AVR-Chat

Thanks for the info, that clean few things up!!!

Steven

-----Original Message-----
From: David VanHorn [mailto:dvanhorn@cedar.net]
Sent: Tuesday, 13 January 2004 8:47 AM
To: avrclub@yahoogroups.com; 'AVR-GCC'; 'AVR-Chat'; 'AVRCLUB'
Subject: [AVR-Chat] Re: [AVR club] STK button detection

>
>
>Can someone explain to me why the keys is compared by using 2, 4, 8, 0x10,
>0x20, 0x40, 0x80!!! And is there other way of detecting it?
>
>

If you look at the port bits as hex numbers, they are

0x80, 0x40, 0x20, 0x10, 0x08, 0x04, 0x02, 0x01

So if I wanted to look for bits 0 and 7, I would read the port, and and it with 0x81
The anding operation causes all "1"; bits in the input except those that are 1 in the mask, to become zeroes.

So if the PIN read in 0xA0: 10100000
in R16,PINB
(R16 now contains 0xA0h)

and we and with 0x80, we get 10000000
andi R16,0x80
(R16 now contains 0x80)

Now if bit 7 was zero, then the result of this operation would have set the zero flag, otherwise zero is cleared, so the next bit of code might be:

brne Button_Pressed ;(Branch Non Equal (or non zero))

Clear as mud? :)


The other way, is to do

sbic PINB,7 ;(Skip if bit in I/O is clear)
rjmp Button_Pressed ;Assuming the button gives a high when pressed.

Note: Make sure to always read the pin states on the PIN register, and make sure that the bits in the corresponding DDR register are zero to make those pins inputs.

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