COM20019IP datasheet(77/81 Pages) SMSC

77 / 81 page

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outside the CPU. It's not synchronized between

the CPU and COM20019. Thus, changing the

CKP3-1 timing does not synchronize with the

internal clocks of Pre-Scalar, and changing

CKP3-1 may cause spike noise to appear on the

output clock line.

Setting the EF bit will include flip-flops inserted

between the Configuration register and Pre-

Scalar for synchronizing the CKP3-1 with Pre-

Scalar’s internal clocks.

Shorten The Write Interval Time To The

Command Register

The COM20019 limits the write interval time for

continuous writing to the Command register. The

minimum interval time is changed by the Data

Rate. It's 800 nS at the 312.5 Kbps and 1.6

μS at

the 156.25 Kbps. This 1.6

μS is very long for

CPU.

Setting the EF bit will change the clock source

from OSCK clock (8 times frequency of data

rate) to XTAL clock which is not changed by the

data rate, such that the minimum interval time

becomes 100 nS.

Eliminate The Write Prohibition Period For

The Enable Tx/Rx Commands

The COM20019 has a write prohibition period for

writing the Enable Transmit/Receive Commands.

This period is started by the TA or RI bit (Status

Reg.) returning to High. This prohibition period is

caused by setting the TA/RI bit with a pulse

signal. It is 3.2

μS at 156.25 Kbps. This period

may

problem

when

using

interrupt

processing. The interrupt occurrs when the RI bit

returns to High. The CPU writes the next Enable

Receive

Command

the

other

page

immediately. In this case, the interval time

between the interrupt and writing Command is

shorter than 3.2

μS.

Setting the EF bit will cause the TA/RI bit to

return to High upon release of the pulse signal

for setting the TA/RI bit, instead of at the start of

the pulse. This is illustrated in figure 23 on the

following page.

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