Motorola MC68824 User Manual page 62

Where:
T T T T
0 0 0 0 Receiver test
0 1 sil/noi
O·
Transmitter test
1 0 0 0 Full duplex loopback test
1 1 0 0 Host interface test
The host interface test exercises the DMA portion of the TBC by checking the path from a memory
buffer to the TBC and back to the memory buffer. The transmitter test is used to check the path
from a transmit queue in memory through the transmitter and back to the receiver. The receiver
test is used to check the path from the receiver to a receive queue in memory. Finally, the full
duplex loopback test is used to check the path from a buffer in memory through the TBC serial
interface and back to the memory buffer.
3.6.1 SET INTERNAUEXTERNAL LOOPBACK MODE Command
The SET INTERNAUEXTERNAL LOOPBACK MODE command is used by the host processor to
enable or disable the internalloopback mode of operation while running the receiver, transmitter,
and full duplex loopback tests. Note that if internal loopback mode is selected, an external clock
has to be provided for the receive clock while running the tests. The default mode of operation
is with internal loop back mode disabled. The coding for this command is CC or C4 and its format
is shown below.
INTERNAL LOOPBACK
I
INTERNAL
I
LOOPBACK
1 Enables internal loopback mode of operation.
o
Disables internal loopback mode of operation.
This command controls the internal loopback from TXSYMO, TXSYM1, and TXSYM2 to
RXSYMO, RXSYM1, and RXSYM2 respectively. In internal loopback mode, transmitted in-
formation is looped back as received information internally to the chip. Also, in internal
loop back mode, the external transmit output pins send silence symbols regardless of the
symbols being generated internally. In external loop back mode, the transmit pins must be
externally connected to the receive pins either directly or through a modem.
3.6.2 HOST INTERFACE TEST Command
The host interface test is used to check the path from the memory buffer to the TBC FIFO and
back to the memory buffer. To initiate this test, the user first builds a 70-byte buffer placing test
data in the first 34 bytes, see Table 3-8 for format. Next, the user loads the function code of the
buffer into CPA VALO of the command parameter area, loads the buffer pointer into CPA VAL 1
(high word) and CPA VAL2 (low word) and issues the test command with the appropriate bits set
for the host interface test. The 34 bytes of data are copied from the first half of the 70-byte buffer
to the TBC and written back to the second half of the 70-byte buffer by the TBC. Upon completion
of the test, the done bit in the status word of the CPA located at offset 9C in the initialization table
is set. The host checks the data written back to the memory buffer against what was transferred.
MC68824 USER'S MANUAL
MOTOROLA
3-19
II