MC68HC908GR4MP datasheet(134/286 Pages) FREESCALE

134 / 286 page

Monitor ROM (MON)

MC68HC908GR8 • MC68HC908GR4 Data Sheet, Rev. 7

134

Freescale Semiconductor

Figure 15-2 shows a simplified diagram of the monitor mode entry when the reset vector is blank and just

1 x VDD voltage is applied to the IRQ pin. An external oscillator of 9.8304 MHz is required for a baud rate

of 9600, as the internal bus frequency is automatically set to the external frequency divided by four.

Enter monitor mode with pin configuration shown in Figure 15-1 by pulling RST low and then high. The

rising edge of RST latches monitor mode. Once monitor mode is latched, the values on the specified pins

can change.

Once out of reset, the MCU waits for the host to send eight security bytes. (See Security.) After the

security bytes, the MCU sends a break signal (10 consecutive logic 0s) to the host, indicating that it is

ready to receive a command.

NOTE

The PTA1 pin must remain at logic 0 for 24 bus cycles after the RST pin

goes high to enter monitor mode properly.

Table 15-1. Monitor Mode Signal Requirements and Options

IRQ

RESET

$FFFE/

$FFFF

PLL

PTB0 PTB1

External

Clock(1)

1. External clock is derived by a 32.768 kHz crystal or a 9.8304 MHz off-chip oscillator

CGMOUT

Bus

Freq

COP

For Serial

Communication

Comment

PTA0 PTA1

Baud

Rate(2) (3)

2. PTA0 = 1 if serial communication; PTA0 = X if parallel communication

3. PTA1 = 0

→ serial, PTA1 = 1 → parallel communication for security code entry

4. DNA = does not apply, X = don’t care

XGND

Disabled

No operation

until reset goes

high

VTST

VDD

VTST

XOFF

9.8304

MHz

4.9152

MHz

2.4576

MHz

Disabled

9600

PTB0 and PTB1

voltages only

required if

IRQ =VTST

DNA

VDD

$FFFF

OFF

9.8304

MHz

4.9152

MHz

2.4576

MHz

Disabled

9600

External

frequency always

divided by 4

DNA

GND

VDD

$FFFF

32.768

kHz

4.9152

MHz

2.4576

MHz

Disabled

9600

PLL enabled

(BCS set) in

monitor code

DNA

VDD

GND

VTST

$FFFF

OFF

—

Enabled

—

Enters user

mode — will

encounter an

illegal address

reset

VDD

GND

VDD

VTST

Not

$FFFF

OFF

—

Enabled

—

Enters

user mode

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