Chapter 16. Serial driver details

Two different classes of serial drivers are provided as a standard part of the eCos system. These are described as “raw serial” (serial) and “tty-like” (tty).

16.1. Raw Serial Driver

Use the include file <cyg/io/serialio.h> for this driver.

The raw serial driver is capable of sending and receiving blocks of raw data to a serial device. Controls are provided to configure the actual hardware, but there is no manipulation of the data by this driver.

There may be many instances of this driver in a given system, one for each serial channel. Each channel corresponds to a physical device and there will typically be a device module created for this purpose. The device modules themselves are configurable, allowing specification of the actual hardware details, as well as such details as whether the channel should be buffered by the serial driver, etc.

16.1.1. Runtime Configuration

Runtime configuration is achieved by exchanging data structures with the driver via the cyg_io_set_config() and cyg_io_get_config() functions.

typedef struct {
    cyg_serial_baud_rate_t baud;
    cyg_serial_stop_bits_t stop;
    cyg_serial_parity_t parity;
    cyg_serial_word_length_t word_length;
    cyg_uint32 flags;
} cyg_serial_info_t;

The field word_length contains the number of data bits per word (character). This must be one of the values:

CYGNUM_SERIAL_WORD_LENGTH_5
CYGNUM_SERIAL_WORD_LENGTH_6
CYGNUM_SERIAL_WORD_LENGTH_7
CYGNUM_SERIAL_WORD_LENGTH_8

The field baud contains a baud rate selection. This must be one of the values:

CYGNUM_SERIAL_BAUD_50
CYGNUM_SERIAL_BAUD_75
CYGNUM_SERIAL_BAUD_110
CYGNUM_SERIAL_BAUD_134_5
CYGNUM_SERIAL_BAUD_150
CYGNUM_SERIAL_BAUD_200
CYGNUM_SERIAL_BAUD_300
CYGNUM_SERIAL_BAUD_600
CYGNUM_SERIAL_BAUD_1200
CYGNUM_SERIAL_BAUD_1800
CYGNUM_SERIAL_BAUD_2400
CYGNUM_SERIAL_BAUD_3600
CYGNUM_SERIAL_BAUD_4800
CYGNUM_SERIAL_BAUD_7200
CYGNUM_SERIAL_BAUD_9600
CYGNUM_SERIAL_BAUD_14400
CYGNUM_SERIAL_BAUD_19200
CYGNUM_SERIAL_BAUD_38400
CYGNUM_SERIAL_BAUD_57600
CYGNUM_SERIAL_BAUD_115200
CYGNUM_SERIAL_BAUD_234000

The field stop contains the number of stop bits. This must be one of the values:

CYGNUM_SERIAL_STOP_1
CYGNUM_SERIAL_STOP_1_5
CYGNUM_SERIAL_STOP_2
[Note]Note

On most hardware, a selection of 1.5 stop bits is only valid if the word (character) length is 5.

The field parity contains the parity mode. This must be one of the values:

CYGNUM_SERIAL_PARITY_NONE
CYGNUM_SERIAL_PARITY_EVEN
CYGNUM_SERIAL_PARITY_ODD
CYGNUM_SERIAL_PARITY_MARK
CYGNUM_SERIAL_PARITY_SPACE

The field flags is a bitmask which controls the behavior of the serial device driver. It should be built from the values CYG_SERIAL_FLAGS_xxx defined below:

#define CYG_SERIAL_FLAGS_RTSCTS 0x0001

If this bit is set then the port is placed in “hardware handshake” mode. In this mode, the CTS and RTS pins control when data is allowed to be sent/received at the port. This bit is ignored if the hardware does not support this level of handshake.

typedef struct {
    cyg_int32 rx_bufsize;
    cyg_int32 rx_count;
    cyg_int32 tx_bufsize;
    cyg_int32 tx_count;
} cyg_serial_buf_info_t;     

The field rx_bufsize contains the total size of the incoming data buffer. This is set to zero on devices that do not support buffering (i.e. polled devices).

The field rx_count contains the number of bytes currently occupied in the incoming data buffer. This is set to zero on devices that do not support buffering (i.e. polled devices).

The field tx_bufsize contains the total size of the transmit data buffer. This is set to zero on devices that do not support buffering (i.e. polled devices).

The field tx_count contains the number of bytes currently occupied in the transmit data buffer. This is set to zero on devices that do not support buffering (i.e. polled devices).

