User Manual
BCM1250/BCM1125/BCM1125H
10/21/02
B r o a d c o m C o r p o r a t i o n
Document
1250_1125-UM100CB-R
Section 15: JTAG and Debug Page
421
S e c t i o n 1 5 : J TA G a n d D e b u g
I
NTRODUCTION
The part includes extensive debugging and performance monitoring features. These can be accessed by the
CPU(s) or through the JTAG port. The JTAG interface supports the required subset of MIPS EJTAG 2.5 and
additions to allow both processors to be debugged. The debug and bus trace unit in the SCD can also be
controlled through the JTAG port.
A JTAG probe may be connected to the part for accessing both the boundary scan and debug functions. There
is a standard connector for the MIPS EJTAG interface. The application note’BCM1250 EJTAG Connector’ #00-
117 from Corelis Inc. (www.corelis.com) includes a recommendation for the connector and terminations for the
BCM1250 and BCM1125/H.
TAP C
ONTROLLER
The SB1250 implements a IEEE 1149.1 compliant Test Access Port (TAP) controller. The TMS pin is used to
control the tap state machine. If TRST_L is asserted then the TAP controller state machine enters the Test-
Logic-Reset state.
shows the state machine used by the JTAG interface. This is the standard JTAG state
machine. TRST_L can be used to reset the state machine to the Test-Logic-Reset state, which will also be
entered if TMS is asserted for five TCK rising edges. The state machine advances each TCK rising edge as
directed by the TMS level. The states are as defined in the JTAG specification. The IR scan branch is used to
scan instructions into the Instruction register described in
Section: “Instruction Register” on page 424
. The DR
branch scans bits into and out of whatever data scan chain has been selected by an instruction.
When TRST_L is asserted or the TAP enters the Test-Logic-Reset state, the instruction register is loaded with
the IDCODE instruction, any EJTAGBOOT indication is removed and the EJTAG Control register is cleared.
The COLDRES_L input is also used to reset the TAP, this ensures that the scan chains and boundary scan
drivers remain in a safe state as the power comes up. A side effect is that the TAP is held in reset and none
of the JTAG machinery can be used while COLDRES_L is asserted. If testing is to be done using the JTAG
interface with the BCM1250 held in reset then RESET_L should be used to keep the system in reset while
allowing the JTAG port to be active. The standard EJTAG connector allows the probe to control RESET_L.
Table 298: JTAG Signals
JTAG Pin
Description
TDI
Test data in. Data is shifted serially in to the part through this input.
TDO
Test data out. Data is shifted serially out of the part through this output.
TMS
Test Mode select. TAP state machine control signal. The value captured on the rising edge of TCK determines
the state transition.
TCK
Test clock. The JTAG interface can be clocked at up to 10 MHz.
TRST_L
Test Reset. Asserting this signal forces the TAP state machine in to the Test-Logic-Reset state.
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