Using the keyboard to enable USB booting

Adam Wang adam at
Thu Aug 27 23:29:47 EDT 2009

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                       
b) 10K                                         Can                    
c)  12.4K                                      Can not                  
d)  20K                                         Can not                  

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?

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