ST72321 datasheet(185/189 Pages) STMICROELECTRONICS

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ST72321

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15 KNOWN LIMITATIONS

15.1 ALL FLASH AND ROM DEVICES

15.1.1 External RC option

The External RC clock source option described in

previous datasheet revisions is no longer support-

ed and has been removed from this specification.

15.1.2 CSS Function

The Clock Security System function is not guaran-

teed. The features described in Section 6.4.3 are

subject to revision.

15.1.3 Safe Connection of OSC1/OSC2 Pins

The OSC1 and/or OSC2 pins must not be left un-

connected otherwise the ST7 main oscillator may

start and, in this configuration, could generate an

fOSC clock frequency in excess of the allowed

maximum (>16MHz.), putting the ST7 in an un-

safe/undefined state. Refer to section 6.2 on page

24.

15.1.4 Unexpected Reset Fetch

If an interrupt request occurs while a “POP CC” in-

struction is executed, the interrupt controller does

not recognise the source of the interrupt and, by

default, passes the RESET vector address to the

CPU.

Workaround

To solve this issue, a “POP CC” instruction must

always be preceded by a “SIM” instruction.

15.1.5

Clearing

active

interrupts

outside

interrupt routine

When an active interrupt request occurs at the

same time as the related flag is being cleared, an

unwanted reset may occur.

Note: clearing the related interrupt mask will not

generate an unwanted reset

Concurrent interrupt context

The symptom does not occur when the interrupts

are handled normally, i.e.

when:

– The interrupt flag is cleared within its own inter-

rupt routine

– The interrupt flag is cleared within any interrupt

routine

– The interrupt flag is cleared in any part of the

code while this interrupt is disabled

If these conditions are not met, the symptom can

be avoided by implementing the following se-

quence:

Perform SIM and RIM operation before and after

resetting an active interrupt request.

Example:

SIM

reset interrupt flag

RIM

Nested interrupt context:

The symptom does not occur when the interrupts

are handled normally, i.e.

when:

– The interrupt flag is cleared within its own inter-

rupt routine

– The interrupt flag is cleared within any interrupt

routine with higher or identical priority level

– The interrupt flag is cleared in any part of the

code while this interrupt is disabled

If these conditions are not met, the symptom can

be avoided by implementing the following se-

quence:

PUSH CC

SIM

reset interrupt flag

POP CC

15.1.6 SCI Wrong Break duration

Description

A single break character is sent by setting and re-

setting the SBK bit in the SCICR2 register. In

some cases, the break character may have a long-

er duration than expected:

- 20 bits instead of 10 bits if M=0

- 22 bits instead of 11 bits if M=1.

In the same way, as long as the SBK bit is set,

break characters are sent to the TDO pin. This

may lead to generate one break more than expect-

ed.

Occurrence

The occurrence of the problem is random and pro-

portional to the baudrate. With a transmit frequen-

19200

baud

(fCPU=8MHz and SCI-

BRR=0xC9), the wrong break duration occurrence

is around 1%.

Workaround

If this wrong duration is not compliant with the

communication protocol in the application, soft-

ware can request that an Idle line be generated

before the break character. In this case, the break

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