[U-Boot] [PATCH 2/5] Enable booting of mx28 without battery

[Graeme Russ]

Hi Marek,

On 22/01/15 10:27, Marek Vasut wrote:
> On Wednesday, January 21, 2015 at 12:55:15 PM, Graeme Russ wrote:
>> Section 4.1.2 of Freescale Application Note AN4199 describes the
>> configuration required to operate the mx28 from a 5V source without a
>> battery. This patch implements the changes to the Freescale bootlets
>> which allow this configuration to properly boot the mx28 processor
>>
>> Signed-off-by: Graeme Russ <gruss at tss-engineering.com>
>
> I have to admit I don't really like this boatload of new ifdeffery. Isn't it
> possible to detect if the battery is or isn't connected at runtime ?

I doubt it. If you look at the circuit in AN4199, the BATTERY and 
DCDC_BATT pins are hooked up to VDD4P2 via 1k resistors. What the mx28 
does internally is measure the voltages on the VDD5V and either 
BATTERY/DCDC_BATT pins and decides which to use (preferencing the 
battery unless it goes below a threshold).

Without this patch, the mx28 sees a good battery voltage and switches to 
battery mode which is drawn of DCDC4P2 which is now trying to be driven 
by the battery input which, of course, will not work.

So the only way to figure it out at runtime would be to have the battery 
inputs at 0v so you could see there is no battery.

The only other option would be to use a GPIO pin

Regards,


Graeme