S42M Hardware User Guide
1VV0301379 Rev. 4
Page
15
of
48
2018-11-28
Serial Interface
The serial interface of S42M is a high-speed UART interface supporting
RTS/CTS flow control and interface-up/down mechanism according to the UICP+ protocol
(refer to [2]).
Electrical interfacing is at CMOS levels (defined by VSUP; see chapter 6.4.1).
Transmission speeds are 9600
– 921600 bps and 1Mbps (asynchronous).
Character representation: 8 Bit, no parity, 1 stop bit (8N1).
Hardware flow-control with RTS and CTS (active low).
Transmission speed may be limited by firmware. See corresponding
AT command reference [3] for further information.
4.3.1.
Basic Serial Interface
The basic serial interface (with RTS/CTS flow control) uses only four signal lines (UART-
RXD, UART-TXD, UART-CTS#, UART-RTS#) and GND. IUR-IN#, IUR-OUT# and GPIO[4]
(see below) can be left unconnected.
S42M
Host
UART-RXD
UART-TXD
UART-CTS#
UART-RTS#
GND
Figure 6: Basic Serial Interface
If the host in question is sufficiently fast, a four-wire scheme may be successful. Connect
the serial lines UART-RXD, UART-TXD as well as UART-RTS# and GND; leave UART-
CTS# open. The host is required to stop sending data within a short time after de-assertion
of UART-RTS# (there is room for up to 4 more characters at the time RTS# drops).
UICP has to be deactivated permanently in this configuration, because
signal UART-CTS# and IUR-IN# become inputs with no PU or PD if
UICP is active. This would cause floating CMOS inputs.
It is strongly recommended to use hardware flow control in both
directions. Not using flow control can cause a loss of data.
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