is in sleep
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REV. 5.0.2