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ATmega128 |
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Bypass Register |
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The Bypass Register consists of a single Shift Register stage. When the Bypass Regis- |
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ter is selected as path between TDI and TDO, the register is reset to 0 when leaving the |
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Capture-DR controller state. The Bypass Register can be used to shorten the scan |
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chain on a system when the other devices are to be tested. |
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Device Identification Register Figure 122 shows the structure of the Device Identification Register. |
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Figure 122. The Format of the Device Identification Register |
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MSB |
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LSB |
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Bit |
31 |
28 |
27 |
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12 |
11 |
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1 |
0 |
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Device ID |
Version |
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Part Number |
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Manufacturer ID |
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1 |
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4 bits |
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16 bits |
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11 bits |
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1-bit |
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Version |
Version is a 4-bit number identifying the revision of the component. The JTAG version |
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number follows the revision of the device. Revision A is 0x0, revision B is 0x1 and so on. |
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Part Number |
The part number is a 16-bit code identifying the component. The JTAG Part Number for |
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ATmega128 is listed in Table 100. |
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Table 100. AVR JTAG Part Number |
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Part Number |
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JTAG Part Number (Hex) |
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ATmega128 |
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0x9702 |
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Manufacturer ID |
The Manufacturer ID is a 11-bit code identifying the manufacturer. The JTAG manufac- |
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turer ID for ATMEL is listed in Table 101. |
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Table 101. |
Manufacturer ID |
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Manufacturer |
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JTAG Manufacturer ID (Hex) |
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ATMEL |
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0x01F |
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Reset Register |
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The Reset Register is a Test Data Register used to reset the part. Since the AVR tri- |
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states Port Pins when reset, the Reset Register can also replace the function of the |
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unimplemented optional JTAG instruction HIGHZ. |
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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 TimeOut Period (refer to “Clock Sources” on page 35) 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 123.
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2467M–AVR–11/04
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ATmega128 |
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• Shift-DR: The IDCODE scan chain is shifted by the TCK input. |
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SAMPLE_PRELOAD; $2 |
Mandatory JTAG instruction for pre-loading the output latches and taking a snap-shot of |
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the input/output pins without affecting the system operation. However, the output |
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latched are not connected to the pins. The Boundary-scan Chain is selected as Data |
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Register. |
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The active states are: |
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• Capture-DR: Data on the external pins are sampled into the Boundary-scan Chain. |
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• Shift-DR: The Boundary-scan Chain is shifted by the TCK input. |
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• Update-DR: Data from the Boundary-scan Chain is applied to the output latches. |
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However, the output latches are not connected to the pins. |
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AVR_RESET; $C |
The AVR specific public JTAG instruction for forcing the AVR device into the Reset |
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mode or releasing the JTAG Reset source. The TAP controller is not reset by this |
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instruction. The one bit Reset Register is selected as Data Register. Note that the Reset |
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will be active as long as there is a logic 'one' in the Reset Chain. The output from this |
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chain is not latched. |
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The active states are:
•Shift-DR: The Reset Register is shifted by the TCK input.
BYPASS; $F |
Mandatory JTAG instruction selecting the Bypass Register for Data Register. |
The active states are:
•Capture-DR: Loads a logic “0” into the Bypass Register.
•Shift-DR: The Bypass Register cell between TDI and TDO is shifted.
Boundary-scan Related
Register in I/O Memory
MCU Control and Status |
The MCU Control and Status Register contains control bits for general MCU functions, |
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Register – MCUCSR |
and provides information on which reset source caused an MCU Reset. |
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Bit |
7 |
6 |
5 |
4 |
3 |
2 |
1 |
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0 |
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JTD |
– |
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JTRF |
WDRF |
BORF |
EXTRF |
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PORF |
MCUCSR |
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Read/Write |
R/W |
R |
R |
R/W |
R/W |
R/W |
R/W |
R/W |
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Initial Value |
0 |
0 |
0 |
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See Bit Description |
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• Bit 7 – JTD: JTAG Interface Disable
When this bit is zero, the JTAG interface is enabled if the JTAGEN fuse is programmed. If this bit is one, the JTAG interface is disabled. In order to avoid unintentional disabling or enabling of the JTAG interface, a timed sequence must be followed when changing this bit: The application software must write this bit to the desired value twice within four cycles to change its value.
If the JTAG interface is left unconnected to other JTAG circuitry, the JTD bit should be set to one. The reason for this is to avoid static current at the TDO pin in the JTAG interface.
• Bit 4 – JTRF: JTAG Reset Flag
This bit is set if a Reset is being caused by a logic one in the JTAG Reset Register selected by the JTAG instruction AVR_RESET. This bit is reset by a Power-on Reset, or by writing a logic zero to the flag.
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2467M–AVR–11/04