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参数资料
| 型号： | 85C30 |
| 厂商： | Advanced Micro Devices, Inc. |
| 英文描述： | Enhanced Serial Communications Controller |
| 中文描述： | 增强的串行通信控制器 |
| 文件页数： | 15/68页 |
| 文件大小： | 528K |
| 代理商： | 85C30 |
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AMD
15
Am85C30
counts 31 and 0. The DPLL looks for edges only during a
time centered on the 15/16 counting transition.
The 32X clock for the DPLL can be programmed to
come from either the
RTxC
input or the output of the
baud rate generator. The DPLL output may be pro-
grammed to be echoed out of the SCC via the
TRxC
pin
(if this pin is not being used as an input).
Crystal Oscillator
When using a crystal oscillator to supply the receive or
transmit clocks to a channel of the SCC, the user should:
1. Select a crystal oscillator that satisfies the following
specifications:
30 ppm @ 25
°
C
50 ppm over temperatures of –20
°
to 70
°
C
5 ppm/yr aging
5-MW drive level
2. Place crystal across
RTxC
and
SYNC
pins.
3. Place 30-pF capacitors to ground from both
RTxC
and
SYNC
pins.
4. Set bit D
7
of WR11 to 1.
Data Encoding
The ESCC may be programmed to encode and decode
the serial data in four different ways (Figure 6). In NRZ
encoding, a 1 is represented by a High level, and a 0 is
represented by a Low level. In NRZI encoding, a 1 is rep-
resented by no change in level, and a 0 is represented
by a change in level. In FM
1
(more properly, biphase
mark), a transition occurs at the beginning of every bit
cell. A 1 is represented by an additional transition at the
center of the bit cell, and a 0 is represented by no
additional transition at the center of the bit cell. In FM
0
(biphase space), a transition occurs at the beginning of
every bit cell. A 0 is represented by an additional transi-
tion at the center of the bit cell, and a 1 is represented by
no additional transition at the center of the bit cell. In ad-
dition to these four methods, the ESCC can be used to
decode Manchester (biphase level) data by using the
DPLL in the FM mode and programming the receiver for
NRZ data. Manchester encoding always produces a
transition at the center of the bit cell. If the transition is
0/1, the bit is a 0. If the transition is 1/0, the bit is a 1.
Auto Echo and Local Loopback
The ESCC is capable of automatically echoing every-
thing it receives. This feature is useful mainly in asyn-
chronous modes but works in SYNC and SDLC modes
as well. In Auto Echo mode, TxD is RxD. Auto Echo
mode can be used with NRZI or FM encoding with no ad-
ditional delay, because the data stream is not decoded
before retransmission. In Auto Echo mode, the
CTS
in-
put is ignored as a transmitter enable (although transi-
tions on this input can still cause interrupts if
programmed to do so). In this mode, the transmitter is
actually bypassed, and the programmer is responsible
for disabling transmitter interrupts and
WAIT
/
REQUEST
on transmit.
The ESCC is also capable of Local Loopback. In this
mode, TxD is RxD just as in Auto Echo mode. However,
in Local Loopback mode, the internal transmit data is
tied to the internal receive data, and RxD is ignored (ex-
cept to be echoed out via TxD). The
CTS
and
DCD
in-
puts are also ignored as transmit and receive enables.
However, transitions on these inputs can still cause in-
terrupts. Local Loopback works in asynchronous,
SYNC, and SDLC modes with NRZ, NRZI, or FM coding
of the data stream.
1
1
0
0
1
0
Data
NRZ
NRZI
(Biphase Mark)
(Biphase Mark)
Manchester
Bit Cell Level
High = 1
Low = 0
No Change = 1
Change = 0
Bit Center Transition
Transition = 1
No Transition = 0
No Transition = 1
Transition = 0
High Low = 1
Low High = 0
Figure 6. Data Encoding Methods
FM
1
FM
0
10216F-10
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