Datasheet

• Programming Enable Register
• Programming Command Register
• Virtual Flash Page Load Register
• Virtual Flash Page Read Register
28.10.8. Reset Register
The Reset Register is a Test Data Register used to reset the part during programming. It is required to
reset the part before entering programming mode.
A high value in the Reset Register corresponds to pulling the external Reset low. The part is reset as long
as there is a high value present in the Reset Register. Depending on the Fuse settings for the clock
options, the part will remain reset for a Reset Time-Out Period (refer to Clock Sources) after releasing the
Reset Register. The output from this Data Register is not latched, so the reset will take place immediately,
as shown in figure Reset Register.
28.10.9. Programming Enable Register
The Programming Enable Register is a 16-bit register. The contents of this register is compared to the
programming enable signature, binary code 1010_0011_0111_0000. When the contents of the register is
equal to the programming enable signature, programming via the JTAG port is enabled. The Register is
reset to 0 on Power-on Reset, and should always be reset when leaving Programming mode.
Figure 28-10. Programming Enable Register
TDI
TDO
D
A
T
A
=
D Q
ClockDR & P ROG_ENABLE
Programming e nable
$A370
28.10.10. Programming Command Register
The Programming Command Register is a 15-bit register. This register is used to serially shift in
programming commands, and to serially shift out the result of the previous command, if any. The JTAG
Programming Instruction Set is shown in the following table. The state sequence when shifting in the
programming commands is illustrated in State Machine Sequence for Changing/Reading the Data Word
further down in this section.
Atmel ATmega644A [DATASHEET]
Atmel-42716C-ATmega644A_Datasheet_Complete-10/2016
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