PIC24FJ64GA004-IPT Datasheet, PDF (13/52 Pages) Microchip Technology – PIC24FJXXXGA0XX Flash Programming Specification

PIC24FJXXXGA0XX
3.2.1
SIX SERIAL INSTRUCTION
EXECUTION
The SIX control code allows execution of the
PIC24FJXXXGA0XX family assembly instructions.
When the SIX code is received, the CPU is suspended
for 24 clock cycles, as the instruction is then clocked into
the internal buffer. Once the instruction is shifted in, the
state machine allows it to be executed over the next four
PGC clock cycles. While the received instruction is
executed, the state machine simultaneously shifts in the
next 4-bit command (see Figure 3-2).
Coming out of Reset, the first 4-bit control code is
always forced to SIX and a forced NOP instruction is
executed by the CPU. Five additional PGCx clocks are
needed on start-up, resulting in a 9-bit SIX command
instead of the normal 4-bit SIX command.
After the forced SIX is clocked in, ICSP operation
resumes as normal. That is, the next 24 clock cycles
load the first instruction word to the CPU.
Note:
To account for this forced NOP, all example
code in this specification begin with a NOP
to ensure that no data is lost.
FIGURE 3-2:
SIX SERIAL EXECUTION
P1
1 2 3 4 56 78 9
1 2 3 4 5 6 7 8 17 18 19 20 21 22 23 24 1 2 3 4
PGCx
P3
P4
P1A
P4A
P1B
P2
PGDx 0 0 0 0 0 0 0 0 0
LSB X X X X X X X X X X X X X X MSB
00 00
Execute PC – 1,
Fetch SIX
Control Code
Only for
Program
Memory Entry
24-Bit Instruction Fetch
PGDx = Input
Execute 24-Bit
Instruction, Fetch
Next Control Code
3.2.1.1
Differences Between Execution of
SIX and Normal Instructions
There are some differences between executing instruc-
tions normally and using the SIX ICSP command. As a
result, the code examples in this specification may not
match those for performing the same functions during
normal device operation.
The important differences are:
• Two-word instructions require two SIX operations
to clock in all the necessary data.
Examples of two-word instructions are GOTO and
CALL.
• Two-cycle instructions require two SIX operations.
The first SIX operation shifts in the instruction and
begins to execute it. A second SIX operation – which
should shift in a NOP to avoid losing data – provides
the CPU clocks required to finish executing the
instruction.
Examples of two-cycle instructions are table read
and table write instructions.
• The CPU does not automatically stall to account
for pipeline changes.
A CPU stall occurs when an instruction modifies a
register that is used for Indirect Addressing by the
following instruction.
During normal operation, the CPU automatically
will force a NOP while the new data is read. When
using ICSP, there is no automatic stall, so any
indirect references to a recently modified
register should be preceded by a NOP.
For example, the instructions, mov #0x0,W0 and
mov [W0],W1, must have a NOP inserted
between them.
If a two-cycle instruction modifies a register that is
used indirectly, it will require two following NOPs: one
to execute the second half of the instruction and a
second to stall the CPU to correct the pipeline.
Instructions such as tblwtl [W0++],[W1]
should be followed by two NOPs.
• The device Program Counter (PC) continues to
automatically increment during ICSP instruction
execution, even though the Flash memory is not
being used.
As a result, the PC may be incremented to point to
invalid memory locations. Invalid memory spaces
include unimplemented Flash addresses and the
vector space (locations 0x0 to 0x1FF).
If the PC points to these locations, the device will
reset, possibly interrupting the ICSP operation. To
prevent this, instructions should be periodically
executed to reset the PC to a safe space. The
optimal method to accomplish this is to perform a
GOTO 0x200.
© 2008 Microchip Technology Inc.
DS39768D-page 13