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:
o stdiodemo.c This is the main example file.
o defines.h Contains some global defines, like the LCD wiring
o hd44780.c Implementation of an HD44780 LCD display driver
o hd44780.h Interface declarations for the HD44780 driver
o lcd.c Implementation of LCD character IO on top of the HD44780 driver
o lcd.h Interface declarations for the LCD driver
o uart.c Implementation of a character IO driver for the internal UART
o 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):
o The 'q' (quit) command has the same effect of leaving the main loop.
o The 'l' (LCD) command takes its second argument, and sends it to the
LCD.
o 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:
o \b (back space) or \177 (delete) deletes the previous character
o ^u (control-U, ASCII NAK) deletes the entire input buffer
o ^w (control-W, ASCII ETB) deletes the previous input word, delimited
by white space
o ^r (control-R, ASCII DC2) sends a \r, then reprints the buffer
(refresh)
o \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
Author
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Version 1.6.8 Thu Aug 12 Using the standard IO facilities(3)