How to use UARTs

Overview

This How-To is meant to be a starting point for people to learn use UART for IGEP v2 devices as quickly and easily as possible. In this how-to, we use Linaro Headless with Kernel 2.6.35.y and UART test program

A user reported that this page might contain some broken, wrong or missing links to external pages. Thank you for your patience as it is being fixed.

More information about UART.

Note: This program has a debug option using GPIOs to debug via oscilloscope, this option is useful to learnt about the Kernel latencies. Debug has set to "write" and "select" functions.

More information

Feedback and Contributing

At any point, if you see a mistake you can contribute to this How-To.

Preparing IGEP

Configure UART1 and UART3 in IGEP with RS232 levels on J960 connector

To test UARTs we use UART1 (dev/ttyS0) and UART3 (dev/ttyS2) via J960 connector using RS232 standard.

NOTE: Make sure that you installed open SSH server. If you don't have it go here.

By default, UART1 is used by RS485 driver, we need to disable it, edit igep.ini file:

board.ei485= yes

Maybe you don't found this line, replaced by or type this:

board.ei485= no

By default, UART3 is used by Kernel serial console, we need to disable it, go to igep.ini file, comment next line:

console=ttyS2,115200n8

Check UARTs devices

Make sure that Kernel detects UARTs, type:

dmesg | grep tty

The result will be similar at that:

root@localhost:~/PROGRAM# dmesg | grep tty
[    0.000000] console [tty0] enabled
[    0.598999] serial8250.0: ttyS0 at MMIO 0x4806a000 (irq = 72) is a ST16654
[    0.784057] serial8250.1: ttyS1 at MMIO 0x4806c000 (irq = 73) is a ST16654
[    0.969085] serial8250.2: ttyS2 at MMIO 0x49020000 (irq = 74) is a ST16654
[    1.153503] serial8250.3: ttyS3 at MMIO 0x49042000 (irq = 80) is a ST16654
root@localhost:~/PROGRAM# 

Get information about UARTs, type:

setserial -g /dev/ttyS[0123]

The result will be similar at that:

root@localhost:~/PROGRAM# setserial -g /dev/ttyS*
/dev/ttyS0, UART: 16654, Port: 0x0000, IRQ: 72
/dev/ttyS1, UART: 16654, Port: 0x0000, IRQ: 73
/dev/ttyS2, UART: 16654, Port: 0x0000, IRQ: 74
/dev/ttyS3, UART: 16654, Port: 0x0000, IRQ: 80
root@localhost:~/PROGRAM# 

If you don't have setserial installed type:

apt-get install setserial

NOTE: Starting from Kernel 2.6.37.y, ttySx are named ttyOx. You need to edit source program.

Connect peripherals

Connect IGEPv2 board using J960 connector like this:

 J960    
                    ------  
    (Not used) x---| 1  2 |---o (Connect to 8)
(Connect to 9) o---| 3  4 |---x (Not used)
         (GND)·|---| 5  6 |---|· (GND)
     (Not used)x---| 7  8 |---o (Connect to 2)
(Connect to 3) o---| 9 10 |---x (Not used)
                    ------  

Send some data

Use minicom to prove it, open two terminals via SSH and log with root user:

Terminal 1:

minicom -D /dev/ttyS0

Terminal 2:

minicom -D /dev/ttyS2 

Type something in one terminal.

Schematics

RS232 Schematic Igep V2

Uart Schematic Igep Module

Compile and run program

Download program source.

As explained above, the program uses GPIO for debug option via oscilloscope, it is necessary install user-gpio-drv.ko driver inside Linux Kernel, more information here .

There are some interesting things to configure, before compiling the program. Open it, seek next lines:

//Define Array char size
#define SIZE 30
//Define Debug mode:0==Disable
#define DEBUG 0

//Define wait select
#define WAIT 5

//Set GPIO Debugger port
//Be sure that mux is correctly configured and GPIOs are not used
#define GPIO1 136
#define GPIO2 137 

Constant SIZE: it determines the max number of char than can be sent

Constant DEBUG: it can break program infinite loop part, it can be useful to debug some errors, 0 (infinite loop) or positive number determine loop max entrances

Constant WAIT: it configurea select max time.

Constants GPIO1 and GPIO2: they configure GPIOs used for debug via oscilloscope. Take care to configure mux and don't use them for other purposes

Once you have installed module. Compile program using your Cross Compiler, I used arm-linux-gnueabi-:

arm-linux-gnueabi-gcc uart-test-beta1.c -o uart-test-beta1 

Testing UART

Some tests do it:

Decode Characters

Decoded with ASCII table.

Decoding "1\n"	Decoding "0\n"	Decoding "100000\n"

Click to enlarge image

select timeout

Program sends information via UART1 but don't receive response via UART3, because only a process is executed. This problem is repeated every 5 seconds (constant WAIT=5) until other process sends information via UART3 at the same speed. If "select" hasn't been implemented, process would have stayed blocked.

UART1=Blue and UART3=Green

Click to enlarge image

RS232 Voltage params

More information here.

Vpp=11.8V aprox , Vmin=-5.6V aprox

Click to enlarge image

Speed Overview

Note that wave is degraded when speed is higher

Overview signal to 300 bauds	Overview signal to 4800 bauds	Overview signal to 115200 bauds	Overview signal to 3000000 bauds
Specific wave to 300 bauds	Specific wave to 4800 bauds	Specific wave to 115200 bauds	Specific wave to 3000000 bauds

Click to enlarge image

Lag

You can see that RS232 have some lag between transmisions. This caused mainly by kernel process management. UART peripheral is controlled by Linux and It isn't a real-time operating system. 

	select: blocked until all data is available.
Lag between UART1 and UART3 (300 bauds)	Lag between UART3 and UART1 (300 bauds)

Click to enlarge image

This program have three I/O functions that connect to UART driver via ioctl: (read(), write() and select()). But select() and write() change process stat to sleep mode until system don't receive or transmit all information via UART. "select" function is used to prevent that read function can be blocked when data aren't available.
The next images (program with debug_gpio enabled and 300 bauds speed) show when process is executed or is waiting. Blue=UART1, Violet="write" and Green="select". When GPIO has HIGH value, process is blocked.

select function time	Time between select and write	write function time	Time between write and select

Click to enlarge image

In this case:

select: blocked until all data is available (LF char received).

write: blocked until all data is send to UART buffer.

Error test

Every full process the number is increased twice. Program checks that this condicion is true, if it isn't accomplished, process will be finish.

Runs 16h without any transfer error (3000000 bauds)