BCM1250/BCM1125/BCM1125H
User Manual
10/21/02
B r o a d c o m C o r p o r a t i o n
Page
426
Section 15: JTAG and Debug
Document
1250_1125-UM100CB-R
The DBBOOT signal will force the CPU to use the alternate debug vector when it enters debug mode. When
the CPUs are next reset, they will sample their DBBOOT signals and if it is asserted they will enter Debug Mode
and start execution from the alternative debug vector (physical
00_1000_0480
). The CPUs will therefore
enter debug mode and execute code from the JTAG probe, without fetching or executing any instructions from
the normal boot vector. This can be used to download code into a system which has no code in ROM.
The EJTAGBOOT indication is effective until the NORMALBOOT instruction is given, TRST_L asserted or a
rising edge of TCK occurs when TAP controller is in the Test-Logic-Reset state. The
PrTrap0
and
PrTrap1
bits may be cleared by scanning in to the EJTAG Control Register and will read back as zeros, but this is not
sufficient to clear the EJTAGBOOT state. The Bypass register is selected when the EJTAGBOOT instruction
is given.
NORMALBOOT Instruction
When the NORMALBOOT instruction is given and Update-IR state is left, then the DBBOOT signal is
deasserted and the PrTrap0, PrTrap1 and ProbEn bits in the EJTAG Control register are set to 0. Since
DBBOOT is deasserted the CPUs will start at the standard Reset vector the next time they are reset. The
Bypass register is selected when the NORMALBOOT instruction is given.
SCAN Instructions (0x26 - 0x38)
When these instructions are given the corresponding scan chain is selected. When the SCANALL instruction
is given then all of the scan chains are selected in series in order TDI to MC to SCD to L2C to IOB1 to IOB0
to CPU1 to CPU0 to TDO. These scan chains are for Broadcom Use Only. Scanning inappropriate values in
to the chains can put the BCM1250 in an UNDEFINED state that requires both a system and a JTAG reset to
clear.
SYSCTRL Instruction
The system control and status register is used to control the part for testing and debug purposes. This register
is defined by
. The System Control Scanchain, summarized in
, includes this
register and additional control bits.