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SFR bit setting

 
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aroonkriss



Joined: 03 Dec 2012
Posts: 19
Location: kerala

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SFR bit setting
PostPosted: Sun Dec 30, 2012 9:49 pm     Reply with quote
Hi all,

I need to set individual bits in SFRs. I tried the following code but it didn't work.
Code:

#byte U1MODE=getenv("SFR:U1MODE")
#byte U2MODE=getenv("SFR:U2MODE")

#use rs232(baud=9600,parity=N,xmit=PIN_C13,rcv=PIN_C14,bits=8,stream=UART1)
#use rs232(baud=2400,parity=N,xmit=PIN_F5,rcv=PIN_F4,bits=8,stream=UART2)


bit_set(U1MODE,10);  // to enable the alternate pins for uart1

bit_clear(U2MODE,15); // to disable the uart

I checked the uart2 part, it was communicating even though I disabled the enable bit.

I need to use the alternate pins of uart1. So I need to set the 'altio' bit of u1mode register.

PLEASE HELP ME. ANY HELP WOULD BE APPRECIATED.
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aroonkriss



Joined: 03 Dec 2012
Posts: 19
Location: kerala

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SFR bit setting
PostPosted: Sun Dec 30, 2012 11:13 pm     Reply with quote
I used the following code and it worked.
Code:

#word U1MODE=0x20C
#word U2MODE=0x216

#bit  ALTIO = U1MODE.10   
#bit  U2EN = U2MODE.15
#bit  U1EN = U1MODE.15

But I wish to know what was my mistake with the earlier code. Thanks in advance.
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Ttelmah



Joined: 11 Mar 2010
Posts: 19504

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PostPosted: Mon Dec 31, 2012 2:11 am     Reply with quote
bit_set(U1MODE,10); // to enable the alternate pins for uart1

Allows you to set a bit in a variable, but is _limited to the size of the variable_.

#byte U1MODE=getenv("SFR:U1MODE")

maps a _byte_ variable at the address - 8 bits

#word U1MODE=getenv("SFR:U1MODE")

maps a _word_ variable at the address - 16 bits

You can't access 'bit 10', in a byte sized variable.

If you code:
Code:

#byte U1MODE=getenv("SFR:U1MODE")
#byte U2MODE=getenv("SFR:U2MODE")

bit_clear(U2MODE,15); // to disable the uart

Then look at the assembler:
Code:

....................    bit_clear(U2MODE,15);  // to enable the alternate pins for uart1
.................... 

No instruction is generated, since you are trying to access a bit beyond the end of the variable.
However code as:
Code:

#word U1MODE=getenv("SFR:U1MODE")
#word U2MODE=getenv("SFR:U2MODE")

bit_clear(U2MODE,15); // to disable the uart

The assembler is then:
Code:

....................    bit_clear(U2MODE,15); // to disable the uart
0130:  BCLR.B  217.7

Which is accessing the seventh bit in the byte after 'U2MODE'.

As a more 'elegant' route, why not use the compiler's knowledge of bits:

Code:

#bit UART_ALTIO=getenv("BIT:ALTIO")
#bit UART2_ENABLE=getenv("BIT:U2MODE.UARTEN")
 
   UART_ALTIO=TRUE;
   UART2_ENABLE=FALSE;

Or use the compiler's ability to handle this for you:
Code:

#use rs232(baud=9600,parity=N,UART1A,bits=8,stream=UART1)
//select UART1 on alternate pins (1A)

#use rs232(baud=2400,parity=N,xmit=PIN_F5,rcv=PIN_F4,bits=8,stream=UART2)

    setup_uart(FALSE,UART2); //turn off UART2


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