Hardware Reference
In-Depth Information
Broadcom also states that there are some differences between its implementation of
the UART and the 16C650 UART. But these are mostly device driver details:
•
Receive FIFO trigger levels are 1/8, 1/4, 1/2, 3/4, and 7/8.
•
Transmit FIFO trigger levels are 1/8, 1/4, 1/2, 3/4, and 7/8.
•
The internal register map address space and the bit function of
each register differ.
•
1.5 stop bits is
not
supported.
•
No
independent receive clock.
The only real concern to the application developer is that the 1.5 stop-bits
configuration option is not available, which is rarely used these days anyway.
If you need the RS-232 DCD, DSR, DTR, and RI signals, these can be implemented
using GPIO input and output pins (along with the appropriate RS-232 line-level shifters).
These are relatively slow-changing signals, which can easily be handled in user space.
The one limitation of this approach, however, is that the hang-up TTY controls provided
by the device driver will be absent. To change that, the device driver source code could be
modified to support these signals using GPIO. The Raspbian Linux module of interest for
this is as follows:
drivers/tty/serial/amba-pl011.c
Exclusive Serial Line Use
As outlined in the “Available Consoles” section in Chapter 5 of
Raspberry Pi System
Software Reference
(Apress, 2014), the serial device
/dev/ttyAMA0
is easily applied as a
serial console device. However, some Raspberry Pi application developers will want to
use that serial interface for application purposes, instead of a console. Without taking
measures for exclusive access, the console will write to your serial peripheral and respond
to its input as well (as root console commands).
Even if you turned off the console, there can still be unwanted interaction from a
login prompt.
Procedure
Use the following steps to configure exclusive serial port access:
1.
Eliminate console references to
console=ttyAMA0,...
in the
files:
a.
/boot/cmline.txt
b.
/boot/config.txt
(check option
cmdline="..."
)
