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Intel
®
IXP45X and Intel
®
IXP46X Product Line of Network Processors—Synchronous Serial Port
Intel
®
IXP45X and Intel
®
IXP46X Product Line of Network Processors
Developer’s Manual
August 2006
870
Order Number: 306262-004US
Bits that cause an interrupt will signal the request as long as the bit is set. Once the bit
is cleared, the interrupt is cleared. Read/write bits are called status bits, read-only bits
are called flags. Status bits are referred to as “sticky” (once set by hardware, must be
cleared by software). Writing a one to a sticky status bit clears it, writing a zero has no
effect. Read-only flags are set and cleared by hardware; writes have no effect.
Additionally some bits that cause interrupts have corresponding mask bits in the
control registers and are indicated in the section headings that follow.
20.5.3.1
Transmit FIFO Not Full Flag (TNF) (Read-Only, Non-Interruptible)
The transmit FIFO not full flag (TNF) is a read-only bit that is set whenever the transmit
FIFO contains one or more entries that do not contain valid data. TNF is cleared when
the FIFO is completely full. This bit can be polled when using programmed I/O to fill the
transmit FIFO over its half-way mark. This bit does not request an interrupt.
20.5.3.2
Receive FIFO Not Empty Flag (RNE) (Read-Only, Non-Interruptible)
The receive FIFO not empty flag (RNE) is a read-only bit that is set whenever the
receive FIFO contains one or more entries of valid data and is cleared when it no longer
contains any valid data. This bit can be polled when using programmed I/O to remove
remaining bytes of data from the receive FIFO since CPU interrupt requests are only
made when the Receive FIFO Threshold has been met or exceeded. This bit does not
request an interrupt.
20.5.3.3
SSP Busy Flag (BSY) (Read-Only, Non-Interruptible)
The SSP busy (BSY) flag is a read-only bit that is set when the SSP is actively
transmitting and/or receiving data, and is cleared when the SSP is idle or disabled
(SSE=0). This bit does not request an interrupt.
20.5.3.4
Transmit FIFO Service Request Flag (TFS)
(Read-Only, Maskable Interrupt)
The Transmit FIFO service request flag (TFS) is a read-only bit that is set when the
transmit FIFO is nearly empty and requires service to prevent an underrun. TFS is set
any time the transmit FIFO has the same or fewer entries of valid data than indicated
by the Transmit FIFO Threshold, and it is cleared when it has more entries of valid data
than the threshold value. When the TFS bit is set, an interrupt request is made unless
the transmit FIFO interrupt request enable (TIE) bit is cleared. After the CPU fills the
FIFO such that it exceeds the threshold, the TFS flag (and the service request and/or
interrupt) is automatically cleared.
20.5.3.5
Receive FIFO Service Request Flag (RFS)
(Read-Only, Maskable Interrupt)
The receive FIFO service request flag (RFS) is a read-only bit that is set when the
receive FIFO is nearly filled and requires service to prevent an overrun. RFS is set any
time the receive FIFO has the same or more entries of valid data than indicated by the
Receive FIFO Threshold, and it is cleared when it has fewer entries than the threshold
value. When the RFS bit is set, an interrupt request is made unless the receive FIFO
interrupt request enable (RIE) bit is cleared. After the CPU reads the FIFO such that it
has fewer entries than the RFT value, the RFS flag (and the service request and/or
interrupt) is automatically cleared.
20.5.3.6
Receiver Overrun Status (ROR)
The receiver overrun status bit (ROR) is a read/write bit that is set when the receive
logic attempts to place data into the receive FIFO after it has been completely filled.
After the FIFO is filled, then each time a new piece of data is received, the set signal to