RAM 6.0 BUX II Series Guia do Utilizador descarregar pdf (Página 117)

3 Porting a BSP to Custom Hardware

3.3 Finalizing Your BSP

109

match value is advanced. To implement the one-shot timer, a match count value is

computed for N ticks. When the system wakes up, the number of ticks slept is

computed using the timer count register. It should be noted that this time

computation requires a divide operation. Depending on your core hardware

design, this operation may be time consuming, but it cannot be avoided. Keeping

the one-shot timer and system timer locked is straightforward in this example

because the same timer is used for both functions. Care is taken to ensure that the

wake up time is in the future.

The ARM Amba timer provides three hardware timers. Each uses a preset value

that is reloaded when the timer reaches zero. Using different clock sources for the

tick and one-shot timers requires a time consuming conversion to compute the

fraction of a tick remaining in the system timer. Timers 1 and 2 use the same clock

source, so they are used for the system and one-shot timers. Because they use the

same source, they can be kept phase-locked with relative ease. The Amba timers

have a limited dynamic range. Changing the prescaler allows you to extend the

timer interrupt to approximately 700 ms. As noted earlier, changing the scalar

values reduces the accuracy of the sleep clock. However, this does not reduce

system clock accuracy because the system tick timer is allowed to run during sleep

periods. When the system wakes up, time is synchronized.

Non-Volatile Memory

The generic NVRAM and flash drivers reside in the target/src/drv/mem directory.

The template file templateNvRam.c is included in this directory. This file provides

the template driver to be used as the basis of non-volatile memory drivers,

including flash. However, do not use this template for the optional True Flash File

System (TrueFFS) product. For information on TrueFFS drivers, refer to the

documentation accompanying special memory technology drivers (MTDs) for

flash devices or see the VxWorks Device Driver Developer’s Guide: Flash (TrueFFS)

Drivers.

All BSPs are required to have some type of non-volatile memory interface, even if

non-volatile memory is not available. The two required routines are

sysNvRamGet( ) and sysNvRamSet( ). Both of these routines require an offset

parameter. Internally, these routines use the offset parameter as follows:

offset += NV_BOOT_OFFSET; /* boot line begins at <offset> = 0 */

if ((offset < 0) || (strLen < 0) || ((offset + strLen) >

NV_RAM_SIZE))

return (ERROR);

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RAM 6.0 BUX II Series Guia do Utilizador Página 117