C8051F360/1/2/3/4/5/6/7/8/9
9.4.6.3. SFR Page Stack Example
The following is an example that shows the operation of the SFR Page Stack during interrupts.
In this example, the SFR Page Control is left in the default enabled state (i.e., SFRPGEN = 1), and the
CIP-51 is executing in-line code that is writing values to OSCICN (SFR “OSCICN”, located at address
0xB6 on SFR Page 0x0F). The device is also using the Programmable Counter Array (PCA) and the 10-bit
ADC (ADC0) window comparator to monitor a voltage. The PCA is timing a critical control function in its
interrupt service routine (ISR), so its interrupt is enabled and is set to high priority. The ADC0 is monitoring
a voltage that is less important, but to minimize the software overhead its window comparator is being used
with an associated ISR that is set to low priority. At this point, the SFR page is set to access the OSCICN
SFR (SFRPAGE = 0x0F). See Figure 9.4 below.
SFR Page
Stack SFR's
0x0F
SFRPAGE
(OSCICN)
SFRNEXT
SFRLAST
Figure 9.4. SFR Page Stack While Using SFR Page 0x0F To Access OSCICN
While CIP-51 executes in-line code (writing values to OSCICN in this example), ADC0 Window Compara-
tor Interrupt occurs. The CIP-51 vectors to the ADC0 Window Comparator ISR and pushes the current
SFR Page value (SFR Page 0x0F) into SFRNEXT in the SFR Page Stack. SFR page 0x00 is then auto-
matically placed in the SFRPAGE register. SFRPAGE is considered the “top” of the SFR Page Stack. Soft-
ware can now access the ADC0 SFR’s. Software may switch to any SFR Page by writing a new value to
the SFRPAGE register at any time during the ADC0 ISR to access SFR’s that are not on SFR Page 0x00.
See Figure 9.5 below.
90
Rev. 1.0
SILICON [ SILICON ]
