M41T83RM6F datasheet(47/64 Pages) STMICROELECTRONICS

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M41T82, M41T83

Clock operation

3.14.2

Backup mode

In backup mode, the operation of the output circuit is as shown in Table 15.

In backup mode, frequency test is disabled. Thus, the FT bit is a ‘don’t care’.

ABE enables interrupts in backup. If it is 0, the output pin is a 1 regardless of the other bits.

The pin is also a 1 when OUT is a 1 and no interrupts are enabled.

When OUT is 0 and ABE is a 1, the pin is 0 regardless of the interrupts.

Thus, in order to enable interrupts in backup mode, OUT must be a 1 and ABE must be a 1,

and one or more of the interrupt enables must be a 1.

Simultaneous interrupts

Since more than one interrupt source can cause the IRQ1/FT/OUT pin to go low, more than

one interrupt may be pending when the microprocessor services the interrupt. Therefore,

the application software should read the flags register (0Fh) to discern which condition or

conditions are causing the pin to be asserted.

Also be aware that once a flag causes the pin to assert, other flags could subsequently also

go true. Since the pin is already low due to the first, no additional output transition will occur.

That is why the software must check the flags register.

Example: If the watchdog is in use and the oscillator fail detect interrupt is enabled, and the

watchdog times out, the IRQ1/FT/OUT pin will go low. If, in the intervening time before the

processor services the interrupt, something disturbs the oscillator, such as a drop of

moisture landing on the crystal pins, the OF bit will also be set. Thus, when the software

services the interrupt, it must service both sources: it must re-initialize the watchdog and

clear the OF bit in order to de-assert the IRQ1/FT/OUT pin. By reading the flags register, the

software will know both flags were set and that both need service.

Table 15. Priority for IRQ1/FT/OUT pin when operating in backup mode

OUT(1)

ABE(2)

A1IE(3)

+ OFIE(4)

Comment

When ABE is 0, the pin is 1 regardless of OUT or

the interrupt sources.

When OUT is 1 and no interrupts are enabled,

the pin is 1. (A1IE and OFIE are the only

interrupts applicable in this mode).

When ABE is 1 and OUT is 0, OUT dominates

and regardless of the interrupt sources.

IRQ

When one or more interrupts are enabled, ABE is

a 1, and OUT is a 1, the pin stays high until one of

the interrupts is asserted.

OUT is bit 7 of register 08h (digital calibration).

ABE is bit 5 of register 0Ah (alarm 1, month).

A1IE is bit 7 of register 0Ah (alarm 1, month).

OFIE is bit 7 of register 09h (watchdog).

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