Datasheet

Central processing unit ST7LITE20F2 ST7LITE25F2 ST7LITE29F2
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by software in the interrupt routine, pending interrupts are serviced regardless of the priority
level of the current interrupt routine.
• Bit 2 = N Negative
This bit is set and cleared by hardware. It is representative of the result sign of the last
arithmetic, logical or data manipulation. It is a copy of the 7
th
bit of the result.
0: The result of the last operation is positive or null
1: The result of the last operation is negative
(i.e. the most significant bit is a logic 1)
This bit is accessed by the JRMI and JRPL instructions.
• Bit 1 = Z Zero
This bit is set and cleared by hardware. This bit indicates that the result of the last
arithmetic, logical or data manipulation is zero.
0: The result of the last operation is different from zero
1: The result of the last operation is zero
This bit is accessed by the JREQ and JRNE test instructions.
•
Bit 0 = C Carry/borrow
This bit is set and cleared by hardware and software. It indicates an overflow or an
underflow has occurred during the last arithmetic operation.
0: No overflow or underflow has occurred
1: An overflow or underflow has occurred
This bit is driven by the SCF and RCF instructions and tested by the JRC and JRNC
instructions. It is also affected by the “bit test and branch”, shift and rotate instructions.
Stack pointer register (SP)
Read / Write
Reset value:
01FFh
The Stack Pointer is a 16-bit register which is always pointing to the next free location in the
stack. It is then decremented after data has been pushed onto the stack and incremented
before data is popped from the stack (see Figure 11).
Since the stack is 128 bytes deep, the 9 most significant bits are forced by hardware.
Following an MCU Reset, or after a Reset Stack Pointer instruction (RSP), the Stack Pointer
contains its reset value (the SP6 to SP0 bits are set) which is the stack higher address.
The least significant byte of the Stack Pointer (called S) can be directly accessed by a LD
instruction.
Note: When the lower limit is exceeded, the Stack Pointer wraps around to the stack upper limit,
without indicating the stack overflow. The previously stored information is then overwritten
and therefore lost. The stack also wraps in case of an underflow.
The stack is used to save the return address during a subroutine call and the CPU context
during an interrupt. The user may also directly manipulate the stack by means of the PUSH
and POP instructions. In the case of an interrupt, the PCL is stored at the first location
15 8
00000001
7 0
1 SP6 SP5 SP4 SP3 SP2 SP1 SP0