Preliminary C8051F120/1/2/3 C8051F124/5/6/7 - KEMT FEI TUKE

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C8051F120/1/2/3 C8051F124/5/6/7 Preliminary 12.2.6.3.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 Port 5 (SFR “P5”, located at address 0xD8 on SFR Page 0x0F). The device is also using the Programmable Counter Array (PCA) and the 10-bit ADC (ADC2) 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 ADC2 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 Port 5 SFR (SFRPAGE = 0x0F). See Figure 12.6 below. Figure 12.6. SFR Page Stack While Using SFR Page 0x0F To Access Port 5 0x0F (Port 5) SFR Page Stack SFR's SFRPAGE SFRNEXT SFRLAST Page 128 DS008-0.8-AUG02 © 2002 Cygnal Integrated Products, Inc.
Preliminary C8051F120/1/2/3 C8051F124/5/6/7 While CIP-51 executes in-line code (writing values to Port 5 in this example), ADC2 Window Comparator Interrupt occurs. The CIP-51 vectors to the ADC2 Window Comparator ISR and pushes the current SFR Page value (SFR Page 0x0F) into SFRNEXT in the SFR Page Stack. The SFR page needed to access ADC2’s SFR’s is then automatically placed in the SFRPAGE register (SFR Page 0x02). SFRPAGE is considered the “top” of the SFR Page Stack. Software can now access the ADC2 SFR’s. Software may switch to any SFR Page by writing a new value to the SFRPAGE register at any time during the ADC2 ISR to access SFR’s that are not on SFR Page 0x02. See Figure 12.7 below. Figure 12.7. SFR Page Stack After ADC2 Window Comparator Interrupt Occurs SFRPAGE pushed to SFRNEXT SFR Page 0x02 Automatically pushed on stack in SFRPAGE on ADC2 interrupt 0x02 (ADC2) 0x0F (Port 5) SFRPAGE SFRNEXT SFRLAST © 2002 Cygnal Integrated Products, Inc. DS008-0.8-AUG02 Page 129
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Preliminary C8051F120/1/2/3 C8051F124/5/6/7 - KEMT FEI TUKE

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