mc68336 Freescale Semiconductor, Inc, mc68336 Datasheet - Page 13

• Manufacturer Part Number: mc68336
• Description: An Introduction To The Mc68331 And Mc68332
• Manufacturer: Freescale Semiconductor, Inc

Freescale Semiconductor, Inc.
+5V
+5V
MCU
10K
820
O.C.
RESET
10–100 F
+5V
LOW VOLTAGE
INHIBIT DEVICE
332TUT LVI/RESET CONN
Figure 8 Typical Reset Circuit
When the internal PLL is used to generate the internal system clock, the RESET pin works as follows. At
power-up, the MCU drives RESET low. When the PLL locks, the MCU releases RESET for two system clock
cycles. If the external pull-up resistor can pull RESET to a logic one during the two cycles, the MCU as-
sumes that the reset is a power-on reset rather than an external reset. However, if RESET does not rise to
a logic one during the two cycles, the MCU assumes that the reset is an external reset and drives RESET
to a logic zero for 512 clock cycles. After 512 cycles have elapsed, the MCU releases RESET for ten clock
cycles. If RESET is a logic one at the end of the ten cycles, the MCU begins program execution. If RESET
is a logic zero at the end of the ten cycles, the MCU once again actively drives RESET low for 512 clock
cycles. This cycle repeats until RESET is finally perceived to be at a logic one. Figure 9 shows the waveform
that is produced on the RESET line when the pull-up resistor is too large and pull-up current is inadequate.
5 VOLTS
10 OR 14 SYSTEM CLOCKS
INDETERMINATE
512 SYSTEM CLOCKS
LOGIC LEVEL
0 VOLTS
332TUT RESET LEVEL TIM
Figure 9 RESET Waveform Caused by Weak Pull-Up
If the PLL circuit is not used, and an external clock at the desired frequency of the system clock is applied
to EXTAL prior to start-up, the start up sequence is the same except that the MCU recognizes the clock
immediately instead of waiting for the PLL to lock.
MC68331/332
For More Information On This Product,
M68331/332TUT/D
13
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