ATmega16U4 datasheet(370/408 Pages) ATMEL

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370

7766D–AVR–01/09

ATmega16U4/ATmega32U4

28.9.8

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” on page 28) 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 8-1 on page 49.

28.9.9

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 0b1010_0011_0111_0000. When the con-

tents 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 28-13. Programming Enable Register

28.9.10

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 Table 28-17. The state sequence when shifting

in the programming commands is illustrated in Figure 28-15.

TDI

TDO

ClockDR & PROG_ENABLE

Programming Enable

0xA370

ATmega16U4 Datasheet(PDF) 370 Page - ATMEL Corporation