16.1.2. API Details

16.1.2.1. cyg_io_write

cyg_io_write(handle, buf, len)

Send the data from buf to the device. The driver maintains a buffer to hold the data. The size of the intermediate buffer is configurable within the interface module. The data is not modified at all while it is being buffered. On return, *len contains the amount of characters actually consumed .

It is possible to configure the write call to be blocking (default) or non-blocking. Non-blocking mode requires both the configuration option CYGOPT_IO_SERIAL_SUPPORT_NONBLOCKING to be enabled, and the specific device to be set to non-blocking mode for writes (see cyg_io_set_config()).

In blocking mode, the call will not return until there is space in the buffer and the entire contents of buf have been consumed.

In non-blocking mode, as much as possible gets consumed from buf. If everything was consumed, the call returns ENOERR. If only part of the buf contents was consumed, -EAGAIN is returned and the caller must try again. On return, *len contains the number of characters actually consumed .

The call can also return -EINTR if interrupted via the cyg_io_get_config()/ABORT key.

16.1.2.2. cyg_io_read

cyg_io_read(handle, buf, len)

Receive data into the buffer, buf, from the device. No manipulation of the data is performed before being transferred. An interrupt driven interface module will support data arriving when no read is pending by buffering the data in the serial driver. Again, this buffering is completely configurable. On return, *len contains the number of characters actually received.

It is possible to configure the read call to be blocking (default) or non-blocking. Non-blocking mode requires both the configuration option CYGOPT_IO_SERIAL_SUPPORT_NONBLOCKING to be enabled, and the specific device to be set to non-blocking mode for reads (see cyg_io_set_config()).

In blocking mode, the call will not return until the requested amount of data has been read.

In non-blocking mode, data waiting in the device buffer is copied to buf, and the call returns immediately. If there was enough data in the buffer to fulfill the request, ENOERR is returned. If only part of the request could be fulfilled, -EAGAIN is returned and the caller must try again. On return, *len contains the number of characters actually received.

The call can also return -EINTR if interrupted via the cyg_io_get_config()/ABORT key.

16.1.2.3. cyg_io_get_config

cyg_io_get_config(handle, key, buf, len)

This function returns current [runtime] information about the device and/or driver.

CYG_IO_GET_CONFIG_SERIAL_INFO
Buf type:
cyg_serial_info_t
Function:
This function retrieves the current state of the driver and hardware. This information contains fields for hardware baud rate, number of stop bits, and parity mode. It also includes a set of flags that control the port, such as hardware flow control.
CYG_IO_GET_CONFIG_SERIAL_BUFFER_INFO
Buf type:
cyg_serial_buf_info_t
Function:
This function retrieves the current state of the software buffers in the serial drivers. For both receive and transmit buffers it returns the total buffer size and the current number of bytes occupied in the buffer. It does not take into account any buffering such as FIFOs or holding registers that the serial device itself may have.
CYG_IO_GET_CONFIG_SERIAL_OUTPUT_DRAIN
Buf type:
void *
Function:
This function waits for any buffered output to complete. This function only completes when there is no more data remaining to be sent to the device.
CYG_IO_GET_CONFIG_SERIAL_OUTPUT_FLUSH
Buf type:
void *
Function:
This function discards any buffered output for the device.
CYG_IO_GET_CONFIG_SERIAL_INPUT_DRAIN
Buf type:
void *
Function:
This function discards any buffered input for the device.
CYG_IO_GET_CONFIG_SERIAL_ABORT
Buf type:
void*
Function:
This function will cause any pending read or write calls on this device to return with -EABORT.
CYG_IO_GET_CONFIG_SERIAL_READ_BLOCKING
Buf type:
cyg_uint32 (values 0 or 1)
Function:
This function will read back the blocking-mode setting for read calls on this device. This call is only available if the configuration option CYGOPT_IO_SERIAL_SUPPORT_NONBLOCKING is enabled.
CYG_IO_GET_CONFIG_SERIAL_WRITE_BLOCKING
Buf type:
cyg_uint32 (values 0 or 1)
Function:
This function will read back the blocking-mode setting for write calls on this device. This call is only available if the configuration option CYGOPT_IO_SERIAL_SUPPORT_NONBLOCKING is enabled.

