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参数资料
型号： ST72F324LK6T6
厂商： STMICROELECTRONICS
元件分类： 微控制器/微处理器
英文描述： 8-BIT, FLASH, 8 MHz, MICROCONTROLLER, PQFP32
封装： 7 X 7 MM, TQFP-32
文件页数： 58/154页
文件大小： 3519K
代理商： ST72F324LK6T6
ST72324Lxx
150/154
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-
cy of 19200 baud (fCPU=8 MHz 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
duration is always correct assuming the applica-
tion is not doing anything between the idle and the
break. This can be ensured by temporarily disa-
bling interrupts.
The exact sequence is:
- Disable interrupts
- Reset and Set TE (IDLE request)
- Set and Reset SBK (Break Request)
- Re-enable interrupts
15.1.7 External interrupt missed
To avoid any risk of generating a parasitic inter-
rupt, the edge detector is automatically disabled
for one clock cycle during an access to either DDR
and OR. Any input signal edge during this period
will not be detected and will not generate an inter-
rupt.
This case can typically occur if the application re-
freshes the port configuration registers at intervals
during runtime.
Workaround
The workaround is based on software checking
the level on the interrupt pin before and after writ-
ing to the PxOR or PxDDR registers. If there is a
level change (depending on the sensitivity pro-
grammed for this pin) the interrupt routine is in-
voked using the call instruction with three extra
PUSH instructions before executing the interrupt
routine (this is to make the call compatible with the
IRET instruction at the end of the interrupt service
routine).
But detection of the level change does not make
sure that edge occurs during the critical 1 cycle du-
ration and the interrupt has been missed. This may
lead to occurrence of same interrupt twice (one
hardware and another with software call).
To avoid this, a semaphore is set to '1' before
checking the level change. The semaphore is
changed to level '0' inside the interrupt routine.
When a level change is detected, the semaphore
status is checked and if it is '1' this means that the
last interrupt has been missed. In this case, the in-
terrupt routine is invoked with the call instruction.
There is another possible case that is, if writing to
PxOR or PxDDR is done with global interrupts dis-
abled (interrupt mask bit set). In this case, the
semaphore is changed to '1' when the level
change is detected. Detecting a missed interrupt is
done after the global interrupts are enabled (inter-
rupt mask bit reset) and by checking the status of
the semaphore. If it is '1' this means that the last
interrupt was missed and the interrupt routine is in-
voked with the call instruction.
To implement the workaround, the following soft-
ware sequence is to be followed for writing into the
PxOR/PxDDR registers. The example is for Port
PF1 with falling edge interrupt sensitivity. The soft-
ware sequence is given for both cases (global in-
terrupt disabled/enabled).
Case 1: Writing to PxOR or PxDDR with Global In-
terrupts Enabled:
LD A,#01
LD sema,A; set the semaphore to '1'
LD A,PFDR
AND A,#02
LD X,A; store the level before writing to PxOR/
PxDDR
LD A,#$90
LD PFDDR,A ; Write to PFDDR
LD A,#$ff
LD PFOR,A ; Write to PFOR
LD A,PFDR
AND A,#02
LD Y,A; store the level after writing to PxOR/PxD-
DR
LD A,X; check for falling edge
cp A,#02
jrne OUT
TNZ Y
1
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