C8051F50x-F51x
SFRPGCN Bit
Interrupt
Logic
SFRPAGE
CIP-51
SFRNEXT
SFRLAST
Figure 13.1. 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 (SFR0CN). 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) are provided in Table 13.3 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. Certain SFRs 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)” designa-
tion, indicating these SFRs are accessible from all SFR pages regardless of the SFRPAGE register value.
13.3. 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 Control register 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 SPI Data Register (SFR “SPI0DAT”, located at address
0xA3 on SFR Page 0x00). The device is also using the CAN peripheral (CAN0) and the Programmable
Counter Array (PCA0) peripheral to generate a PWM output. The PCA is timing a critical control function in
its interrupt service round so its associated ISR that is set to low priority. At this point, the SFR page is set
to access the SPI0DAT SFR (SFRPAGE = 0x00). See Figure 13.2.
Rev. 1.1
101
SILICON [ SILICON ]