16.1.2.4. cyg_io_set_config

cyg_io_set_config(handle, key, buf,len)

This function is used to update or change runtime configuration of a port.

CYG_IO_SET_CONFIG_SERIAL_INFO
Buf type:
cyg_serial_info_t
Function:
This function updates the information for the driver and hardware. The information contains fields for hardware baud rate, number of stop bits, and parity mode. It also includes a set of flags that control the port, such as hardware flow control.
CYG_IO_SET_CONFIG_SERIAL_READ_BLOCKING
Buf type:
cyg_uint32 (values 0 or 1)
Function:
This function will set the blocking-mode for read calls on this device. This call is only available if the configuration option CYGOPT_IO_SERIAL_SUPPORT_NONBLOCKING is enabled.
CYG_IO_SET_CONFIG_SERIAL_WRITE_BLOCKING
Buf type:
cyg_uint32 (values 0 or 1)
Function:
This function will set the blocking-mode for write calls on this device. This call is only available if the configuration option CYGOPT_IO_SERIAL_SUPPORT_NONBLOCKING is enabled.

16.2.  TTY driver

Use the include file <cyg/io/ttyio.h> for this driver.

This driver is built on top of the simple serial driver and is typically used for a device that interfaces with humans such as a terminal. It provides some minimal formatting of data on output and allows for line-oriented editing on input.

16.2.1. Runtime configuration

Runtime configuration is achieved by exchanging data structures with the driver via the cyg_io_set_config() and cyg_io_get_config() functions.

typedef struct {
    cyg_uint32 tty_out_flags;
    cyg_uint32 tty_in_flags;
} cyg_tty_info_t;

The field tty_out_flags is used to control what happens to data as it is send to the serial port. It contains a bitmap comprised of the bits as defined by the CYG_TTY_OUT_FLAGS_xxx values below.

#define CYG_TTY_OUT_FLAGS_CRLF 0x0001 // Map '\n' => '\r\n' on output

If this bit is set in tty_out_flags, any occurrence of the character "\n" will be replaced by the sequence "\r\n" before being sent to the device.

The field tty_in_flags is used to control how data is handled as it comes from the serial port. It contains a bitmap comprised of the bits as defined by the CYG_TTY_IN_FLAGS_xxx values below.

#define CYG_TTY_IN_FLAGS_CR 0x0001 // Map '\r' => '\n' on input

If this bit is set in tty_in_flags, the character "\r" (“return” or “enter” on most keyboards) will be mapped to "\n".

#define CYG_TTY_IN_FLAGS_CRLF 0x0002 // Map '\r\n' => '\n' on input

If this bit is set in tty_in_flags, the character sequence "\r\n" (often sent by DOS/Windows based terminals) will be mapped to "\n".

#define CYG_TTY_IN_FLAGS_ECHO 0x0004 // Echo characters as processed

If this bit is set in tty_in_flags, characters will be echoed back to the serial port as they are processed.

#define CYG_TTY_IN_FLAGS_BINARY 0x0008 // No input processing

If this bit is set in tty_in_flags, the input will not be manipulated in any way before being placed in the user’s buffer.

16.2.2. API details

cyg_io_read(handle, buf, len)

This function is used to read data from the device. In the default case, data is read until an end-of-line character ("\n" or "\r") is read. Additionally, the characters are echoed back to the [terminal] device. Minimal editing of the input is also supported.

[Note]Note

When connecting to a remote target via GDB it is not possible to provide console input while GDB is connected. The GDB remote protocol does not support input. Users must disconnect from GDB if this functionality is required.

cyg_io_write(handle, buf, len)

This function is used to send data to the device. In the default case, the end-of-line character "\n" is replaced by the sequence "\r\n".

cyg_io_get_config(handle, key, buf, len)

This function is used to get information about the channel’s configuration at runtime.

CYG_IO_GET_CONFIG_TTY_INFO
Buf type:
cyg_tty_info_t
Function:
This function retrieves the current state of the driver.

Serial driver keys (see above) may also be specified in which case the call is passed directly to the serial driver.

cyg_io_set_config(handle, key, buf, len)

This function is used to modify the channel’s configuration at runtime.

CYG_IO_SET_CONFIG_TTY_INFO
Buf type:
cyg_tty_info_t
Function:
This function changes the current state of the driver.

Serial driver keys (see above) may also be specified in which case the call is passed directly to the serial driver.