The PNX8310 comes with two on-chip UARTs for serial communication.
Other PNX83xx processors come with compatible devices. The
CYGPKG_DEVS_SERIAL_MIPS_PNX8310 package provides an
eCos serial device driver. It can support up to three UARTs, depending
on the processor and on which of the UARTs are connected on any given
platform. If the CTS and RTS lines are connected then hardware flow
control is supported. Line status is supported for line breaks and for
certain communication errors. The UARTs do not have any support for
DTR, DSR, DCD or RI lines. On some platforms these lines may be
emulated using GPIO pins, but the driver does not currently have any
support for this.
Once application code accesses a UART through the serial driver, for example by opening a device /dev/ser0, the driver assumes that it has sole access to the hardware. This means that the UART should not be used for any other purpose, for example HAL diagnostics or gdb debug traffic. Instead such traffic has to go via another communication channel such as ethernet.
The PNX8310 serial driver should be loaded automatically when
selecting a platform containing a suitable processor, and it should
never be necessary to load it explicitly. The driver as a whole is
inactive unless the generic serial support,
CYGPKG_IO_SERIAL_DEVICES, is enabled. Exactly which
UART or UARTs are accessible on a given platform is determined by the
platform because even if the processor contains a UART the platform
may not provide a connector. Support for a given UART, say uart0, is
controlled by a configuration option
CYGPKG_DEVS_SERIAL_PNX8310_SERIAL0, which will be
active only if the platform enables the device. If a given UART is of
no interest to an application developer then it is possible to save
some memory by disabling this option.
For every enabled UART there are a further four configuration options:
Each serial device should have a unique name so that application code can open it. The default device names are /dev/ser0, /dev/ser1, and so on. It is only necessary to change these if the platform contains additional off-chip UARTs with clashing names.
By default the driver arranges for the UARTs to interrupt at a low interrupt priority. Usually there will be no need to change this because the driver does not actually do very much processing at ISR level, and anyway UARTs are not especially fast devices so do not require immediate attention.
Each UART will be initialized to a given baud rate. The default baud rate is 38400 because in most scenarios this is fast enough yet does not suffer from excess data corruption. Lower baud rates can be used if the application will operate in an electrically noisy environment, or higher baud rates up to 230400 can be used if 38400 does not provide sufficient throughput.
The serial driver will maintain software buffers for incoming and outgoing data. The former allows data to continue to arrive even if the application is still busy processing the previous transfer, and thus potentially improves throughput. The latter allows the application to transmit data without immediately blocking until the transfer is complete, often eliminating the need for a separate thread. The size of these buffers can be controlled via this configuration option, or alternatively these buffers can be disabled completely to save memory.
There are additional options in the generic serial I/O package
CYGPKG_IO_SERIAL which will affect this driver. For
CYGPKG_IO_SERIAL_FLOW_CONTROL and its
sub-options determine what flow control mechanism (if any) should be
This package also defines some configuration options related to testing. Usually these options are of no interest to application developers and can be ignored.
The generic driver needs some information from other packages about the exact hardware, for example how many UARTs are available and whether or not they are connected.
Another package, usually the platform HAL, should implement one or more of the interfaces
CYGINT_DEVS_SERIAL_PNX8310_UART2. Typically this is left to the platform HAL because even if the processor contains the UART device it may not be accessible on a given platform because there is no suitable connector.
If the RTS and CTS are connected for a given UART then the platform HAL should also implement the appropriate interface, for example
CYGINT_DEVS_SERIAL_PNX8310_UART0_RS232_RTSCTS. This will enable driver support for hardware handshaking.
If a given UART is supported then the generic driver will need to know where it is mapped in the address space. Typically this is handled by the processor or variant HAL package via a definition
HAL_PNX8310_UART0_BASEin cyg/hal/proc_io.h or cyg/hal/var_io.h.
On some platforms or processors additional initialization may be needed, for example to connect certain pins to the internal UART rather than to other on-chip devices. The processor or platform HAL can define a macro
HAL_PNX8310_UART0_PROC_INITfor this purpose.