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MOTOROLA M68040 USER’S MANUAL C- 13
while the bypass register is selected as the serial path between TDI and TDO. The signals
driven from the MC68040V and MC68EC040V pins do not change while the CLAMP
instruction is selected.
C.6.1.5 BYPASS. The BYPASS instruction selects the single-bit bypass register, creating
a single-bit shift-register path from TDI to the bypass register to TDO. The instruction
enhances test efficiency when a component other than the MC68040V and MC68EC040V
becomes the device under test. When the bypass register is initially selected, the
instruction shift register stage is set to a logic zero on the rising edge of TCK following
entry into the capture-DR state. Therefore, the first bit to be shifted out after selecting the
bypass register is always a logic zero. Figure C-5 illustrates the bypass register.
1MUX
1
G1
1D
C1
CLOCK DR
FROM TDI
0
SHIFT DR
TO TDO
Figure C-5. Bypass Register
C.6.2 Boundary Scan Register
The 188-bit boundary scan register uses the TAP controller to scan user-defined values
into the output buffers, capture values presented to input pins, and control the direction of
bidirectional pins. The instruction shift register cell nearest TDO (i.e., first to be shifted out)
is defined as bit zero. The last bit to be shifted out is bit 187. This register includes cells
for all device signal pins and clock pins along with associated control signals.
The MC68040V and MC68EC040V boundary scan register consists of three cell structure
types, O.Latch, I.Pin, and IO.Ctl, that are associated with a boundary scan register bit. All
boundary scan output cells capture the logic level of the device output latch during the
capture-DR state. Figures C-6 through C-9 illustrate these three cell types. Figure 6-6
illustrates the general arrangement of these cells.
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