External Reset Pin (Reset) And Interrupt Pin (Irq) - Freescale Semiconductor MC9S08PT60 Reference Manual

Pin functions

2.2.4 External reset pin (RESET) and interrupt pin (IRQ)

A low on the RESET pin forces the MCU to an known startup state. RESET is
bidirectional, allowing a reset of the entire system. It is driven low when any internal
reset source is asserted. This pin contains an internal pullup resistor.
RESET shares an I/O pin with PTA5. The RESET pin function is enabled by default after
POR reset, because internal power-on reset and low-voltage reset circuitry typically make
external reset circuitry unnecessary. This pin is normally connected to the standard 6-pin
background debug connector so that a development system can directly reset the MCU
system. If RESET function of PTA5/IRQ/TCLK0/RESET pin is enabled, a manual
external reset can be added by supplying a simple switch to ground (pull reset pin low to
force a reset). When the RESET pin function is enabled, an internal pullup resistor is
connected to this pin and a reset signal can feed into MCU with an input hysteresis. POR
reset brings RESET pin into its default configuration, reset other than POR has no effect
on the RESET pin function configuration.
When PTA5/IRQ/TCLK0/RESET is enabled as IRQ pin, it is the input source for the
IRQ interrupt and is also the input for the BIH and BIL instructions. IRQ is asynchronous
external interrupt pins.
In EMC-sensitive applications, an external RC filter is recommended on the reset pin.
See the following figure for example.
MCU
PTA5/IRQ/TCLK0/RESET V
SS
0.1 F
4.7k 10k
V
DD
Figure 2-8. PTA5/IRQ/TCLK0/RESET external RC filter
MC9S08PT60 Reference Manual, Rev. 4, 08/2014
48 Freescale Semiconductor, Inc.