AN1769 Freescale Semiconductor / Motorola, AN1769 Datasheet - Page 33

• Manufacturer Part Number: AN1769
• Description: Designing a Minimal PowerPC System
• Manufacturer: Freescale Semiconductor / Motorola

MOTOROLA
Part 5 Reset
In order to properly condition a PowerPC processor, the HRESET signal must be asserted whenever the
system initially powers up and whenever the processor power supply (or supplies) fall below -5% of the
nominal voltage described in the hardware speciÞcation. The JTAG TRST signal must be asserted at reset
as well, to initialize the scan chain to a known state.
In addition, the initial power-up sequence requires that the HRESET signal be asserted for a minimum of
255 clocks in order to properly initialize the clock PLL and initialize hardware signals.
The simplest way to achieve all of these goals is to use one of many inexpensive devices available to drive
the reset lines at the proper time. Called Òreset controllersÓ or Òsystem supervisory controllersÓ, these
devices are typically very inexpensive (less than US$0.50), have small footprints (SOT23 to SO8), and are
widely available from Texas Instruments, Maxim Semiconductor, and others. Figure 20 shows an example
using these types of circuits.
If the reliability of the power supply can be assured, or if the power supply provides a failure output, then
the reset controller can be reduced to a simple R-C network, as shown in Figure 21.
Because the drain on the HRESET signal is negligible, the simple equation t = RC sufÞces to calculate the
necessary values. The above values shown give a 10
25 MHz.
Digital Reset Input
(optional)
Power Supply with open-drain
Power Failure Output
Freescale Semiconductor, Inc.
Figure 20. Reset Using Supervisory Controller
For More Information On This Product,
POWERFAIL
Figure 21. Simple Reset Controller
Minimal PowerPC System Design
Go to: www.freescale.com
Supervisory Circuit
Dallas DS1834
V
T
Open Drain
µ
TH
RST
+3.3V
s reset, which is sufÞcient for all bus speeds faster than
= 3.08V
= 1 ms
+3.3V
R = 10K
C = 1µF
+3.3V
HRESET
10K
PowerPC
HRESET
TRST
PowerPC
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