ST72F324BK6TAS STMicroelectronics, ST72F324BK6TAS Datasheet - Page 189

• Manufacturer Part Number: ST72F324BK6TAS
• Description: 8-BIT MCU
• Manufacturer: STMicroelectronics

ST72324B-Auto
15
15.1
15.1.1
15.1.2
Known limitations
All Flash and ROM devices
Safe connection of OSC1/OSC2 pins
The OSC1 and/or OSC2 pins must not be left unconnected, otherwise the ST7 main
oscillator may start and, in this configuration, could generate an f
excess of the allowed maximum (> 16 MHz), putting the ST7 in an unsafe/undefined state.
Refer to
External interrupt missed
To avoid any risk of generating a parasitic interrupt, 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 interrupt.
This case can typically occur if the application refreshes 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
writing to the PxOR or PxDDR registers. If there is a level change (depending on the
sensitivity programmed for this pin) the interrupt routine is invoked 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
one cycle duration 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 interrupt 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 disabled (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 (interrupt 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 invoked with the call instruction.
To implement the workaround, the following software sequence is to be followed for writing
into the PxOR/PxDDR registers. The example is for Port PF1 with falling edge interrupt
sensitivity. The software sequence is given for both cases (global interrupt
disabled/enabled).
Section 6.3 on page
32.
Doc ID13466 Rev 4
OSC
clock frequency in
Known limitations
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