MIC74 datasheet(13/20 Pages) MICREL

13 / 20 page

August 1, 2000

MIC74

Micrel

Polled Mode

Input state changes on I/O’s configured as inputs will be

reflected in the status register regardless of the state of the

global interrupt enable bit (IE) and the individual interrupt

mask bits in INT_MASK. In a system utilizing polling to

monitor for input changes, the status register is periodically

read to check for input events. The act of reading STATUS

clears it in preparation for detecting future events. The status

bits corresponding to I/O’s configured as outputs or corre-

sponding to P[7:4] when in fan mode will not be set by state

changes on these pins. It is always good practice, however,

to mask the value obtained when reading STATUS to elimi-

nate any bits, output or otherwise, that are not of immediate

concern. This will help avoid problems if software changes

are made in the future.

The flowchart shown in Figure 9a illustrates the steps in-

volved in initializing the MIC74 for polled operation. The

flowchart in Figure 10 illustrates the corresponding polling

routine. The process for writing output data is straightfor-

ward—simply write the desired bit pattern to DATA. (Special

precautions may be required when changing output data in

an interrupt driven system, however. See the discussion

below under “Writing to the Data Register.”)

Interrupt Mode

Input state changes on I/Os configured as inputs will be

reflected in the status register regardless of the state of the

global interrupt enable bit (IE) and the individual interrupt

mask bits in INT_MASK. In a system utilizing interrupts to

detect input changes, one or more of the bits in the interrupt

mask register, INT_MASK, are set to allow interrupts on

/ALERT to be generated by input events. The global interrupt

enable bit, IE, in the device configuration register must also

be set to enable interrupts.

The flowchart shown in Figure 9b illustrates the steps in-

volved in initializing the MIC74 for interrupt-driven operation.

The flowchart in Figure 11 illustrates the corresponding inter-

rupt service routine using the SMBus ARA (alert response

address). The corresponding timing diagram is shown in

Figure 7. The flowchart in Figure 12 illustrates the corre-

sponding interrupt service routine using polling to determine

the interrupt source. Figure 8 illustrates the timing. Utilizing

the ARA greatly speeds identification of the interrupting slave

device and lowers latency, as only a single transaction on the

bus is necessary to identify the interrupt source.

Using either method, STATUS must be read to determine the

exact source of the interrupt within the MIC74. The act of

Read

STATUS

x set

Yes

Polling the

MIC74

Service function

STATUS

≠ 00

Yes

m set

Yes

Service function

n set

Yes

Service function

Figure 10: Polling the MIC74

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