MOTOROLA
Chapter 13. External Signals
13-23
Part IV. Hardware Interface
13.4.1 Reset ConÞguration
Some external pin conÞguration is determined at reset by the hard reset conÞguration word.
Thus, some decisions as to system conÞguration (for example, location of BDM pins)
should be made before required application of pull-up and pull-down resistors can be
determined.
RSTCONF should be grounded if the hard reset conÞguration word is used to conÞgure the
MPC860 or should be connected to VCC if the default conÞguration is used.
Pull-up resistors may not be used on D[0Ð31] to set the hard reset conÞguration word, as
the values of the internal pull-down resistors are not speciÞed or guaranteed. To change a
data bus signal from its default logic low state during reset, actively drive that signal high.
MODCK[1Ð2] must be used to determine the default clocking mode for the MPC860. After
power-on reset, the MODCK[1Ð2] pins change function and become outputs. Thus, if these
alternate functions are also desired, then the MODCK[1Ð2] conÞguration should be set
with three-state drivers that turn off after PORESET is negated; however, if MODCK[1Ð2]
pinsÕ alternate output functions are not used in the system, they can be conÞgured with
pull-up and pull-down resistors.
13.4.1.1 Bus Control Signals and Interrupts
Signals with open-drain buffers and active pull-up buffers (HRESET, SRESET, TEA, TS,
TA, BI, and BB) must have external pull-up resistors.
Some other input signals do not absolutely require a pull-up resistor, as they may be
actively driven by external logic. However, if they are not used externally, or if the external
logic connected to them is not always actively driving, they may need external pull-up
resistors to hold them negated. These signals include the following:
¥
PORESET
¥
AS
¥
CR/IRQ3
¥
KR/RETRY/IRQ4/SPKROUT (if conÞgured as KR/RETRY or IRQ4)
¥
Any IRQx (if conÞgured as IRQx)
¥
BR (if the MPC860Õs internal bus arbiter is used)
¥
BG (if an external bus arbiter is used)
13.4.2 JTAG and Debug Ports
Recommendations on conÞguration of the JTAG pins (including TMS, TRST, TDI, TDO,
and TCK) are made in Section 38.6, ÒRecommended TAP ConÞguration.Ó
TCK/DSCK or ALE_B/DSCK/AT1 (depending on the conÞguration of the DSCK
function) should be connected to ground through a pull-down resistor to disable Debug
Содержание MPC860 PowerQUICC
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