Intel 537EX manual 9.2.8Interrupt Enable Register IER, 9.2.9Transmitter Holding Register THR

Parallel Host Interface 16C450/16C550A UART

Bit 3	Interrupt ID Bit 2—In 16C450 mode, this bit is always a ‘0’.
	In FIFO mode, both this bit and bit IIR2 are set whenever a time-out interrupt is pending.
Bits 2:1	Interrupt ID Bits ID0 and ID1—These two bits are used to identify the highest-priority interrupt as shown in Table 28.
	Interrupt Pending—This bit indicates when a modem interrupt is pending. Whenever this bit is equal to ‘0’, then one
Bit 0	or more interrupts are pending. Whenever this bit is equal to ‘1’, then no interrupts are pending. When an interrupt has
	occurred, the host can determine the cause of the interrupt by looking at the IIR interrupt ID bits 0 and 1 (and interrupt
	ID bit 2 for FIFO mode).

9.2.8Interrupt Enable Register (IER)

Figure 21. Interrupt Enable Register (IER)

Register 1

(DLAB = 0)

0

0

0

0

MSIE

RLSIE

THREIE

RDAIE

	This register is used to enable up to five types of UART interrupts: receiver line status, received
	data available, character time-out indication (FIFO mode only), Transmitter Holding register
	empty, and modem status. Each enabled interrupt can individually cause an interrupt to host on the
	∝P HINT output pin. To cause an interrupt to the host (HINT), both the interrupt enable bit and
	OUT2 (MCR2) must be set to ‘1’.
Bits 7:4	Not used—These bits are permanently set to ‘0’.
Bit 3	MSIE (Modem Status Interrupt Enabled)—when set to ‘1’, this bit enables the modem status interrupt.
Bit 2	RLSIE (Receiver Line Status Interrupt Enabled)—when set to ‘1’, this bit enables the receiver line status interrupt.
Bit 1	THREIE (Transmitter Holding Register Empty Interrupt Enabled)—when set to ‘1’, this bit enables the Transmitter
	Holding register empty interrupt.
Bit 0	RDAIE (Received Data Available Interrupt Enabled)—when set to ‘1’, this bit enables the received data available
	interrupt.

9.2.9Transmitter Holding Register (THR)

Figure 22. Transmitter Holding Register (THR)

Register 0

(DLAB = 0)

THR

The THR (Transmitter Holding register) is a write-only register used for sending data and AT commands to the modem.

100 536EX Chipset Developer’s Manual

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