MC68000 MOTOROLA [Motorola, Inc], MC68000 Datasheet - Page 404

• Manufacturer Part Number: MC68000
• Manufacturer: MOTOROLA [Motorola, Inc]

MC68302 Applications
Figure D-25 shows how CTS can be used in the NMSI transmit case. NTSYN and EXSYN
are set to enable transparent mode. Instead of software operation for CTS and CD, normal
(automatic) operation is chosen. RTS is asserted when the transmit FIFO is full. From then
on, data is held off until CTS is sampled low. From that sample point, there is a 3.5 TCLK
delay before the first bit of the data buffer is transmitted. Ones are transmitted until the first
bit of the data buffer is transmitted.
In the case shown in Figure D-25, it is important that CTS not go high for the duration of the
buffer transmission. If multiple buffers are all ready with their L bits cleared, transmission of
frames will continue back-to-back. If CTS negated during any of these buffers, transmission
will cease, and that buffer will report a CTS lost condition. Ones will be transmitted at that
time. Once a restart transmit command is given, transmission of the next buffer can begin
once CTS is reasserted.
Once CTS deasserts after RTS, the RTS-CTS protocol can begin again as soon as the next
buffer is made ready, but a minimum of 17 idle bits will occur between frames, regardless of
how soon CTS is reasserted. Remember that when EXSYN is set, CD (sync) must be low
for transmission to begin. In this case, it is grounded; whereas, in the following case, EXSYN
is actively switching.
Figure D-26 shows how CD (sync) can be used to control transmission. EXSYN and NTSYN
are once again set to enable transparent mode, and the L bit is set. Since software operation
mode (DIAG1 = 1 and DIAG0 = 1) is chosen, the CTS pin value is ignored. Once CD (sync)
is latched low, data begins transmission in 6.5 TCLKs. Notice that the rising edge of CD
(sync) and subsequent falling edges of CD (sync) (not shown) have no effect, since synchro-
nization has already been achieved.
D-54
CD (SYNC)
(OUTPUT)
(OUTPUT)
(INPUT)
(INPUT)
RCLK
TCLK
(I/O)
(I/O)
TXD
RTS
CTS
EXSYN = 1
NTSYN = 1
DIAG1-DIAG0 BITS = 00
L = 1 IN THE Tx BD
ONLY ONE Tx BD IS READY
Figure D-25. Using CTS In the NMSI Transmit Case
DATA READY TO TRANSMIT HERE
3.5 TCLKs
CTS SAMPLED AS LOW HERE
MC68302 USER’S MANUAL
FIRST BIT OF DATA
IN BUFFER.
LAST BIT OF DATA
MUST NOT BE NEGATED
UNTIL RTS IS NEGATED,
OR CTS LOST ERROR WILL
RESULT
MOTOROLA