Provided by: avr-libc_1.8.0-4.1_all 
NAME
Using the standard IO facilities - This project illustrates how to use the standard IO
facilities (stdio) provided by this library. It assumes a basic knowledge of how the stdio
subsystem is used in standard C applications, and concentrates on the differences in this
library's implementation that mainly result from the differences of the microcontroller
environment, compared to a hosted environment of a standard computer.
This demo is meant to supplement the documentation, not to replace it.
Hardware setup
The demo is set up in a way so it can be run on the ATmega16 that ships with the STK500
development kit. The UART port needs to be connected to the RS-232 'spare' port by a
jumper cable that connects PD0 to RxD and PD1 to TxD. The RS-232 channel is set up as
standard input (stdin) and standard output (stdout), respectively.
In order to have a different device available for a standard error channel (stderr), an
industry-standard LCD display with an HD44780-compatible LCD controller has been chosen.
This display needs to be connected to port A of the STK500 in the following way:
PortHeaderFunction A0 1 LCD D4 A1 2 LCD D5 A2 3 LCD D6 A3 4 LCD D7 A4 5 LCD R/~W A5 6 LCD
E A6 7 LCD RS A7 8 unused GND9 GND VCC10Vcc
Wiring of the STK500Wiring of the STK500
The LCD controller is used in 4-bit mode, including polling the 'busy' flag so the R/~W
line from the LCD controller needs to be connected. Note that the LCD controller has yet
another supply pin that is used to adjust the LCD's contrast (V5). Typically, that pin
connects to a potentiometer between Vcc and GND. Often, it might work to just connect that
pin to GND, while leaving it unconnected usually yields an unreadable display.
Port A has been chosen as 7 pins are needed to connect the LCD, yet all other ports are
already partially in use: port B has the pins for in-system programming (ISP), port C has
the ports for JTAG (can be used for debugging), and port D is used for the UART
connection.
Functional overview
The project consists of the following files:
• stdiodemo.c This is the main example file.
• defines.h Contains some global defines, like the LCD wiring
• hd44780.c Implementation of an HD44780 LCD display driver
• hd44780.h Interface declarations for the HD44780 driver
• lcd.c Implementation of LCD character IO on top of the HD44780 driver
• lcd.h Interface declarations for the LCD driver
• uart.c Implementation of a character IO driver for the internal UART
• uart.h Interface declarations for the UART driver
A code walkthrough
stdiodemo.c
As usual, include files go first. While conventionally, system header files (those in
angular brackets < ... >) go before application-specific header files (in double quotes),
defines.h comes as the first header file here. The main reason is that this file defines
the value of F_CPU which needs to be known before including <utils/delay.h>.
The function ioinit() summarizes all hardware initialization tasks. As this function is
declared to be module-internal only (static), the compiler will notice its simplicity, and
with a reasonable optimization level in effect, it will inline that function. That needs
to be kept in mind when debugging, because the inlining might cause the debugger to 'jump
around wildly' at a first glance when single-stepping.
The definitions of uart_str and lcd_str set up two stdio streams. The initialization is
done using the FDEV_SETUP_STREAM() initializer template macro, so a static object can be
constructed that can be used for IO purposes. This initializer macro takes three
arguments, two function macros to connect the corresponding output and input functions,
respectively, the third one describes the intent of the stream (read, write, or both).
Those functions that are not required by the specified intent (like the input function for
lcd_str which is specified to only perform output operations) can be given as NULL.
The stream uart_str corresponds to input and output operations performed over the RS-232
connection to a terminal (e.g. from/to a PC running a terminal program), while the lcd_str
stream provides a method to display character data on the LCD text display.
The function delay_1s() suspends program execution for approximately one second. This is
done using the _delay_ms() function from <util/delay.h> which in turn needs the F_CPU
macro in order to adjust the cycle counts. As the _delay_ms() function has a limited range
of allowable argument values (depending on F_CPU), a value of 10 ms has been chosen as the
base delay which would be safe for CPU frequencies of up to about 26 MHz. This function is
then called 100 times to accomodate for the actual one-second delay.
In a practical application, long delays like this one were better be handled by a hardware
timer, so the main CPU would be free for other tasks while waiting, or could be put on
sleep.
At the beginning of main(), after initializing the peripheral devices, the default stdio
streams stdin, stdout, and stderr are set up by using the existing static FILE stream
objects. While this is not mandatory, the availability of stdin and stdout allows to use
the shorthand functions (e.g. printf() instead of fprintf()), and stderr can mnemonically
be referred to when sending out diagnostic messages.
Just for demonstration purposes, stdin and stdout are connected to a stream that will
perform UART IO, while stderr is arranged to output its data to the LCD text display.
Finally, a main loop follows that accepts simple 'commands' entered via the RS-232
connection, and performs a few simple actions based on the commands.
