ICL3225 Datasheet, PDF (10/18 Pages) Intersil Corporation – 1 Microamp, +3V to +5.5V, 1Mbps, RS-232 Transceivers with Enhanced Automatic Powerdown

ICL3225, ICL3245
RECEIVER
INPUTS
TRANSMITTER
INPUTS
TRANSMITTER
OUTPUTS
INVALID
OUTPUT
t_INVL
READY
OUTPUT
V+
V_CC
0
V-
t_INVH
t_AUTOPWDN
} INVALID
REGION
t_WU
t_AUTOPWDN
t_WU
FIGURE 8. ENHANCED AUTOMATIC POWERDOWN, INVALID AND READY TIMING DIAGRAMS
Figure 7 illustrates the enhanced powerdown control logic.
Note that once the ICL32XX enters powerdown (manually or
automatically), the 30 second timer remains timed out (set),
keeping the ICL32XX powered down until FORCEON
transitions high, or until a transition occurs on a receiver or
transmitter input.
The INVALID output signal switches low to indicate that
invalid levels have persisted on all of the receiver inputs for
more than 30µs (see Figure 8), but this has no direct effect
on the state of the ICL32XX (see the next sections for
methods of utilizing INVALID to power down the device).
INVALID switches high 1µs after detecting a valid RS-232
level on a receiver input. INVALID operates in all modes
(forced or automatic powerdown, or forced on), so it is also
useful for systems employing manual powerdown circuitry.
The time to recover from automatic powerdown mode is
typically 100µs.
Emulating Standard Automatic Powerdown
If enhanced automatic powerdown isn’t desired, the user can
implement the standard automatic powerdown feature
(mimics the function on the ICL3221/ICL3223/ICL3243) by
connecting the INVALID output to the FORCEON and
FORCEOFF inputs, as shown in Figure 9. After 30µs of
invalid receiver levels, INVALID switches low and drives the
ICL32XX into a forced powerdown condition. INVALID
switches high as soon as a receiver input senses a valid RS-
232 level, forcing the ICL32XX to power on. See the
“INVALID DRIVING FORCEON AND FORCEOFF” section
of Table 2 for an operational summary. This operational
mode is perfect for handheld devices that communicate with
another computer via a detachable cable. Detaching the
cable allows the internal receiver pull-down resistors to pull
the inputs to GND (an invalid RS-232 level), causing the
30µs timer to time-out and drive the IC into powerdown.
Reconnecting the cablerestores valid levels, causing the IC
to power back up.
CPU
I/O
UART
ICL32XX
FIGURE 9. CONNECTIONS FOR AUTOMATIC POWERDOWN
WHEN NO VALID RECEIVER SIGNALS ARE
PRESENT
Hybrid Automatic Powerdown Options
For devices which communicate only through a detachable
cable, connecting INVALID to FORCEOFF (with FORCEON
= 0) may be a desirable configuration. While the cable is
attached INVALID and FORCEOFF remain high, so the
enhanced automatic powerdown logic powers down the RS-
232 device whenever there is 30 seconds of inactivity on the
10