u-boot/doc/README.omap-reset-time
Lokesh Vutla 0b1b60c779 ARM: OMAP5: Fix warm reset with USB cable connected
Warm reset on OMAP5 freezes when USB cable is connected.
Fix requires PRM_RSTTIME.RSTTIME1 to be programmed
with the time for which reset should be held low for the
voltages and the oscillator to reach stable state.

There are 3 parameters to be considered for calculating
the time, which are mostly board and PMIC dependent.
-1- Time taken by the Oscillator to shut + restart
-2- PMIC OTP times
-3- Voltage rail ramp times, which inturn depends on the
PMIC slew rate and value of the voltage ramp needed.

In order to keep the code in u-boot simple, have a way
for boards to specify a pre computed time directly using
the 'CONFIG_OMAP_PLATFORM_RESET_TIME_MAX_USEC'
option. If boards fail to specify the time, use a default
as specified by 'CONFIG_DEFAULT_OMAP_RESET_TIME_MAX_USEC' instead.
Using the default value translates into some ~22ms and should work in
all cases.
However in order to avoid this large delay hiding other bugs,
its recommended that all boards look at their respective data
sheets and specify a pre computed and optimal value using
'CONFIG_OMAP_PLATFORM_RESET_TIME_MAX_USEC'

In order to help future board additions to compute this
config option value, add a README at doc/README.omap-reset-time
which explains how to compute the value. Also update the toplevel
README with the additional option and pointers to
doc/README.omap-reset-time.

Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
[rnayak@ti.com: Updated changelog and added the README]
Signed-off-by: Rajendra Nayak <rnayak@ti.com>
2013-05-10 08:25:55 -04:00

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README on how reset time on OMAPs should be calculated
CONFIG_OMAP_PLATFORM_RESET_TIME_MAX_USEC:
Most OMAPs' provide a way to specify the time for
which the reset should be held low while the voltages
and Oscillator outputs stabilize.
This time is mostly board and PMIC dependent. Hence the
boards are expected to specify a pre-computed time
using the above option, (the details on how to compute
the value are given below) without which a default time
as specified by CONFIG_DEFAULT_OMAP_RESET_TIME_MAX_USEC
is used.
The value for CONFIG_OMAP_PLATFORM_RESET_TIME_MAX_USEC
can be computed using a summation of the below 3 parameters
-1- Time taken by the Osciallator to stop and restart
-2- PMIC OTP time
-3- Voltage ramp time, which can be derived using the
PMIC slew rate and value of voltage ramp needed.