Motorola MC68824 User Manual page 78

4.1.1.2 RECEIVE STATUS WORD. The receive status word is updated by the TBC in the receive
case. The host must clear this word in the frame descriptor before placing the frame descriptor
into the free frame descriptor pool. A one in the corresponding bit of the receive status error
mask located in the private area will cause the frame to be accepted according to the following
criteria:
• If a CRC error occurred the frame is stored in its entirety. The frame data length (offset 10 in
FD) reflects the actual data length including the CRC length if the RCDS (RX CRC to data
storage mode) is enabled.
• If an E bit error was detected, the frame is stored in its entirety and the frame data length is
updated to reflect the number of bytes the TBC heard.
• If a FIFO overflow occurred (i.e., overrun error), the data is copied to the data buffer(s) up to
the point where the overrun error occurred. The frame data length reflects the number of
bytes copied to the data buffer(s).
• If noise was detected, the data is copied to the data buffer(s) up to the point where the noise
occurred. The frame data length reflects the number of bytes copied to the data buffer(s).
• If a frame >8K was detected, the data is copied to the data buffer(s) in its entirety. The frame
data length reflects the number of bytes copied to the data buffer(s).
• If not enough buffers are available in the free buffer pool, the TBC receives the frame's header
information as long as free FDs are available. The correct frame length is stored in the FD.
o c
B
A
*Reserved
CRCE
EBIT
FOVF
NOISE
FTL
NOBUF
- CRC error occurred in the frame received
- The E bit in the end delimiter is set
- FIFO overflow
- Noise
- Frame too long (the received frame is longer than 8K bytes)
- Not enough buffers in buffer pool
4.1.1.3 CONTROL FOR NEXT FRAME DESCRIPTOR POINTER. This word is updated by the host
and its format is shown below:
F E D C B A
I
NPV
I
W_FD
I
*
I
*
I
*
I
*
*Reserved
NPV - Next Pointer Valid
Valid in TX and RX Cases
*
I
*
o
This is the last valid FD in this queue. The TBC will set the FD pool empty bit in
interrupt status word 0 upon using this FD for receiving.
The next FD in this queue is valid.
W-FD - Warning Frame Descriptor Pool Low
Valid in free pool case. This bit should be used only while in a free frame descriptor pool.
o
During transmission, the W-FD bit must be set to zero. Note that in the last FD the
free pool (NPV
=
0) W-FD must be a zero.
FD pool is almost empty, the FD pool low bit in the interrupt status word 0 will be
set upon the TBC's detection of this condition.
MC68824 USER'S MANUAL
MOTOROLA
JLk
II