Using the keyboard to enable USB booting

Carlos Camargo cicamargoba at
Fri Aug 28 06:34:02 EDT 2009

Hi Adam
Can you add more components to the board ? I mean IC, push buttons?
Because the best way maybe is add a little push button accesible from
battery hole. Or making available through an external hole? is
possible do thta at this moment?

On 8/27/09, Adam Wang <adam at> wrote:
> Hi Carlos,
> Carlos Camargo wrote:
>> Reading the Jz4720 chip, I can see that all GPIOs have an internal
>> Pull-up resistor enabled at reset.
>> So, We need probe some Resistor values, if with a 330 resistor from
>> KIN3 to GROUND the processor enter in USB mode, you can try with 2k,
>> 10k, etc
> I tried:
> _resistor from KIN3 to GND      Can USB boot or not while power on
>                      Keys work well_
> a) 2.4K                                        Can
>                                               fail
> b) 10K                                         Can
>                                                  fail
> c)  12.4K                                      Can not
>                                              unstable
> d)  20K                                         Can not
>                                              unstable
> a) & b) I measured the KEYOUT7 have a high pulse with 50us in 16.6KHz
> frequency while don't push "u" key.
> In fact, the scan frequency should show up only when you push keys.
> _c) & d)_ same as _a) & b_) but both they of course have different
> voltages(0.5V and 1.6V for example).
> Under 0.8V the KEYIN3 will recognize it's in low level, but even if
> choose lower resistor will let usb boot successfully but can not
> detect a normal key pushing.
> Do you think if i make a circuit to meet the BOOT_SEL1 its IO Schmitt
> trig level (0.88~0.99V, 1.44~1.56V) and also meet GPIO's pull-up
> resistor can
> solve this kind boundary situation?
> Thanks,
> Adam

Carlos Iván Camargo Bareño
Profesor Asistente
Departamento de Ingeniería Eléctrica y Electrónica
Universidad Nacional de Colombia
cicamargoba at

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