RTC: Difference between revisions

at offset 0xA, 0xB, and 0xC in the CMOS RAM. To write 0x20 to Status Register A you would do this:

 

disable_ints(); // important that no interrupts happen (perform a CLI)

outportb(0x70, 0x8A); // select Status Register A, and disable NMI (by setting the 0x80 bit)

outportb(0x71, 0x20); // write to CMOS/RTC RAM

enable_ints(); // (perform an STI) and reenable NMI if you wish

 

Other bytes of the CMOS RAM are used by the RTC for other functions, or by the [[BIOS]] and other such services.

To turn on the periodic interrupt, all you have to do is:

 

disable_ints(); // disable interrupts

outportb(0x70, 0x8B); // select register B, and disable NMI

outportb(0x71, prev | 0x40); // write the previous value ORed with 0x40. This turns on bit 6 of register B

enable_ints();

 

This will turn on the IRQ with the default 1024 Hz rate. Be sure that you install the IRQ handler before you enable the RTC

A value of 0 disables the interrupt. To calculate a new frequency:

 

frequency = 32768 >> (rate-1);

 

"Rate" is the divider setting. If you select a rate of 1 or 2, the RTC will have problems and "roll over" so that it

rate you can select is 3. This will generate interrupts of 8 kHz or 122 uS. To change the rate:

 

rate &= 0x0F; // rate must be above 2 and not over 15

disable_ints();

outportb(0x71, (prev & 0xF0) | rate); //write only our rate to A. Note, rate is the bottom 4 bits.

enable_ints();

 

===Interrupts and Register C===

it is, just attach this code to the bottom of your IRQ handler to be sure you get another interrupt. If you're using Bochs, it is also recommended to read Status Register C after initialising the RTC, to make sure any RTC interrupts that were pending before/while the RTC was initialised are acknowledged (not doing this has lead to the RTC timer not sending any interrupts after rebooting in at least one case, observed on Bochs 2.6.6, [http://forum.osdev.org/viewtopic.php?f=1&t=30091 see this thread]).

 

outportb(0x70, 0x0C); // select register C

inportb(0x71); // just throw away contents

 

==Problems?==