ST92141 datasheet(75/179 Pages) STMICROELECTRONICS

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ST92141 - RESET AND CLOCK CONTROL UNIT (RCCU)

RESET/STOP MANAGER (Cont’d)

The on-chip Timer/Watchdog generates a reset

condition if the Watchdog mode is enabled

(WCR.WDEN cleared, R252 page 0), and if the

programmed period elapses without the specific

code (AAh, 55h) written to the appropriate register.

The input pin RESET is not driven low by the on-

chip reset generated by the Timer/Watchdog.

When the Reset pin goes high again, a delay occurs

before exiting the Reset state. Subsequently a short

Boot routine is executed from the device internal

Boot ROM, and control then passes to the user pro-

gram.

The Boot routine sets the device characteristics

and loads the correct values in the Memory Man-

agement Unit’s pointer registers, so that these

point to the physical memory areas as mapped in

the specific device. The precise duration of this

short Boot routine varies from device to device,

depending on the Boot ROM contents.

At the end of the Boot routine the Program Coun-

ter will be set to the location specified in the Reset

Vector located in the lowest two bytes of memory.

5.6.1 Reset Pin Timing

To improve the noise immunity of the device, the

Reset pin has a Schmitt trigger input circuit with

hysteresis. In addition, a filter will prevent an un-

wanted reset in case of a single glitch of less than

50 ns on the Reset pin. The device is certain to re-

set if a negative pulse of more than 20µs is ap-

plied. When the reset pin goes high again, a delay

of up to 4µs will elapse before the RCCU detects

this rising front. From this event on, 20478 (about

5 ms with a 4MHz quartz) oscillator clock cycles

(CLOCK1) are counted before exiting the Reset

state (+-1 CLOCK1 period depending on the delay

between the positive edge the RCCU detects and

the first rising edge of CLOCK1)

If the ST9 is a ROMLESS version, without on-chip

program memory, the memory interface ports are

set to external memory mode (i.e Alternate Func-

tion) and the memory accesses are made to exter-

nal Program memory with wait cycles insertion.

Figure 38. Recommended Signal to be Applied

on Reset Pin

VRESET

VCC

0.7 VCC

0.3 VCC

20 µs

Minimum

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