User's Manual

Control Signals
Rev 1.0 Apr.02 Proprietary and Confidential 73
buffer powered by the MCU's 3.3 V V
CC
rail is
suggested. Connect the
/OE1 and /OE2 pins to
GND.
This buffer is there mainly to protect the host
MCU if it is ever powered down while the SB555
remains powered up. Only omit this buffer if all
MCU input pins can tolerate 3.0 V applied to
them while the MCU is powered down (MCU
V
CC
= 0 V) without back-powering the MCU, and
if the MCU input pins don’t have pullups which
could back-power the modem when it is
powered down.
Shutdown and reset
control
To correctly shutdown the modem prior to
powering it off or resetting it, the host device
must implement a shutdown request and
acknowledge handshake. This can be done in
software or hardware. See the Design Guide for
details.
The hardware mechanism to handshake a
shutdown requires the implementation of the
Shutdown Request (
/ShutDown) and Shutdown
Acknowledge (
/Shdn_Ack) signals.
Table 6-3: Shutdown control pinouts
Pin Name Description Type Termination
if not used
37 /Shdn_Ack Shutdown Acknowledge Output Not connected
38 /ShutDown Shutdown Request Input 3.0 V
39 /Reset Reset Input 3.0 V