Specifications

Table Of Contents
ARM Debugger 54 ARM specific SYStem Commands
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<parameter> describing the “JTAG” scan chain and signal behavior
With the JTAG interface you can access a Test Access Port controller (TAP) which has implemented a state
machine to provide a mechanism to read and write data to an Instruction Register (IR) and a Data Register
(DR) in the TAP. The JTAG interface will be controlled by 5 signals: nTRST(reset), TCK (clock), TMS (state
machine control), TDI (data input), TDO (data output). Multiple TAPs can be controlled by one JTAG
interface by daisy-chaining the TAPs (serial connection). If you want to talk to one TAP in the chain you need
to send a BYPASS pattern (all ones) to all other TAPs. For this case the debugger needs to know the
position of the TAP he wants to talk to which can be notified with the first four commands in the table below.
... DRPOST <bits> Defines the TAP position in a JTAG scan chain. Number of TAPs
in the JTAG chain between the TDI signal and the TAP you are
describing. In BYPASS mode each TAP contributes one data
register bit. See possible TAP types and example below.
Default: 0.
... DRPRE <bits> Defines the TAP position in a JTAG scan chain. Number of TAPs
in the JTAG chain between the TAP you are describing and the
TDO signal. In BYPASS mode each TAP contributes one data
register bit. See possible TAP types and example below.
Default: 0.
... IRPOST <bits> Defines the TAP position in a JTAG scan chain. Number of
Instruction Register (IR) bits of all TAPs in the JTAG chain
between TDI signal and the TAP you are describing. See
possible TAP types and example below.
Default: 0.
... IRPRE <bits> Defines the TAP position in a JTAG scan chain. Number of
Instruction Register (IR) bits of all TAPs in the JTAG chain
between the TAP you are describing and the TDO signal. See
possible TAP types and example below.
Default: 0.
CHIPDRLENGTH <bits> Number of Data Register (DR) bits which needs to get a certain
BYPASS pattern.
CHIPDRPATTERN [Stan-
dard | Alternate <pattern>]
Data Register (DR) pattern which shall be used for BYPASS
instead of the standard (1...1) pattern.
CHIPIRLENGTH <bits> Number of Instruction Register (IR) bits which needs to get a
certain BYPASS pattern.
CHIPIRPATTERN [Standard |
Alternate <pattern>]
Instruction Register (IR) pattern which shall be used for BYPASS
instead of the standard pattern.