X5643P datasheet(3/19 Pages) RENESAS

3 / 19 page

X5643, X5645 (Replaces X25643, X25645)

FN8135 Rev 1.00

Page 3 of 19

July 18, 2005

PRINCIPLES OF OPERATION

Power-on Reset

Application of power to the X5643/X5645 activates a

power-on reset circuit. This circuit goes active at about

1V and pulls the RESET/RESET pin active. This signal

prevents the system microprocessor from starting to oper-

ate with insufficient voltage or prior to stabilization of the

oscillator. When VCC exceeds the device VTRIP value for

200ms (nominal) the circuit releases RESET/RESET,

allowing the processor to begin executing code.

Low Voltage Monitoring

During operation, the X5643/X5645 monitors the VCC

level and asserts RESET/RESET if supply voltage falls

below a preset minimum VTRIP. The RESET/RESET sig-

nal prevents the microprocessor from operating in a

power fail or brownout condition. The RESET/RESET

signal remains active until the voltage drops below 1V. It

also remains active until VCC returns and exceeds VTRIP

for 200ms.

Watchdog Timer

The watchdog timer circuit monitors the microprocessor

activity by monitoring the WDI input. The microprocessor

must toggle the CS/WDI pin periodically to prevent a

RESET/RESET signal. The CS/WDI pin must be toggled

from HIGH to LOW prior to the expiration of the watchdog

time out period. The state of two nonvolatile control bits

in the status register determine the watchdog timer

period. The microprocessor can change these watchdog

bits, or they may be “locked” by tying the WP pin LOW

and setting the WPEN bit HIGH.

VCC Threshold Reset Procedure

The X5643/X5645 has a standard VCC threshold (VTRIP)

voltage. This value will not change over normal operat-

ing and storage conditions. However, in applications

where the standard VTRIP is not exactly right, or for

higher precision in the VTRIP value, the X5643/X5645

threshold may be adjusted.

Setting the VTRIP Voltage

This procedure sets the VTRIP to a higher voltage value.

For example, if the current VTRIP is 4.4V and the new

VTRIP is 4.6V, this procedure directly makes the change.

If the new setting is lower than the current setting, then it

is necessary to reset the trip point before setting the new

value.

To set the new VTRIP voltage, apply the desired VTRIP

threshold to the Vcc pin and tie the CS/WDI pin and the

WP pin HIGH. RESET/RESET and SO pins are left

unconnected. Then apply the programming voltage VP to

both SCK and SI and pulse CS/WDI LOW then HIGH.

Remove VP and the sequence is complete.

Figure 1. Set VTRIP Voltage

Resetting the VTRIP Voltage

This procedure sets the VTRIP to a “native” voltage level.

For example, if the current VTRIP is 4.4V and the VTRIP is

reset, the new VTRIP is something less than 1.7V. This

procedure must be used to set the voltage to a lower

value.

To reset the VTRIP voltage, apply a voltage between 2.7

and 5.5V to the VCC pin. Tie the CS/WDI pin, the WP pin,

and the SCK pin HIGH. RESET/RESET and SO pins are

left unconnected. Then apply the programming voltage VP

to the SI pin ONLY and pulse CS/WDI LOW then HIGH.

Remove VP and the sequence is complete.

Figure 2. Reset VTRIP Voltage

SCK

VCC

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