RTLinux (continued)

Setting up a Real-Time Clock Interrupt Handler

In sound.c, init_module does the following:

rtl_request_irq (8, intr_handler);
save_cmos_A = CMOS_READ(RTC_REG_A);
save_cmos_B = CMOS_READ(RTC_REG_B);
CMOS_WRITE(0x23, RTC_REG_A);  	/* 32kHz Time Base, 8192 Hz interrupt frequency */
ctemp = CMOS_READ(RTC_REG_B);
ctemp &= 0x8f;                	/* Clear */
ctemp |= 0x40;                	/* Periodic interrupt enable */
CMOS_WRITE(ctemp, RTC_REG_B); 
rtl_hard_enable_irq (8);
(void) CMOS_READ(RTC_REG_C);

The net effect is to:

• Install an RTL (indirect) interrupt handler for IRQ 8
• Program the RTC to interrupt at 8192 Hz
• Enable the RTC interrupt

To program the clock to interrupt at a different rate, you use a different value in the place of "0x23" above.

The handler is not installed directly at the hardware level. RTLinux installs its own handler at the lowest level. That handler in turn calls intr_handler.

RTLinux IRQ Handler Form

unsigned int intr_handler(unsigned int irq, struct pt_regs *regs) {
 	char data;
	char temp;
 	(void) CMOS_READ(RTC_REG_C);        /* clear IRQ */ 
...
	rtl_hard_enable_irq (8);
	return 0;
}

If you track down the definition of rtl_had_enable_irq you will eventually get to , in rtl_sync.h.

To understand why this all works, we need to understand something about the CMOS timer chip used on most PCs.

The following information is for people who like to understand how things work. One doesn't actually need to understand all this to reuse the RTLinux example code, if you are satisfied with cargo cult programming.

CMOS Timer

• Motorola MC 146818A chip, or equivalent
• Programming this chip can be dangerous!
• It is also the BIOS nonvolatile memory, containing critical system parameters, such as memory speed and size.
• Has two registers connected to I/O ports:
  • 0x70 = index register
    you set this to a value to read/write other registers
  • 0x71 = data register
    you read/write values of some other register via this one;
    which other register depends on the value of the index register
• They must always be touched in sequence:
  1. set index register
  2. read or write data register
• Warnings:
  • Never forget step 2.
  • Mask all interrupts (even the "nonmaskable" interrupt) when you do this, as a critical section.
  • Always restore the value of the index register to 0x0D.
    (This means you need to always do four steps.)

• combine setting index and data registers
• See mc146818rtc.h for code.
• don't provide interrupt masking
• following link to code of CMOS_READ for details

It seems that the RTLinux code it careless about changing the CMOS register values. It does not mask interrupts while it is changing the values:

#define CMOS_WRITE(val, addr) ({ \
outb_p((addr),RTC_PORT(0)); \
outb_p((val),RTC_PORT(1)); \
})

Register A

• Register A selects interrupt rate
• Basic oscillator frequency = 323,768 Hz
• Bits 4-8 should not be touched, or you will interfere with the normal timekeeping function of the chip
• Bits 0-3 are the Rate Select (RS) bits
  They specify a divider, which you can safely modify
• The RS bits specify the power of two that should be used as a divider
• e.g., the value RS=0x0011 (decimal 3) means the interrupt frequency will be 32,768 / 2^(3-1) = 8192
  (the maximum)

For other values and their effects, see the table below:

Note that the behaviors for the values 0x0001 and 0x0010 are anomalous.

Register B

• contains several flags, which control the action of the chip
• one is the Periodic Interrupt Enable (PIE) bit
• turning this on starts the periodic interrupts
• turning it off stops the periodic interrupts

The clock interrupt number is INT 0x70 (only coincidence that this is the same as the status register port number), which the PIC maps to PIC IRQ 8.

Register C

• tells what kind of interrupt occurred
• sideffect of reading this: reenable interrupt

If you don't Register C, there will not be another interrupt

For further information see notes from http://www.geocities.com/SiliconValley/Campus/1671/docs/rtc2.html, reformatted to remove annoying Javascript side-effects, and HREF="rtc.html">http://www.geocities.com/SiliconValley/Campus/1671/docs/rtc.htm. The links precedingare actually to copies of these files which I have reformatted in HTML and cached locally to remove annoying Javascript side-effects.