Maxim Integrated DS4830A Optical Microcontroller - Manual (Page 166)

DS4830A User’s Guide
166
TDI TDO
9 0 9 0
X X s1 s0
Host Command / Data Input Status
DS4830A
DS4830A Data Output
Figure 21-2: 10-Bit Word Format
Table 21-1: Status Bits
s1:s0 STATUS/CONDITION
00 Non-Debug. Default condition, Background mode, or debug engine inactive.
01 Debug Idle. Debug engine is ready to receive data from the host (command, data).
10 Debug Busy. Debug engine is busy without valid data (i.e. ROM code execution, trace operations).
11 Debug Valid. Debug engine is busy with valid data
The data byte portion of the 10-bit shift register is interfaced directly to the ICDB parallel register. The ICDB register
functions as the holding data register for both transmit and receive operations. On the falling edge of TCK in the
Update-DR state, the outgoing data is loaded from the ICDB parallel register to the debug shift register and the
incoming shift register data is latched in the ICDB parallel register.
21.1 – Background Mode Operation
When the instruction register is loaded with the Debug instruction (IR2:0 = 010b), the host can communicate with the
DS4830A in a background mode using TAP DR-Scan sequences without disturbing CPU operation. Note, however,
that JTAG in-system programming also requires use of the 10-bit debug shift register and, if enabled (JTAG_SPE=1,
PSS1:0= 0), takes precedence over background mode communication. When operating in background mode, the
status bits are always cleared to 00b (non-debug), which indicates that the DS4830A is ready to receive background
mode commands.
The host can perform the following operations from background mode:
• read/write internal breakpoint registers (BP0–BP5)
• read/write internal in-circuit debug registers (ICDC, ICDF, ICDA, ICDD)
• monitor to determine when a breakpoint match has occurred
• directly invoke debug mode
Table 21-2 shows the background mode commands supported by the DS4830A. Encodings not listed in this table
are not supported in background mode and are treated as no operations.