Datasheet

19
7764A–8051–11/07
3. Troubleshooting
3.1 MCU not starting or operating in a reliable way
Possible root
cause I/O External Code Internal Code
POWERS and
GND delivered
to the device
Input
- Must be stables
- Monitor power supplies
- Check if GND is properly supplied to device
XTA2 pin Output - Monitor XTAL2 to see if the oscillator has started and is working properly. Check if your crystal is not an overtone part.
RST pin I/O
- If RST pin is not correctly driven, MCU is not properly reset.
- If the device does not have an internal POR, read application note: “how to calculate the capacitor of the reset input of
a C51 microcontroller” (doc 4284)”
- If an external Brown-out device is used, an intermediate resistor or diode must be implemented between RST pin and
the Brown-out device
EA pin Input - Must be connected to GND
- Must be connected to VCC, directly or through a 10K
resistor.
- MCU may not work properly, if this pin is left floatting
ALE pin
Output
signal
- This signal is used to clock the Least
Significant Byte into the address latch
- the frequency of this signal enables to guess the
frequency that the core runs at.
(See “Relationship between oscillator and ALE
frequencies” on page 11.)
- Not used.
- Must be left unconnected in normal operation
- If ALE is not disabled by firmware, the frequency of this
signal enables to guess the frequency that the core runs at.
See “Relationship between oscillator and ALE frequencies”
on page 11.)
PSEN pin
Output
signal
- This signal is used to strobe the external program
memory when the MCU fetches the code byte.
- Monitor ALE and PSEN pins to get the status of the
MCU. ( See “Microcontroller status” on page 13.)
- Not used.
- Must be left unconnected in normal operation
- If ALE is not disabled by firmware, monitor ALE and PSEN
pins to get the status of the MCU. ( See “Microcontroller
status” on page 13.)
P0
I/O
port
Port 0 serves as a multiplexed address/data bus. It
emits the low byte of the Program Counter (PCL) as an
address, and then goes into a float state awaiting the
arrival of the code byte from the Program Memory
Can be used as a general purpose I/O port.
In this case external pull-up’s must be provided.
P2
I/O
port
Port 2 emits the high byte of the Program Counter
(PCH)
Can be used as a general purpose I/O port.
No external pull-up’s are needed as this port owns internal
pull-up’s
HSB set to maximum
security
Does not work No impact
Device is damaged Try another device