Fast AMBA Peripherals
12-48
GMS30C7201 Data Sheet
transmitting sixteen preambles and a start flag followed by the transmission of data
from the buffer. When the TUR bit is set an interrupt request is made unless it is
masked. When TUS=0 the interrupt is masked, when TUS=1 it is enabled. Note that
underruns are not generated when the FIr transmitter is first enabled and is in the idle
state (continuously transmits flags).
Receiver Abort Status (RAB) (read/write, non-maskable interrupt)
The receiver abort status bit (RAB) is set when an abort is detected during receipt of
an incoming frame. An abort is signalled when two or more symbols which do not
contain any pulses (0000) or symbols containing 0011, 1001, 1010, or 0101(invalid
symbols which are not contained within the stop flag) are detected after a valid start
flag has been detected but before a complete stop flag has been received (i.e. an
incorrect chip in the stop flag generates an abort as well). When an abort is received,
the EOF tag is set in the buffer entry which corresponds to the last piece of data which
was received before the frame was aborted. The receiver then enters hunt mode,
searching for the preamble.
Transmit buffer Service Request Flag (TFS) (read-only, maskable interrupt)
The transmit buffer service request flag (TFS) is a read-only bit which is set when the
transmit buffer is not full and requires service. When the TFS bit is set, an interrupt
request is made unless the transmit buffer interrupt request mask (TIM) bit is cleared.
The state of TFS is also sent to the DMA controller, and may be used to signal a DMA
service request. Note that TIM has no effect on the generation of the DMA service
request. After the DMA or CPU fills the buffer, the TFS flag (and the service request
and/or interrupt) is automatically cleared.
Receive buffer Service Request Flag (RFS) (read-only, maskable interrupt)
The receive buffer service request flag (RFS) is a read-only bit which is set when the
receive buffer is not empty and requires service. When the RFS bit is set, an interrupt
request is made unless the receive buffer interrupt request mask (RIM) bit is cleared.
The state of RFS is also sent to the DMA controller, and may be used to signal a DMA
service request. Note that RIM has no effect on the generation of the DMA service
request. After the DMA or CPU fills the buffer, the RFS flag (and the service request
and/or interrupt) is automatically cleared.
Framing Error Status (FRE) (read/write, non-maskable interrupt)
The framing error status (FRE) bit is set when a frame alignment error is detected by
the receive logic. A frame alignment error is detected on received data when a
preamble is followed by something other than another preamble or a start flag.
Figure 12-14: FIr status register 0 bit locations
shows the bit locations
corresponding to the status and flag bits within FIr status register 0. Note that the reset
state of all writable status bits is unknown and must be cleared (by writing a one to
them) before enabling the FIr. Also note that writes to reserved bits are ignored and
reads return zeros.