PXS20RM Datasheet, PDF (1141/1368 Pages) Freescale Semiconductor, Inc – PXS20 Microcontroller

Nexus Port Controller (NPC)
34.5.3.4 Selecting a Nexus Client Register
When the NEXUS-ENABLE instruction is decoded by the TAP controller, the input port allows
development tool access to all Nexus registers. Each register has a 7-bit address index.
All register access is performed via the SELECT-DR-SCAN path. The Nexus Controller defaults to the
REG_SELECT state when enabled. Accessing a register requires two passes through the
SELECT-DR-SCAN path: one pass to select the register and the second pass to read/write the register.
The first pass through the SELECT-DR-SCAN path is used to enter an 8-bit Nexus command consisting
of a read/write control bit in the LSB followed by a 7-bit register address index, as illustrated in
Figure 34-11. The read/write control bit is set to 1 for writes and 0 for reads.
MSB
LSB
7-bit register index
R/W
Figure 34-11. IEEE 1149.1 Controller Command Input
The second pass through the SELECT-DR-SCAN path is used to read or write the register data by shifting
in the data (LSB first) during the SHIFT-DR state. When reading a register, the register value is loaded into
the IEEE 1149.1-2001 shifter during the CAPTURE-DR state. When writing a register, the value is loaded
from the IEEE 1149.1-2001 shifter to the register during the UPDATE-DR state. When reading a register,
there is no requirement to shift out the entire register contents. Shifting may be terminated once the
required number of bits have been acquired.
Table 34-11 illustrates a sequence which writes a 32-bit value to a register
Table 34-11. Write to a 32-Bit Nexus Client Register
Clock
0
1
2
3
12
13
14
15
16
48
49
50
TMS
0
1
0
0
1
1
1
0
0
1
1
0
IEEE 1149.1 State
RUN-TEST/IDLE
SELECT-DR-SCAN
CAPTURE-DR
SHIFT-DR
7 TCKs
EXIT1-DR
UPDATE-DR
SELECT-DR-SCAN
CAPTURE-DR
SHIFT-DR
31 TCKs
EXIT1-DR
UPDATE-DR
RUN-TEST/IDLE
Nexus State
Description
REG_SELECT
REG_SELECT
REG_SELECT
REG_SELECT
IEEE 1149.1-2001 TAP controller in idle state
First pass through SELECT-DR-SCAN path
Internal shifter loaded with current value of
controller command input.
TDO becomes active, and write bit and 6 bits of
register index shifted in.
REG_SELECT Last bit of register index shifted into TDI
REG_SELECT Controller decodes and selects register
DATA_ACCESS Second pass through SELECT-DR-SCAN path
DATA_ACCESS Internal shifter loaded with current value of register
DATA_ACCESS TDO becomes active, and outputs current value of
register while new value is shifted in through TDI
DATA_ACCESS Last bit of current value shifted out TDO. Last bit of
new value shifted in TDI.
DATA_ACCESS Value written to register
REG_SELECT Controller returned to idle state. It could also return
to SELECT-DR-SCAN to write another register.
Freescale Semiconductor
PXS20 Microcontroller Reference Manual, Rev. 1
34-17