I/O Programming Functional Description
AMD
3–16
D7
D6
D5
D4
D3
D2
D1
DTR/
REQ
Funct.
D0
Wait/
DMA REQ
Funct.
Wait/
DMA REQ
Enable
Wait/
DMA REQ
Rx/Tx
D7
D6
D5
D4
D3
D2
D1
D0
WR14 Register Layout
WR1 Register Layout
Each channel in the SCC has two pins,
DTR
/
REQ
and
W
/
REQ
, which may be used to
control the block transfer of data. Both pins in each channel may be programmed to act
as DMA Request signals, and one pin (
W
/
REQ
) in each channel may be programmed to
act as a Wait signal for the CPU. In either mode, it is advisable to select and enable the
mode in two separate accesses of the appropriate register. The first access should select
the mode and the second access should enable the function. This procedure prevents
glitches on the output pins. Reset forces Wait mode, with
W
/
REQ
open-drain.
3.9.1
The Wait function on transmit is selected by programming WR1 as shown below.
Wait on T ransmit
1
0
0
D7
D6 D5
D4 D3
D2 D1
D0
WR1—Wait on Transmit Function Selection
In this mode, the
W
/
REQ
pin carries the Wait signal, and is open-drain when inactive and
Low when active. When the processor attempts to write to WR8 (Transmit Buffer) and it is
full, the SCC will assert
W
/
REQ
until the buffer is empty. This allows the use of a block-
move instruction to transfer the transmit data.
W
/
REQ
will go active in response to
WR
going active but only if WR8 (Transmit Buffer) is being accessed, either directly or via the
pointers. The
W
/
REQ
pin is released in response to the falling edge of PCLK. Details of
the timing are shown in Figure 3–6.
3.9.2
The Wait function on receive is selected by programming WR1 as shown below.
Wait on Rec eive
1
0
1
D7
D6 D5
D4 D3
D2 D1
D0
WR1—Wait on Receive Function Selection