
Chapter 8. System Considerations
8-5
8.5 Pull-Up Resistors
The following signals are driven by various bus devices: TS, XATS, ABB, TA, DBB,
ARTRY, SHD, and DRTRY. When control of these signals is passed from one device to
another, the device that is releasing control always deasserts them before release. The
signals are then left in high-impedance state before being driven by another device. Pull-up
resistors are required on these signals to keep them in the negated state for this interval.
Note that this can be a fairly high value resistor because it does not cause a transition and
only retains a value.
8.6 Features for Improved Bus Performance
The following 60x processor features help improve bus performance:
Disabling the DRTRY feature (603 and 604e) decreases read latency by one cycle.
The use of ABB and DBB are optional on 60x processors. Because of the fractional-
cycle restoration to the high state, this helps achieve shorter cycle time.
Consecutive read data transfers can be sent without a dead cycle (604). This
increases maximum bandwidth by 25%.
8.7 IEEE 1149.1-Compliant Interface
The 604 boundary-scan interface is a fully-compliant implementation of the IEEE 1149.1
standard. This section describes the 604 IEEE 1149.1(JTAG) interface.
8.7.1 IEEE 1149.1 Interface Description
Table 8-1 describes the 604’s five dedicated JTAG signals. The TDI and TDO scan ports are
used to scan instructions and data into the various scan registers for JTAG operations. The
scan operation is controlled by the test access port (TAP) controller which in turn is
controlled by the TMS input sequence. Scan data is latched at the rising edge of TCK.
TRST is a JTAG-optional signal used to reset the TAP controller asynchronously; it ensures
that JTAG logic does not interfere with normal chip operation. It can be asserted coincident
with HRESET.
Table 8-1. IEEE Interface Signal Descriptions
Signal Name
Input/Output
Weak Pullup Provided
IEEE 1149.1 Function
TDI
Input
Yes
Serial scan input signal
TDO
Output
No
Serial scan output signal
TMS
Input
Yes
TAP controller mode signal
TCK
Input
Yes
Scan clock
TRST
Input
Yes
TAP controller reset