ST92F124 datasheet(424/429 Pages) STMICROELECTRONICS

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ST92F124/F150/F250 - KNOWN LIMITATIONS

KNOWN LIMITATIONS (Cont’d)

During reset, the risk of power consumption in the

input stage due to floating inputs is avoided by a

design feature.

However, if the application requires pull-ups

during reset (for instance, in order to send known

logic values to external devices), external pull-ups

must be provided. When the I/O port outputs a

zero, there will be some additional power con-

sumption as these external pull-ups are not

switched off.

These ports behave in the same way following an

external, watchdog or software reset.

13.8.2 High Drive I/Os when BSZ=1

Description

If the BSZ bit in the EMR1 register (bit 1 of R245,

page 21) is set so as to use high-drive output

buffers for P4[7:6] and P6[5:4], all I/O ports as well

as AS, DS and RW will also use high-drive output

buffers.

Impact On Application

P0[7:0], AS, DS and RW have the same VOH pa-

rameter value as P6[5:4].

P0[7:0]-P2[3:2], AS, DS and RW have the same

VOL and IIO parameter values as the P4[7:6] and

P6[5:4].

These I/Os using high-drive output buffers will

g ene rate m or e noise tha n tho se using th e

standard low-noise output buffers.

13.8.3 ADC PARASITIC DIODE

Description

A parasitic diode is present between an ADC input

and AVDD.

As described in the datasheet, the user has the

possibility to switch off AVDD when he switches off

the ADC to save power consumption. However, if

AVDD is connected to ground and a voltage is

present on the Input Port, an increase in power

consumption can occur.

The Input Port affected by this diode is the one

pointed to by the analog multiplexer of the ADC, if

the port is set up as AF analog input. When the

ADC is stopped, the multiplexer points to the first

input to be converted in a scan (i.e. the channel

pointed to by the SC[3:0] bits).

Workaround

In order to avoid this problem, the I/O connected to

the ADC has to be set up in any mode except AF

analog input (i.e. any combination of PxC2.. PxC0

except 111).

1. Deprogram analog input mode from the I/O port

which is pointed to by the SC[3:0] bits (start

conversion channel, b7..b4 of CLR1).

For example the I/O can be reprogrammed as

an open drain output, with the data at 1. The

high impedance of the output stage then

avoids a conflict with the external voltage

source. In order to avoid potential power con-

sumption in the input buffer of this I/O,

depending on the external voltage applied to

the pin, it is wise to set the 'start conversion

channel' to a channel which carries levels

below 800 mV or above (VDD -800 mV).

Another possibility is to modify the SC[3:0]

bits so that they point to an I/O Port which is

not used as an analog input.

2. Next, switch off the A/D Converter.

The current in AVDD will be zero, whatever the

logic levels on the analog inputs, and whatever the

voltage level applied to AVDD (between 0 and

VDD).

13.8.4

ADC

ACCURACY

VS.

NEGATIVE

INJECTION CURRENT

Description

If a negative current is injected to an input pin (i.e.

input signal voltage below -0.3V), a part of this cur-

rent will be drawn from the adjacent I/Os. The fol-

lowing curve quantifies this current:

ST92F124 Datasheet(PDF) 424 Page - STMicroelectronics

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