st62t62b STMicroelectronics, st62t62b Datasheet - Page 46

• Manufacturer Part Number: st62t62b
• Description: 8-bit Otp/eprom Mcus With A/d Converter, Auto-reload Timer And Eeprom
• Manufacturer: STMicroelectronics

46
ST62T52B ST62T62B/E62B
A/D CONVERTER (Cont’d)
Since the ADC is on the same chip as the micro-
processor, the user should not switch heavily
loaded output signals during conversion, if high
precision is required. Such switching will affect the
supply voltages used as analog references.
The accuracy of the conversion depends on the
quality of the power supplies (V
user must take special care to ensure a well regu-
lated reference voltage is present on the V
V
less than 5V/ms). This implies, in particular, that a
suitable decoupling capacitor is used at the V
pin.
The converter resolution is given by::
The Input voltage (Ain) which is to be converted
must be constant for 1µs before conversion and
remain constant during conversion.
Conversion resolution can be improved if the pow-
er supply voltage (V
lowered.
In order to optimise conversion resolution, the
user can configure the microcontroller in WAIT
mode, because this mode minimises noise distur-
bances and power supply variations due to output
switching. Nevertheless, the WAIT instruction
should be executed as soon as possible after the
beginning of the conversion, because execution of
the WAIT instruction may cause a small variation
of the V
iation is minimized at the beginning of the conver-
sion when the converter is less sensitive, rather
than at the end of conversion, when the less sig-
nificant bits are determined.
The best configuration, from an accuracy stand-
point, is WAIT mode with the Timer stopped. In-
deed, only the ADC peripheral and the oscillator
are then still working. The MCU must be woken up
from WAIT mode by the ADC interrupt at the end
of the conversion. It should be noted that waking
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SS
pins (power supply voltage variations must be
DD
voltage. The negative effect of this var-
V
--------------------------- -
DD
DD
256
) to the microcontroller is
–
V
S S
DD
and V
SS
DD
). The
and
DD
up the microcontroller could also be done using
the Timer interrupt, but in this case the Timer will
be working and the resulting noise could affect
conversion accuracy.
A/D Converter Control Register (ADCR)
Address: 0D1h — Read/Write
Bit 7 = EAI: Enable A/D Interrupt. If this bit is set to
“1” the A/D interrupt is enabled, when EAI=0 the
interrupt is disabled.
Bit 6 = EOC: End of conversion. Read Only . This
read only bit indicates when a conversion has
been completed. This bit is automatically reset to
“0” when the STA bit is written. If the user is using
the interrupt option then this bit can be used as an
interrupt pending bit. Data in the data conversion
register are valid only when this bit is set to “1”.
Bit 5 = STA : Start of Conversion. Write Only . Writ-
ing a “1” to this bit will start a conversion on the se-
lected channel and automatically reset to “0” the
EOC bit. If the bit is set again when a conversion is
in progress, the present conversion is stopped
and a new one will take place. This bit is write on-
ly, any attempt to read it will show a logical zero.
Bit 4 = PDS : Power Down Selection. This bit acti-
vates the A/D converter if set to “1”. Writing a “0” to
this bit will put the ADC in power down mode (idle
mode).
Bit 3-0 = D3-D0. Not used
A/D Converter Data Register (ADR)
Address: 0D0h — Read only
Bit 7-0 = D7-D0 : 8 Bit A/D Conversion Result.
EAI
D7
7
7
EOC
D6
STA
D5
PDS
D4
D3
D3
D2
D2
D1
D1
D0
D0
0
0