Swarm M138 - Power On;Off Control; Serial Data Interface

4.4 Power On/Off Control
The Modem can be powered down with the use of the $PO*1F command. Once powered down,
it can be externally switched on/off by a user-supplied load switch on the VDD power rail.
After power to the Modem has been removed, power should not be restored until at least 1
second has elapsed.
If a Modem does not respond to software commands, remove power from the module, wait for
at least 1 second, and then power it back on. If the 1 second minimum wait time is not adhered
to, the reset circuit may not operate and the Modem could be placed in a non-operational state.
The state is not permanent and can be rectified by the above procedure.
4.5 Serial Data Interface
The serial data interface is a CMOS serial UART 3-wire (serial Rx, serial Tx, and ground)
interface at 3.3V digital signal levels over which the Modem transfers commands, responses,
and message data. The serial communication parameters can be found in Table 12.
Parameter
Value
Baud Rate
115200
Data Bits
8 Bits
Parity
None
Stop bits
1 Bit
Flow Control
None
Table 12: Serial communication parameters.
In addition, the electrical characteristics for SERIAL_RX, SERIAL_TX, and GPIO1 can be found
in Table 13.
Symbol
Parameter
Min
Typ.
Max
Unit
VIL
I/O input low level voltage
-0.3
-
0.3*3.3V
V
VIH
I/O input high level voltage
0.7*3.3V
-
3.3V
V
RPU/RPD
Weak pull up/down equivalent resistor
(for GPIO1)
25
40
55
kΩ
Table 13: Electrical characteristics for SERIAL_RX, SERIAL_TX, and GPIO1.
November 2021 Swarm M138 Modem Manual - Rev 1.00 18/77