实现MAXQ2000微控制器的JTAG加载主机
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作为一个例子，在表4中，JTAG主机将数值100b (系统编程模式)移入到IR寄存器中。IR寄存器开始设置旁路值011b；TAP控制器最初为Run-Test-Idle状态。如下面的图4所示，比特以最低有效位在前、最高有效位在后移入(移出)TAP控制器。因此，在第一个移位周期，新数值的第0位被移入，旧数值的第0位被移出。
表4. 指令寄存器移位实例
| TCK | TMS | TDI | TDO | TAP State | Shift Register | Instruction Register | ||||
| bit 2 | bit 1 | bit 0 | bit 2 | bit 1 | bit 0 | |||||
| 0 | 1 | x | x | Run-Test-Idle | x | x | x | 0 | 1 | 1 |
| 1 | 1 | x | x | Select-DR-Scan | x | x | x | 0 | 1 | 1 |
| 0 | 1 | x | x | Select-DR-Scan | x | x | x | 0 | 1 | 1 |
| 1 | 1 | x | x | Select-IR-Scan | x | x | x | 0 | 1 | 1 |
| 0 | 0 | x | x | Select-IR-Scan | x | x | x | 0 | 1 | 1 |
| 1 | 0 | x | x | Capture-IR | 0 | 0 | 1 | 0 | 1 | 1 |
| 0 | 0 | x | x | Capture-IR | 0 | 0 | 1 | 0 | 1 | 1 |
| 1 | 0 | x | x | Shift-IR | 0 | 0 | 1 | 0 | 1 | 1 |
| 0 | 0 | 0 | x | Shift-IR | 0 | 0 | 1 | 0 | 1 | 1 |
| 1 | 0 | 0 | x | Shift-IR | 0 | 0 | 1 | 0 | 1 | 1 |
| 0 | 0 | 0 | 1 | Shift-IR | 0 | 0 | 0 | 0 | 1 | 1 |
| 1 | 0 | 0 | 1 | Shift-IR | 0 | 0 | 0 | 0 | 1 | 1 |
| 0 | 1 | 1 | 0 | Shift-IR | 0 | 0 | 0 | 0 | 1 | 1 |
| 1 | 1 | 1 | 0 | Exit1-IR | 0 | 0 | 0 | 0 | 1 | 1 |
| 0 | 1 | x | 0 | Exit1-IR | 1 | 0 | 0 | 0 | 1 | 1 |
| 1 | 1 | x | 0 | Update-IR | 1 | 0 | 0 | 1 | 0 | 0 |
| 0 | 0 | x | x | Update-IR | 1 | 0 | 0 | 1 | 0 | 0 |
| 1 | 0 | x | x | Run-Test-Idle | x | x | x | 1 | 0 | 0 |
下面是执行这一操作的例程shiftIR3。
;==============================================================================;=;= shiftIR3;= clock0, clock1, shift;=;= Shifts a 3-bit value into the IR register.;=;= Inputs : A[0] - Low three bits contain value to shift into IR;= Outputs : None;= Destroys : AP, APC, A[0], PSW, LC[0];=shiftIR3:move APC, #80h ; Acc => A[0], turn off auto inc/deccall clock1 ; (Select DR Scan)call clock1 ; (Select IR Scan)call clock0 ; (Capture IR - loads 001b to shift register)call clock0 ; (Shift IR)move TMS, #0 ; Drive TMS low move C, TDO ; xxxxx210 c = srrc ; sxxxxx21 c = 0call shift ; Shift in IR bit 0rrc ; ssxxxxx2 c = 1call shift ; Shift in IR bit 1rrc ; sssxxxxx c = 2move TMS, #1 ; Drive TMS high for last bitcall shift ; Shift in IR bit 2 (Exit1 IR)call clock1 ; (Update IR)call clock0 ; (Run Test Idle)ret
写入TAP数据寄存器
数值移入或者移出TAP控制器DR的操作与装入/卸载IR的方式相似。通常，只有当IR被设置为两个数值之一时才能进行这一操作：100b (使TAP控制器进入系统编程模式)或者010b (使TAP控制器进入调试模式)。激活系统编程模式后，装入和卸载DR寄存器的操作如下。
- DR寄存器移入和移出宽度为3比特。所有三个比特都代表有效数据。
- 当到达Update-DR状态时，送入DR寄存器的数值被复制到从机微控制器的内部系统编程寄存器中。这三个比特的用法如下。
- DR寄存器传送出去的数值是系统编程寄存器以前的数值(当进入Capture-DR状态后，锁存到移位寄存器中)。
- DR寄存器移入和移出宽度为10比特。对于移出数据，所有10个比特都代表有效数据(8个数据位和2个状态位)。对于移入数据，只使用了8个数据位；没有使用两个状态位。
- 然后，将移入DR寄存器的高8位卸载，作为启动加载程序命令的一部分，被启动加载程序(从程序ROM中运行)读取。
- 移出DR寄存器的10个比特含有8位由启动加载程序装入的数值(作为命令输出的一部分)，以及由TAP控制器设置的两个状态信息位。
;==============================================================================;=;= shiftDR3;=;= Shifts a 3-bit value into the DR register. This operation should only be;= performed when IR =100b (System Programming Mode).;=;= Inputs : A[0] - Low 3 bits contain value to shift into SPB (PSS1:PSS0:SPE);= Outputs : None;= Destroys : AP, APC, A[0], PSW, LC[0]shiftDR3:move APC, #80h ; Acc => A[0], turn off auto inc/deccall clock1 ; (Select DR Scan)call clock0 ; (Capture DR)call clock0 ; (Shift DR)move TMS, #0 ; Drive TMS low move C, TDO ; xxxxx210 c = srrc ; sxxxxx21 c = 0call shift ; Shift in DR bit 0rrc ; ssxxxxx2 c = 1call shift ; Shift in DR bit 1rrc ; sssxxxxx c = 2move TMS, #1 ; Drive TMS high for last bitcall shift ; Shift in DR bit 2 (Exit1 DR)call clock1 ; (Update DR)call clock0 ; (Run Test Idle)ret;==============================================================================;=;= shiftDR;= clock0, clock1, shift;=;= Shifts a 10-bit value into and out of the DR register. This operation ;= should only be performed when IR = 010b (Debug/Loader Mode).;=;= Inputs : A[0] - Byte value (input) to shift into DR;= Outputs : A[0] - Byte value (output) shifted out of DR;= A[1] - Low two bits are status bits 1:0 shifted out of DR;= A[15] - Byte value shifted in, cached for use by shiftDR_next;= Destroys : AP, APC, PSW, LC[0]shiftDR:move APC, #80h ; Acc => A[0], turn off auto inc/decmove A[15], A[0] ; Cache input byte value for use by shiftDR_nextsla2 ; Add two empty bits (for status)call clock1 ; (Select DR Scan)call clock0 ; (Capture DR)call clock0 ; (Shift DR)move TMS, #0 ; Drive TMS low move C, TDO ; xxxxxxxx76543210 c = s
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