Understanding this embedded c code

Question

I'm trying to understand this embedded c code. I think it means it is connecting port bits to some register in a bus. Correct me if I'm wrong. And what ever changes we make to the ports will be reflected on the bus registers. Here is the snippet of the code. Thanks.

/*--------------------------------------------------------------------------
Local Variables
--------------------------------------------------------------------------*/
// Port bits assigned to Amba Peripheral Bus (APB)
// P0^7..P0^0               // output=reg_addr, input=data_in (APB prdata)
sbit APB_SEL = P1^7;        // select a bus transaction
sbit APB_EN  = P1^6;        // enable/activate a component 0 = disable, 1 = enable

The code you posted is highly macro'ified / typedef'd, and it's pretty much impossible to tell from the small fragment you posted. I suggest you look for the definitions of sbit and P1 to give you some clues. — DaveR
– DaveR, Commented Jul 7, 2011 at 20:15
@Dave - apparently it's worse than you (or I) thought (see Praetorian's answer below). Apparently the "sbit" and "^" are compiler-specific extensions (no, that ^ isn't an XOR operator...) Ugh. — Dan
– Dan, Commented Jul 7, 2011 at 23:11

Praetorian · Accepted Answer · 2011-07-07 20:18:45Z

9

The code is defining bit positions to be read from registers. sbit defines a bit within a special function register (SFR).

sbit APB_SEL = P1^7;

Here P1 is a previously defined SFR. The line defines APB_SEL as bit 7 (zero-based numbering) of P1.

This link has additional details on the syntax.

answered Jul 7, 2011 at 20:18

Praetorian

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8 Comments

Jonathan Leffler Over a year ago

+1 for the URL. It isn't completely clear to me how an xor operation does that, but that is less relevant than that it is what seems to be the documented behaviour.

Praetorian Over a year ago

@Jonathan Leffler: I don't think that is an XOR operation, it looks like some hokey syntax invented just for this purpose.

Praetorian Over a year ago

C that is only understood by the 8051 (or its variants) compiler :-) Embedded compilers make up such keywords and syntax all the time. For instance, the TI compiler I use at work includes keywords such as cregister and interrupt.

Vikyboss Over a year ago

Thank you very much Praetorian. Your reply was really helpful! :)

Jonathan Leffler Over a year ago

@Praetorian: You are probably right about it being special syntax for that particular compiler. It's a different world, it seems. I have to work across platforms; clearly, code using that notation more or less cannot work across platforms. (It wouldn't work as described on the compilers I work with on the machines I work on). Something like that could be carefully put into a ghetto for the machine-specific parts of some program, but we seldom get quite that close to the hardware. Other people probably do it frequently; the world is a big place, but this was eye-opening, even so.

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Community · Accepted Answer · 2020-06-20 09:12:55Z

0

The sbit type defines a bit within a special function register (SFR). It is used in one of the following ways:

sbit name = sfr-name ^ bit-position;
sbit name = sfr-address ^ bit-position;
sbit name = sbit-address;

Where

name is the name of the SFR bit.

sfr-name is the name of a previously-defined SFR.

bit-position is the position of the bit within the SFR.

sfr-address is the address of an SFR.

sbit-address is the address of the SFR bit.

With typical 8051 applications, it is often necessary to access individual bits within an SFR. The sbit type provides access to bit-addressable SFRs and other bit-addressable objects. For example:

sbit EA = 0xAF;

This declaration defines EA as the SFR bit at address 0xAF. On the 8051, this is the enable all bit in the interrupt enable register.

edited Jun 20, 2020 at 9:12

CommunityBot

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answered Aug 10, 2015 at 8:47

Moorthi Muthu

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