ATTINY20-EK1 Datasheet, PDF (13/220 Pages) ATMEL Corporation – 8-bit AVR Microcontroller with 2K Bytes In-System Programmable Flash

4.8 Register Description
4.8.1 CCP – Configuration Change Protection Register
Bit
0x3C
Read/Write
Initial Value
7
6
5
4
3
2
1
0
CCP[7:0]
CCP
W
W
W
W
W
W
W
R/W
0
0
0
0
0
0
0
0
 Bits 7:0 – CCP[7:0]: Configuration Change Protection
In order to change the contents of a protected I/O register the CCP register must first be written with the correct
signature. After CCP is written the protected I/O registers may be written to during the next four CPU instruction cycles.
All interrupts are ignored during these cycles. After these cycles interrupts are automatically handled again by the CPU,
and any pending interrupts will be executed according to their priority.
When the protected I/O register signature is written, CCP0 will read as one as long as the protected feature is enabled,
while CCP[7:1] will always read as zero.
Table 4-1 shows the signatures that are recognised.
Table 4-1. Signatures Recognised by the Configuration Change Protection Register
Signature
0xD8
Group
IOREG: CLKMSR, CLKPSR, WDTCSR⁽¹⁾, MCUCR⁽²⁾
Description
Protected I/O register
Notes: 1. Only WDE and WDP[3:0] bits are protected in WDTCSR.
2. Only BODS bit is protected in MCUCR.
4.8.2 SPH and SPL — Stack Pointer Registers
Initial Value
Read/Write
Bit
0x3E
0x3D
Bit
Read/Write
Initial Value
0
R
15
–
SP7
7
R/W
RAMEND
0
R
14
–
SP6
6
R/W
RAMEND
0
R
13
–
SP5
5
R/W
RAMEND
0
R
12
–
SP4
4
R/W
RAMEND
0
R
11
–
SP3
3
R/W
RAMEND
0
R
10
–
SP2
2
R/W
RAMEND
0
R
9
–
SP1
1
R/W
RAMEND
0
R
8
–
SP0
0
R/W
RAMEND
SPH
SPL
 Bits 7:0 – SP[7:0]: Stack Pointer
The Stack Pointer register points to the top of the stack, which is implemented growing from higher memory locations to
lower memory locations. Hence, a stack PUSH command decreases the stack pointer.
The stack space in the data SRAM must be defined by the program before any subroutine calls are executed or
interrupts are enabled.
In ATtiny20, the SPH register has not been implemented.
ATtiny20 [DATASHEET] 13
8235E–AVR–03/2013