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struct pointing to RTC in far memory location.

I have a Real Time Clock (RTC) located at address 0x180000

I would like to create a struct of the RTC registers and map it to 0x180000 where the RTC registers start, located in far xdata memory or (HDATA).

I want to access the RTC such as

 RTC.Month = 8;

Would I use the FVAR or far * in some way to accomplish this?

Thanks for your help..

0 Drew Davis over 22 years ago
I think that would be a reasonable thing to do, yes. Something like:

#define RtcAddr 0x180000 typedef struct { U16 year; U8 month; U8 day; } RtcReg; #define Rtc FVAR (RtcReg, RtcAddr) ... Rtc.month = 8;

If you peek at the definition of FVAR, you'll see that it's a macro that really just adjusts an address for the Keil tag byte coding scheme, casts it to a pointer to the type given, and dereferences that pointer. So it's a way to claim that a variable of a particular type lives at a particular address, without actually declaring a variable.

The way I read the manual, you should also be able to use the _at_ extension to declare a variable at the right location:

RtcReg far Rtc _at_ 0x180000UL; Rtc.month = 8;

But I've never done it that way myself.

A variation would be to put the variable in a source file by itself, so that it will wind up in a data segment by itself, and then use the linker to specify the location of that data segment. A bit more trouble, but it avoids the use of one non-standard extension if portability is a concern.

(The first method is, IMO, the most portable; people tend to forget about the linker configuration files, and there's no commonality at all between linker control file / directive syntax from one linker to another. Most projects I've worked on use the cast-an-address route to physically locate a variable.)
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0 Jim Leifker over 22 years ago

This is what I came up with that works, it is accessing the RTC at 0x180000.

#define RTC_ADDRESS (0x180000ul)

typedef struct RTC_REGS
{
	unsigned char hsec;
	unsigned char sec;
	unsigned char min;
	unsigned char min_alarm;
	unsigned char hour;
	unsigned char hour_alarm;
	unsigned char day;
	unsigned char day_alarm;
	unsigned char date;
	unsigned char month;
	unsigned char year;
	unsigned char cmd;
	unsigned char wd_hsec;
	unsigned char wd_sec;
	unsigned char dummy1;
	unsigned char dummy2;
	unsigned char user[48];
} RTC_regs;

RTC_regs far *RTC_registers = &FVAR(RTC_regs, RTC_ADDRESS);

void RTC_Init( void )
{
		RTC_registers->hsec = 0;
		RTC_registers->sec  = 0;
		RTC_registers->min  = 0;
}