ST6200 STMICROELECTRONICS [STMicroelectronics], ST6200 Datasheet - Page 50

• Manufacturer Part Number: ST6200
• Description: 8-BIT MCUs WITH A/D CONVERTER, TWO TIMERS, OSCILLATOR SAFEGUARD & SAFE RESET
• Manufacturer: STMICROELECTRONICS [STMicroelectronics]

ST6200C/ST6201C/ST6203C
A/D CONVERTER (Cont’d)
8.3.4 Recommendations
The following six notes provide additional informa-
tion on using the A/D converter.
1.The A/D converter does not feature a sample
and hold circuit. The analog voltage to be meas-
ured should therefore be stable during the entire
conversion cycle. Voltage variation should not ex-
ceed ±1/2 LSB for optimum conversion accuracy.
A low pass filter may be used at the analog input
pins to reduce input voltage variation during con-
version.
2. When selected as an analog channel, the input
pin is internally connected to a capacitor C
typically 9pF. For maximum accuracy, this capaci-
tor must be fully charged at the beginning of con-
version. In the worst case, conversion starts one
instruction (6.5 µs) after the channel has been se-
lected. The impedance of the analog voltage
source (ASI) in worst case conditions, is calculat-
ed using the following formula:
(capacitor charged to over 99.9%), i.e. 30 k
cluding a 50% guardband.
The ASI can be higher if C
a longer period by adding instructions before the
start of conversion (adding more than 26 CPU cy-
cles is pointless).
3. Since the ADC is on the same chip as the micro-
processor, the user should not switch heavily load-
ed output signals during conversion, if high preci-
sion is required. Such switching will affect the sup-
ply voltages used as analog references.
4. Conversion accuracy depends on the quality of
the power supplies (V
take special care to ensure a well regulated refer-
ence voltage is present on the V
(power supply voltage variations must be less than
0.1V/ms). This implies, in particular, that a suitable
decoupling capacitor is used at the V
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.
5. Conversion resolution can be improved if the
power supply voltage (V
is lowered.
6. In order to optimize the conversion resolution,
the user can configure the microcontroller in WAIT
mode, because this mode minimises noise distur-
50/100
1
6.5µs = 9 x C
V DD V SS
------------------------------- -
DD
256
ad
DD
–
and V
ad
x ASI
) to the microcontroller
has been charged for
SS
DD
). The user must
and V
DD
pin.
SS
ad
pins
in-
of
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 least sig-
nificant bits are determined.
The best configuration, from an accuracy stand-
point, is WAIT mode with the Timer stopped. In
this case only the ADC peripheral and the oscilla-
tor are then still working. The MCU must be woken
up from WAIT mode by the ADC interrupt at the
end of the conversion. The microcontroller can
also be woken up by the Timer interrupt, but this
means the Timer must be running and the result-
ing noise could affect conversion accuracy.
Caution: When an I/O pin is used as an analog in-
put, A/D conversion accuracy will be impaired if
negative current injections (V
adjacent I/O pins with analog input capability. Re-
fer to
– Use another I/O port located further away from
– Increase the input resistance R
Figure 29. Leakage from Digital Inputs
the analog pin, preferably not multiplexed on the
A/D converter
current injections) and reduce R
conversion accuracy).
V
V
AIN
Figure
INJ
DD
voltage. The negative effect of this var-
R
29. To avoid this:
Analog
Input
R
INJ
Digital
Input
ADC
PBy/AINy
PBx/AINx
Leakage Current
if V
INJ
INJ
< V
IN J
ADC
< V
SS
(to reduce the
SS
A/D
Converter
(to preserve
I/O Port
(Digital I/O)
) occur from