C511 datasheet(32/128 Pages) SIEMENS

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System Reset

Semiconductor Group

5-1

System Reset

5.1

Hardware Reset

The hardware reset function built in the SAB-C511/513 microcontrollers allows for an easy

automatic start-up at a minimum of additional hardware and forces the controller to a predefined

default state. The hardware reset function can also be used during normal operation in order to

restart the device. This is particularly done when the power-down mode is to be terminated (see

power-down description chapter 8).

The reset input is an active high input. An internal Schmitt trigger is used at the input for noise

rejection. Since the reset is synchronized internally, the RESET pin must be held high for at least

two machine cycles (12 oscillator periods) while the oscillator is running. With the oscillator running

the internal reset is executed during the second machine cycle and is repeated every cycle until

RESET goes low again.

During RESET active, the pins ALE and PSEN are configured as inputs and should not be active

driven externally. An external stimulation at these lines during reset activates several test modes

which are reserved for test purposes. This in turn may cause unpredictable output operations at

several port pins.

At the RESET pin a pulldown resistor is internally connected to

SS to allow a power-up reset

operation with an external capacitor only. An automatic reset can be obtained when power supply

is applied by connecting the reset pin via an external capacitor to

CC. After the power supply has

been turned on, the capacitor must hold the voltage level at the reset pin for a specified time to effect

a complete reset.

A correct reset leaves the processor in a defined state. The program execution starts at location

0000H. After reset is internally accomplished the port latches of ports 0, 1, 2 and 3 default in FFH.

This leaves port 0 floating, since it is an open drain port when not used as data/address bus. All

other I/O port lines (ports 1, 2 and 3) output a one (1).

The contents of the internal RAM (conventional and XRAM) of the SAB-C511/513 is not affected by

a reset. After power-up the contents is undefined, while it remains unchanged during a reset if the

power supply is not turned off.

Note:

For the SAB-C513A-H (EEPROM version) the RESET signal has to be activated for at least 10 ms

if power is applied to the device. The reason for this is that the reference voltage generator of the

EEPROM device needs some time to come up from power-down state. In the power-on behaviour

there are no differences between the EEPROM and ROM versions.

This reset behaviour of the EEPROM version has to be taken into account for systems that also will

uses ROM versions of the SAB-C511/513 family and that use its software power down features.

C511 Datasheet(PDF) 32 Page - Siemens Semiconductor Group