AM85C30 datasheet(15/68 Pages) AMD

15 / 68 page

AMD

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 D7 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 FM1 (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 FM0

(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.

001

Data

NRZ

NRZI

(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

FM1

FM0

10216F-10

AM85C30 Datasheet(PDF) 15 Page - Advanced Micro Devices

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