HAL Port — Implementation Details
This documentation explains how the eCos HAL specification has been mapped onto the ST40EB hardware, and should be read in conjunction with that specification. The ST40EB platform HAL package complements the SH architectural HAL and the SH4 variant HAL. It provides functionality which is specific to the target board.
Following a hard or soft reset the HAL will initialize or reinitialize most of the on-chip peripherals. There is an exception for RAM startup applications which depend on a ROM monitor for certain services.
For ROM startup, the HAL will perform additional initialization,
setting up the external SDRAM and programming the various internal
registers including clocks, EMI and LMI. The values used for most
of these registers are assigned fixed values from a table in the
The platform HAL provides configuration options for the eCos system clock. This always uses the hardware timer 0, which should not be used directly by application code. Timer 1 is used to implement a microsecond resolution busy delay service. Timer 2 is not used by eCos so application code is free to manipulate this as required. The actual HAL macros for managing the clock are provided by the SH architecture processor HAL.
There is a software model of the structure of the SH family clock supply subsystem which performs the correct calculations to yield not only the inputs for the CPU clock but also the peripheral clocks fed to the serial device, memory controllers and other devices. The values for the master crystal, the PLL multipliers and various dividers are supplied by the platform HAL. Some care must be taken in defining these since wrong values will cause the timers and the SCIF baud rate to be miscalculated (resulting visibly in garbage on the serial output).
The ST40 extends the SH family clock model by providing a CLOCKGEN subsystem allowing the hardware clock frequency to be controlled. The CLOCKGENA.PLL1CR register is the primary means to do this, and is initialised by switches 1, 2 and 3 on DIP switch block SW3. As the ST40EB is fitted with an ST40RA166 processor, it is assumed that a speed of 166MHz has been selected. This corresponds to SW3-1 set to OFF, SW3-2, set to OFF and SW3-3 set to ON.
If the DIP switches are changed from the
default then the value of
must be changed to match. Consult the ST40RA documentation on appropriate
values for the clock and associated divider options for the subclocks
if you wish these to be altered from the default.
The ST40EB platform HAL does not affect the implementation of other parts of the eCos HAL specification. The SH4 variant HAL, and the SH architectural HAL documentation should be consulted for further details.
It should be noted that the floating point support in the SH HAL has a
caveat that, if the FPSCR register is changed, it may get reverted at a
later stage by certain operations performed by the GCC compiler. This
behaviour is intentional as the alternative would be to update the GCC
compiler's internal state about the FPSCR at every context switch which
would be expensive for a feature that is unlikely to be used frequently.
If the FPSCR is to be changed by the application, the developer
should call the function
__set_fpscr(int), passing it
the new FPSCR value.
|2021-10-22||eCosPro Non-Commercial Public License|