AM85C30 datasheet(158/194 Pages) AMD

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SCC Application Notes

AMD

7–9

Table 7–4. Polled Asynchronous Initialization Procedure

Value

Comments

WR9

C0H

Force Hardware Reset

WR4

4CH

x16 clock, 2 stop bits, no parity

WR3

C0H

Rx 8 bits, Rx disabled

WR5

60H

Tx 8 bits, DTR, RTS, Tx off

WR9

00H

Int. Disabled

WR10

00H

NRZ

WR11

56H

Tx & Rx = BRG out, TRxC = BRG out

WR12

06H

Time constant = 6

WR13

00H

Time constant high = 0

WR14

10H

BRG in =

RTxC, BRG off, loopback

Enables

WR14

11H

BRG enable

WR3

C1H

Rx enable

WR5

68H

Tx enable

7.2.3.1

SCC Operating Mode Programming

WR9 resets the SCC to a known state by writing a C0 hex. The Force Hardware Reset

command is identical to a hardware reset. It will reset both channels.

WR4 selects the Asynchronous, x 16 mode, with 2 stop bits and no parity. The x 16 mode

means that clock rate is 16 times the data rate.

WR3 selects 8 bits per character and does not enable the receive. The 8 bits per charac-

ter allows 8 bits to be assembled from the data stream. The receiver is not enabled at this

time because the SCC has not been initialized.

WR5 selects 8 bits per character and does not enable the transmitter. The 8 bits per char-

acter allows 8 bits to be sent, as data, with the least significant bit first. The transmitter is

not enabled at this time because the SCC has not been initialized.

WR9 selects that there are no interrupts enabled. This inhibits the SCC from requesting

an interrupt from the CPU.

WR10 selects NRZ encoding. This NRZ coding is used on the transmitter as well as the

receiver.

WR11 selects the

RTxC pin to TTL clock; the baud-rate generator is the transmit and re-

ceive clocks source, and the

TRxC pin is used as a baud-rate generator output.

WR12 & WR13 select the baud-rate generator’s time constant. The WR13 time constant

is determined by the equation:

Time Constant =

Clock Frequency

2 x Baud Rate x clock mode

–2

In this example, the clock frequency is 2.4576 MHz, the baud rate is 9600, the clock

mode is 16. The time constant is, therefore, 6; expressed as a 16-bit, hexadecimal num-

ber, it is 0006H. The time constant Low (WR12) is, therefore 06H and the time constant

High (WR13) is 00H. The baud rate for this example can be varied, as long as the data

rate is less than 1/4 of the PCLK rate. Table 7–5 shows the time constants for other com-

mon baud rates.

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