Datasheet

7764A–8051–11/07

1.1.2 Type of package

In most of the products, two kind of packages are proposed: the HPC (High Pin Count) and the

LPC (Low Pin Count).

The HPC packages support the external code and therefore provide the appropriate pins (EA,

ALE, PSEN, P0, P2), while the LPC packages do not support the external code and do not pro-

vide any of the dedicated pins.

1.1.3 Hardware Security Byte (HSB)

1.1.4 Reduced EMI Mode

The ALE signal is used to demultiplex address and data buses on port 0 when used with exter-

nal program or data memory. Nevertheless, during internal code execution, ALE signal is still

generated. In order to reduce EMI, ALE signal can be disabled by setting AO bit.

The AO bit is located in AUXR register at bit location 0. As soon as AO is set, ALE is no longer

output but remains active during MOVX and MOVC instructions and external fetches. During

ALE disabling, ALE pin is weakly pulled high.

1.2 Hardware reset

1.2.1 Internal features

Some C51 devices may implement the following features:

• Power On Reset (POR)

• Power Fail Detector (PFD)

• Hardware Watch-Dog Timer (WDT)

• Internal Pull-up on reset pin

There are different ways to drive the reset pin depending on which features are implemented in

the device. The user is invited to carefully read the product datasheet to know which features are

or are not implemented.

WARNING

Parts are delivered by default with HSB set to maximum security (see product datasheet for

further information). This configuration prevents the MCU from fetching external code.

On ROM products the user will need to take care about the HSB configuration at the time he

orders the mask ROM if the external code mode is needed on his application.

On FLASH products the user will need to perform a full chip erase by means of an external

programmer before using the parts if the external code mode is needed on his application.