THROUGH THE LOOKING GLASS

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THROUGH THE LOOKING GLASS

Lessons from the Trenches Taking a Look at the PIC18Cxxx Series
by David Brobst

Start ı PIC18Cxxx Chips ı Memory ı Data Memory ı Advanced Indirect Addressing ı Deep and Accessible Stack ı Interrupts ı Power-On Features ı Clock Speed ı 10-Bit A/D ı Hardware Multiplier ı Timers ı CCP/PWM ı USART ı I²C Master ı Table Read/Write ı Current Status ı Sources and PDF

ADVANCED INDIRECT ADDRESSING

Hand in hand with the deep data memory, the PIC18Cxxx family provides three separate indirect addressing pointers, each with five modes of operation. The indirect address pointers are composed of two registers that are concatenated to form a 12-bit wide word that can handle the complete data space, so no paging is necessary when using indirect addressing (a blessed relief). The five modes of operation are: do nothing, post-decrement, post-increment, pre-increment, and add an offset. The three independent indirect pointers in conjunction with the operating modes make manipulating and tracking such things as arrays and large amounts of data manageable. Each of these operating modes is described in Table 2.

Operating mode	Description
Do nothing	After accessing the indirect data, the pointer is left as is. This is the current operating mode in the low- and mid-range PIC devices.
Post-decrement	After accessing the indirect data, the pointer is decremented by one. All carries between the two pointer registers are serviced internally.
Post-increment	After accessing the indirect data, the pointer is incremented by one. All carries between the two pointer registers are serviced internally.
Pre-increment	Before accessing the indirect data, the pointer is incremented. All carries between the two pointer registers are serviced internally.
Add an offset	The data in the indirect pointer is modified by the value in the working register (W) before the data access. The modification is a signed byte (2ıs complement) added to the indirect pointer, so the pointer can be offset by 127 to ı128. The value in W and the pointer remains the same after the operation as before.

Table 2ıThe indirect memory modes of the 18Cxxx family encourage creative memory manipulation and data storage.

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