Motorola MC68340 User Manual

7- 4

MOTOROLA

currently active interrupt conditions. The interrupt enable register (IER) is programmable
to mask any events that can cause an interrupt.

The serial module is code compatible with the MC68681 with some modifications. The
following paragraphs describe the differences.

The programming model is slightly altered. The supervisor/user block in the MC68340
closely follows the MC68681. The supervisor-only block has the following changes:

• The interrupt vector register is moved from supervisor/user to supervisor only at a

new address.

• MR2A and MR2B are moved from a hidden address location to a location at the

bottom of the programming model.

The timer/counter is eliminated as well as all associated command and status registers.

Only certain output port pins are available.

There are no IP pins on the MC68340.

RxRTS and TxRTS are more automated on the MC68340.

The XTAL_RDY bit in the ISR should be polled until it is cleared to prevent an unstable
frequency from being applied to the baud rate generator. The following code is an
example:

if (XTAL_RDY==0)

begin

write CSR

end

else

begin

wait
jump loop

end

The following paragraphs contain a brief description of the serial module signals. Figure 7-
2 shows both the external and internal signal groups.

The terms 

assertion and negation are used throughout this

section to avoid confusion when dealing with a mixture of
active-low and active-high signals. The term 

assert or assertion

indicates that a signal is active or true, independent of the level
represented by a high or low voltage. The term 

negate or

negation indicates that a signal is inactive or false.

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Freescale Semiconductor, Inc.

For More Information On This Product,

   Go to: www.freescale.com

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