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PEF 81912/81913
Functional Description
Data Sheet
129
2001-03-30
–
XPR
(
T
ransmit
P
ool
R
eady) interrupt, indicating that a data block of up to 16 or 32
bytes (block size selected via EXMR:XFBS) can be written to the XFIFO.
A XPR interrupt is generated either
– after a XRES (Transmitter Reset) command (which is issued for example for frame
abort) or
– when a data block from the XFIFO is transmitted and the corresponding FIFO
space is released to accept further data from the host.
–
XDU
(
T
ransmit
D
ata
U
nderrun) interrupt, indicating that the transmission of the
current frame has been aborted (seven consecutive ’1’s are transmitted) as the XFIFO
holds no further transmit data. This occurs if the host fails to respond to a XPR
interrupt quick enough.
–
XMR
(
T
ransmit
M
essage
R
epeat) interrupt, indicating that the transmission of the
complete last frame has to be repeated as a collision on the S bus has been detected
while the first data bytes have already been overwritten with new data. So the XFIFO
does not hold the first data bytes of the frame (the HDLC transmitter is stopped if a
collision on the S bus has been detected).
The occurrence of a XDU or XMR interrupt clears the XFIFO and a XPR interrupt is
issued together with a XDU or XMR interrupt, respectively. Data cannot be written to
the XFIFO as long as a XDU/XMR interrupt is pending.
Three different control commands are used for transmission of data:
–
XTF
(
T
ransmit
T
ransparent
F
rame) command, telling the Q-SMINT
IX that up to 16
or 32 bytes (according to selected block size) have been written to the XFIFO and
should be transmitted. A start flag is generated automatically.
–
XME
(
T
ransmit
M
essage
E
nd) command, telling the Q-SMINT
IX that the last data
block written to the XFIFO completes the corresponding frame and should be
transmitted. This implies that according to the selected mode a frame end (CRC +
closing flag) is generated and appended to the frame.
–
XRES
(
T
ransmitter
R
eset) command, resetting the HDLC transmitter and clearing the
transmit FIFO of any data. After a XRES command the transmitter always sends an
abort sequence, i.e. this command can be used to abort a transmission. XRES does
not clear pending interrupt indications of the transmitter, but has to be be cleared by
reading these interrupts.
Optionally two additional status conditions can be read by the host:
–
XDOV
(
T
ransmit
D
ata
O
verflow), indicating that the data block size has been
exceeded, i.e. more than 16 or 32 bytes were entered and data was overwritten.
–
XFW
(
T
ransmit
F
IFO
W
rite Enable), indicating that data can be written to the XFIFO.
This status flag may be polled instead of or in addition to XPR.
Note: The significant interrupts and commands are underlined as only these are usually
used during a normal transmission sequence.