First, a prompt is sent out using printf_P() (which takes a program space string). The
string is read into an internal buffer as one line of input, using fgets(). While it would
be also possible to use gets() (which implicitly reads from stdin), gets() has no control
that the user's input does not overflow the input buffer provided so it should never be
used at all.
If fgets() fails to read anything, the main loop is left. Of course, normally the main
loop of a microcontroller application is supposed to never finish, but again, for
demonstrational purposes, this explains the error handling of stdio. fgets() will return
NULL in case of an input error or end-of-file condition on input. Both these conditions
are in the domain of the function that is used to establish the stream, uart_putchar() in
this case. In short, this function returns EOF in case of a serial line 'break' condition
(extended start condition) has been recognized on the serial line. Common PC terminal
programs allow to assert this condition as some kind of out-of-band signalling on an
RS-232 connection.
When leaving the main loop, a goodbye message is sent to standard error output (i.e. to
the LCD), followed by three dots in one-second spacing, followed by a sequence that will
clear the LCD. Finally, main() will be terminated, and the library will add an infinite
loop, so only a CPU reset will be able to restart the application.
There are three 'commands' recognized, each determined by the first letter of the line
entered (converted to lower case):
• The 'q' (quit) command has the same effect of leaving the main loop.
• The 'l' (LCD) command takes its second argument, and sends it to the LCD.
• The 'u' (UART) command takes its second argument, and sends it back to the UART
connection.
Command recognition is done using sscanf() where the first format in the format string
just skips over the command itself (as the assignment suppression modifier * is given).
defines.h
This file just contains a few peripheral definitions.
The F_CPU macro defines the CPU clock frequency, to be used in delay loops, as well as in
the UART baud rate calculation.
The macro UART_BAUD defines the RS-232 baud rate. Depending on the actual CPU frequency,
only a limited range of baud rates can be supported.
The remaining macros customize the IO port and pins used for the HD44780 LCD driver. Each
definition consists of a letter naming the port this pin is attached to, and a respective
bit number. For accessing the data lines, only the first data line gets its own macro
(line D4 on the HD44780, lines D0 through D3 are not used in 4-bit mode), all other data
lines are expected to be in ascending order next to D4.
hd44780.h
This file describes the public interface of the low-level LCD driver that interfaces to
the HD44780 LCD controller. Public functions are available to initialize the controller
into 4-bit mode, to wait for the controller's busy bit to be clear, and to read or write
one byte from or to the controller.
As there are two different forms of controller IO, one to send a command or receive the
controller status (RS signal clear), and one to send or receive data to/from the
controller's SRAM (RS asserted), macros are provided that build on the mentioned function
primitives.
Finally, macros are provided for all the controller commands to allow them to be used
symbolically. The HD44780 datasheet explains these basic functions of the controller in
more detail.
hd44780.c
This is the implementation of the low-level HD44780 LCD controller driver.
On top, a few preprocessor glueing tricks are used to establish symbolic access to the
hardware port pins the LCD controller is attached to, based on the application's
definitions made in defines.h.
The hd44780_pulse_e() function asserts a short pulse to the controller's E (enable) pin.
Since reading back the data asserted by the LCD controller needs to be performed while E
is active, this function reads and returns the input data if the parameter readback is
true. When called with a compile-time constant parameter that is false, the compiler will
completely eliminate the unused readback operation, as well as the return value as part of
its optimizations.
As the controller is used in 4-bit interface mode, all byte IO to/from the controller
needs to be handled as two nibble IOs. The functions hd44780_outnibble() and
hd44780_innibble() implement this. They do not belong to the public interface, so they are
declared static.
Building upon these, the public functions hd44780_outbyte() and hd44780_inbyte() transfer
one byte to/from the controller.
The function hd44780_wait_ready() waits for the controller to become ready, by
continuously polling the controller's status (which is read by performing a byte read with
the RS signal cleard), and examining the BUSY flag within the status byte. This function
needs to be called before performing any controller IO.
Finally, hd44780_init() initializes the LCD controller into 4-bit mode, based on the
initialization sequence mandated by the datasheet. As the BUSY flag cannot be examined yet
at this point, this is the only part of this code where timed delays are used. While the
controller can perform a power-on reset when certain constraints on the power supply rise
time are met, always calling the software initialization routine at startup ensures the
controller will be in a known state. This function also puts the interface into 4-bit mode
(which would not be done automatically after a power-on reset).
lcd.h
This function declares the public interface of the higher-level (character IO) LCD driver.
lcd.c
The implementation of the higher-level LCD driver. This driver builds on top of the
HD44780 low-level LCD controller driver, and offers a character IO interface suitable for
direct use by the standard IO facilities. Where the low-level HD44780 driver deals with
setting up controller SRAM addresses, writing data to the controller's SRAM, and
controlling display functions like clearing the display, or moving the cursor, this high-
level driver allows to just write a character to the LCD, in the assumption this will
somehow show up on the display.
