UM0851 datasheet(100/245 Pages) STMICROELECTRONICS

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Communication device drivers

UM0851

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Doc ID 16604 Rev 2

Figure 26.

UART software system architecture

There are different types of TTY drivers: console & serial port. The console driver is used at

two different places in Linux. Firstly, at boot time it is used before the initialization of the

serial TTY framework as it takes some time for the serial TTY framework to initialize during

Linux boot up. Secondly, after Linux boot up, the console device sits in the lowest levels of

Linux in order to bring critical information out of the system as soon as possible. It is not

involved in all the complexity of TTY management.

The serial ports are named ttyS0, ttyS1, etc. (and usually correspond respectively to COM1,

COM2, etc.). For each such serial port, there is a special file in the /dev (device) directory.

Major number 4 is associated to the ttyS driver.

For the UART layer major number is 4 and the minor number ranges between 64-255.

Note:

DMA support is not present in the driver.

4.8.3

TTY framework in Linux

The TTY driver serves as an intermediary device between hardware device drivers and user

applications to provide line buffering of input and management of input and output. The

driver is purely software oriented and makes no direct communication with physical

hardware. Instead, the TTY driver relies on an underlying device driver to communicate

directly with the hardware. It has the following system calls:

Open

Open associates a TTY with an underlying char-oriented hardware device. The device

should already be opened and initialized before the TTY is opened. When the port is

opened for the first time you can do the necessary hardware initialization and memory

allocation.

Since a serial port is a file, the open() function is used to access it.

●

Open() returns a file descriptor, which is just an int.

●

Open() returns -1 if any error occurs.

To open a serial port:

#include <fcntl.h>

int fd;

/* The name argument is the filename

"/dev/ttyS0" in case of SPEAr300

Application

Hardware

Kernel space

User space

Console driver

Linux TTY layer

UART driver

UART hardware ARM PL011

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