MK68564 datasheet(13/46 Pages) STMICROELECTRONICS

13 / 46 page

For example, when the time constant register is loa-

ded with ”01H” and divide by four is selected, one

output clock will occur for every four input clocks. If

the time constant value loaded is ”00H” (256 deci-

mal) instead of ”01H” and divide by 64 is selected,

one output clock will occur for every 16384 input

clocks. Note that the minimum count value is ”01H”

(1 decimal), and the maximum count value is ”00H”

(256 decimal).

The output of the baud rate generator may be pro-

grammed to drive the transmitter (BRG output on

TxC), the receiver (BRG output on RxC), both (BRG

output on TxC and RxC), or neither (TxC and RxC

are inputs). After a reset, the baud rate generator is

disabled, divide by four is selected, and TxC and

RxC are inputs.

The baud rate generator should be disabled before

the CPU writes to the time constant register. This is

necessary because no attempt was made to syn-

chronize the loading of a new time constant with the

clock used to drive the BRG.

Figure 8 indicates the external components needed

to connect a crystal oscillator to the SIO XTAL in-

puts. The allowed crystal parameters are also listed.

For a 3.6864MHz input signal to the baud rate ge-

nerator, the time constants, listed in table 2, are loa-

ded to obtain the desired baud rates (in x1 clock

mode).

Figure 8 : SIO External Oscillator Components.

Table 2 : Time-Constant Values.

R a te

Tim e Constant

D ivide By

Error

19200

9 600

7 200

4 800

3 600

2 400

2 000

1 800

1 200

600

300

128

192

256

192

69 %

CRYSTAL PARAMETERS :

Parallel Resonance, FundamentalMode AT Cut

Rs â 150

Ω (Fr = 2.8 - 5.0MHz)

Rs â 300

Ω (Fr = 2.0 - 2.7MHz)

CI = 18pf ; Cm = 0.02pF ; Ch = 5pF ; Lm = 96MHz

Fr (typ) = 2.457MHz

ASYNCHRONOUS OPERATION

INTRODUCTION

Many types of Asynchronous operations are perfor-

med by the MK68564 SIO. Figure 9 represents a ty-

pical Asynchronous message format and some of

the options available on the SIO. The transmit pro-

cess inserts start, stop, and parity bits to a variable

data format and supplies a serial data stream to the

Transmit Data output (TxD). The receiver takes the

data from the Receive Data input (RxD) and strips

away expected start and stop bits at a programmed

clock rate. It provides error checking for overrun, pa-

rity, and carrier-loss errors, and, if desired, provides

interrupts for these conditions.

To set up the SIO for Asynchronous operation, the

following registers need to be initialized : Mode

Control Register, Interrupt Control Register, Recei-

ver Control Register, and Transmitter Control

programmed before the other registers to assure

proper operation of the SIO. The following registers

are used to transfer data or to communicate status

between the SIO and the CPU or other bus master

when

operating

Asynchronous

modes

Command Register, Status Register 0, Status Re-

gister 1, Data Register, and the Vector Register.

MK68564

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