Rainbow Electronics ATmega128RFA1 - Manual (Page 14)

14
8266A-MCU Wireless-12/09
ATmega128RFA1
7.6.2 SPL – Stack Pointer Low
Bit 7 6 5 4 3 2 1 0
$3D ($5D) SP7 SP6 SP5 SP4 SP3 SP2 SP1 SP0 SPL
Read/Write RW RW RW RW RW RW RW RW
Initial Value 1 1 1 1 1 1 1 1
The AVR Stack Pointer is implemented as two 8-bit registers SPL and SPH in the I/O
space. The number of bits actually used is implementation dependent. Note that the
data space in some implementations of the AVR architecture is so small that only SPL
is needed. In this case, the SPH Register will not be present.
• Bit 7:0 – SP7:0 - Stack Pointer Low Byte
7.6.3 RAMPZ – Extended Z-pointer Register for ELPM/SPM
Bit 7 6 5 4 3 2 1 0
$3B ($5B) Res5 Res4 Res3 Res2 Res1 Res0 RAMPZ1 RAMPZ0 RAMPZ
Read/Write RW RW RW RW RW RW RW RW
Initial Value 0 0 0 0 0 0 0 0
For ELPM/SPM instructions, the Z-pointer is a concatenation of RAMPZ, ZH, and ZL.
Note that LPM is not affected by the RAMPZ setting.
• Bit 7:2 – Res5:0 - Reserved
For compatibility with future devices, be sure to write these bits to zero.
• Bit 1:0 – RAMPZ1:0 - Extended Z-Pointer Value
These two bits represent the MSB's of the Z-Pointer.
Table 7-2 RAMPZ Register Bits
Register Bits Value Description
RAMPZ1:0 0 Default value of Z-pointer MSB's.
For ELPM/SPM instructions, the Z-pointer is a concatenation of RAMPZ, ZH, and ZL,
as shown in
Figure 7-3 below . Note that LPM is not affected by the RAMPZ setting.
Figure 7-3. The Z-pointer used by ELPM and SPM
The actual number of bits is implementation dependent. Unused bits in an
implementation will always read as zero. For compatibility with future devices, be sure
to write these bits to zero.