P90CL301 datasheet(13/92 Pages) PHILIPS

13 / 92 page

1996 Dec 11

Philips Semiconductors

Preliminary speciﬁcation

Low voltage 16-bit microcontroller

P90CL301BFH (C100)

Table 15 Selection of prescaler divisor values

PCLK3

PCLK1

PCLK0

DIVISOR (D)

DIVISOR FOR UART IN MODE 0

000

001

010

5 (default value after a CPU reset)

101

110

6.5

Reset operation

The reset circuitry of the P90CL301BFH is connected to

the pins RESET, HALT, RESETIN and to the internal

Watchdog Timer. A Schmitt trigger is used at the input pin

for noise rejection. After Power-on a CPU reset is

accomplished by holding the RESET pin and the HALT pin

LOW for at least 50 oscillator clocks after the oscillator has

stabilized.

For further information on the clock generation, see

Section 6.6. The CPU responds by reading the reset

vectors; the long word at address 000000H is loaded into

the Supervisor stack and the long word data at address

000004H is loaded into the program counter PC. The

interrupt level is set to 7 in the Status Register and

execution starts at the PC location. By pulling the RESET

pin LOW and keeping HALT HIGH, only the peripherals

are reset.

When VDD is turned on and its rise time does not exceed

10 ms, an automatic reset can be performed by

connecting the RESETIN pin to VDD via an external

capacitor. The external capacitor is charged via an internal

pull-down resistor.

The RESET pin can also be pulled LOW internally by a

pull-down transistor activated by an overflow of the

Watchdog Timer. When the CPU executes a RESET

instruction, the RESET pin is pulled LOW. When the CPU

is internally halted (at double bus fault), the HALT pin is

pulled LOW and only a CPU reset can restart the

processor.

The internal signal RESET_AS (Reset Asynchronous)

resets the core and all registers.

When an internal Watchdog Timer overflow occurs, an

internal CPU reset is generated which resets all registers

except the SYSCON, PCON, PRL and PRH registers and

pulls the RESET pin LOW during 12 clock cycles.

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