C8051F120 Datasheet, PDF (139/356 Pages) Silicon Laboratories – Mixed Signal ISP Flash MCU Family

C8051F120/1/2/3/4/5/6/7
C8051F130/1/2/3
SFR page is automatically restored to the SFR Page in use prior to the interrupt. This is accomplished via
a three-byte SFR Page Stack. The top byte of the stack is SFRPAGE, the current SFR Page. The second
byte of the SFR Page Stack is SFRNEXT. The third, or bottom byte of the SFR Page Stack is SFRLAST.
On interrupt, the current SFRPAGE value is pushed to the SFRNEXT byte, and the value of SFRNEXT is
pushed to SFRLAST. Hardware then loads SFRPAGE with the SFR Page containing the flag bit associated
with the interrupt. On a return from interrupt, the SFR Page Stack is popped resulting in the value of SFRN-
EXT returning to the SFRPAGE register, thereby restoring the SFR page context without software interven-
tion. The value in SFRLAST (0x00 if there is no SFR Page value in the bottom of the stack) of the stack is
placed in SFRNEXT register. If desired, the values stored in SFRNEXT and SFRLAST may be modified
during an interrupt, enabling the CPU to return to a different SFR Page upon execution of the RETI instruc-
tion (on interrupt exit). Modifying registers in the SFR Page Stack does not cause a push or pop of the
stack. Only interrupt calls and returns will cause push/pop operations on the SFR Page Stack.
SFRPGCN Bit
Interrupt
Logic
CIP-51
SFRPAGE
SFRNEXT
SFRLAST
Figure 11.5. SFR Page Stack
Automatic hardware switching of the SFR Page on interrupts may be enabled or disabled as desired using
the SFR Automatic Page Control Enable Bit located in the SFR Page Control Register (SFRPGCN). This
function defaults to ‘enabled’ upon reset. In this way, the autoswitching function will be enabled unless dis-
abled in software.
A summary of the SFR locations (address and SFR page) is provided in Table 11.2. in the form of an SFR
memory map. Each memory location in the map has an SFR page row, denoting the page in which that
SFR resides. Note that certain SFR’s are accessible from ALL SFR pages, and are denoted by the “(ALL
PAGES)” designation. For example, the Port I/O registers P0, P1, P2, and P3 all have the “(ALL PAGES)”
designation, indicating these SFR’s are accessible from all SFR pages regardless of the SFRPAGE regis-
ter value.
Rev. 1.3
139