Control characters can be handled at this level, and used to perform specific actions on
the LCD. Currently, there is only one control character that is being dealt with: a
newline character (\n) is taken as an indication to clear the display and set the cursor
into its initial position upon reception of the next character, so a 'new line' of text
can be displayed. Therefore, a received newline character is remembered until more
characters have been sent by the application, and will only then cause the display to be
cleared before continuing. This provides a convenient abstraction where full lines of text
can be sent to the driver, and will remain visible at the LCD until the next line is to be
displayed.
Further control characters could be implemented, e. g. using a set of escape sequences.
That way, it would be possible to implement self-scrolling display lines etc.
The public function lcd_init() first calls the initialization entry point of the lower-
level HD44780 driver, and then sets up the LCD in a way we'd like to (display cleared,
non-blinking cursor enabled, SRAM addresses are increasing so characters will be written
left to right).
The public function lcd_putchar() takes arguments that make it suitable for being passed
as a put() function pointer to the stdio stream initialization functions and macros
(fdevopen(), FDEV_SETUP_STREAM() etc.). Thus, it takes two arguments, the character to
display itself, and a reference to the underlying stream object, and it is expected to
return 0 upon success.
This function remembers the last unprocessed newline character seen in the function-local
static variable nl_seen. If a newline character is encountered, it will simply set this
variable to a true value, and return to the caller. As soon as the first non-newline
character is to be displayed with nl_seen still true, the LCD controller is told to clear
the display, put the cursor home, and restart at SRAM address 0. All other characters are
sent to the display.
The single static function-internal variable nl_seen works for this purpose. If multiple
LCDs should be controlled using the same set of driver functions, that would not work
anymore, as a way is needed to distinguish between the various displays. This is where the
second parameter can be used, the reference to the stream itself: instead of keeping the
state inside a private variable of the function, it can be kept inside a private object
that is attached to the stream itself. A reference to that private object can be attached
to the stream (e.g. inside the function lcd_init() that then also needs to be passed a
reference to the stream) using fdev_set_udata(), and can be accessed inside lcd_putchar()
using fdev_get_udata().
uart.h
Public interface definition for the RS-232 UART driver, much like in lcd.h except there is
now also a character input function available.
As the RS-232 input is line-buffered in this example, the macro RX_BUFSIZE determines the
size of that buffer.
uart.c
This implements an stdio-compatible RS-232 driver using an AVR's standard UART (or USART
in asynchronous operation mode). Both, character output as well as character input
operations are implemented. Character output takes care of converting the internal newline
\n into its external representation carriage return/line feed (\r\n).
Character input is organized as a line-buffered operation that allows to minimally edit
the current line until it is 'sent' to the application when either a carriage return (\r)
or newline (\n) character is received from the terminal. The line editing functions
implemented are:
• \b (back space) or \177 (delete) deletes the previous character
• ^u (control-U, ASCII NAK) deletes the entire input buffer
• ^w (control-W, ASCII ETB) deletes the previous input word, delimited by white space
• ^r (control-R, ASCII DC2) sends a \r, then reprints the buffer (refresh)
• \t (tabulator) will be replaced by a single space
The function uart_init() takes care of all hardware initialization that is required to put
the UART into a mode with 8 data bits, no parity, one stop bit (commonly referred to as
8N1) at the baud rate configured in defines.h. At low CPU clock frequencies, the U2X bit
in the UART is set, reducing the oversampling from 16x to 8x, which allows for a 9600 Bd
rate to be achieved with tolerable error using the default 1 MHz RC oscillator.
The public function uart_putchar() again has suitable arguments for direct use by the
stdio stream interface. It performs the \n into \r\n translation by recursively calling
itself when it sees a \n character. Just for demonstration purposes, the \a (audible bell,
ASCII BEL) character is implemented by sending a string to stderr, so it will be displayed
on the LCD.
The public function uart_getchar() implements the line editor. If there are characters
available in the line buffer (variable rxp is not NULL), the next character will be
returned from the buffer without any UART interaction.
If there are no characters inside the line buffer, the input loop will be entered.
Characters will be read from the UART, and processed accordingly. If the UART signalled a
framing error (FE bit set), typically caused by the terminal sending a line break
condition (start condition held much longer than one character period), the function will
return an end-of-file condition using _FDEV_EOF. If there was a data overrun condition on
input (DOR bit set), an error condition will be returned as _FDEV_ERR.
Line editing characters are handled inside the loop, potentially modifying the buffer
status. If characters are attempted to be entered beyond the size of the line buffer,
their reception is refused, and a \a character is sent to the terminal. If a \r or \n
character is seen, the variable rxp (receive pointer) is set to the beginning of the
buffer, the loop is left, and the first character of the buffer will be returned to the
application. (If no other characters have been entered, this will just be the newline
character, and the buffer is marked as being exhausted immediately again.)
The source code
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