Microchip Technology dsPIC30F - Page 70

dsPIC30F Family Reference Manual
DS70050C-page 3-10 © 2004 Microchip Technology Inc.
An additional condition exists for indirect read operations performed immediately after writing to
the modulo address SFRs:
•XMODSRT
• XMODEND
•YMODSRT
• YMODEND
If modulo addressing has already been enabled in MODCON, then a write to the X (or Y) modulo
address SFRs should not be immediately followed by an indirect read, using the W register
designated for modulo buffer access from X-data space (or Y-data space). The code segment in
Example 3-3 shows how initializing the modulo SFRs associated with the X-data space, could
lead to unexpected results. A similar example can be made for initialization in Y-data space.
Example 3-3: Incorrect Modulo Addressing Setup
To work around this issue, insert a NOP, or perform any operation other than an indirect read that
uses the W register designated for modulo buffer access, after initializing the modulo address
SFRs. This is demonstrated in Example 3-4. Another alternative would be to enable modulo
addressing in MODCON after initializing the modulo start and end address SFRs.
Example 3-4: Correct Modulo Addressing Setup
MOV #0x8FF4, w0 ;Modulo addressing enabled
MOV w0, MODCON ;in X-data space using w4
;for buffer access
MOV #0x1200, w4 ;XMODSRT is initialized
MOV w4, XMODSRT
MOV #0x12FF, w0 ;XMODEND is initialized
MOV w0, XMODEND
MOV [w4++], w5 ;Incorrect EA generated
MOV #0x8FF4, w0 ;Modulo addressing enabled
MOV w0, MODCON ;in X-data space using w4
;for buffer access
MOV #0x1200, w4 ;XMODSRT is initialized
MOV w4, XMODSRT
MOV #0x12FF, w0 ;XMODEND is initialized
MOV w0, XMODEND
NOP ;See Note below
MOV [w4++], w5 ;Correct EA generated here
Note: Alternatively, execute other instructions that do not perform indirect read operations,
using the W register designated for modulo buffer access.