Rev. 1.00
122
September 11, 2018
Rev. 1.00
123
September 11, 2018
HT45F4050
A/D NFC Flash MCU
HT45F4050
A/D NFC Flash MCU
Bit 3~2
SAVRS1~SAVRS0
: A/
D
converter reference voltage selection
00:
Internal A/D converter power, AV
DD
01: External
VREF
pin
1x:
Internal PGA output voltage,
V
R
These bits are used to select the A/D converter reference voltage. When the internal
A/D converter power or the internal PGA output voltage is selected as the reference
voltage, the hardware will automatically disconnect the external VREF input.
Bit 1~0
PGAGS1~PGAGS0
: PGA gain selection
00: Gain=1
01: Gain=1.66
7
–
V
R
=2V
as
V
RI
=1.2V
10: Gain=2.5
–
V
R
=3V
as
V
RI
=1.2V
11: Gain=3.333 –
V
R
=4V
as
V
RI
=1.2V
These bits are used to select the PGA gain. Note that here the gain is guaranteed only
when the PGA input voltage is equal to 1.2V.
• VBGRC Register
Bit
7
6
5
4
3
2
1
0
Name
—
—
—
—
—
—
—
VBGREN
R/W
—
—
—
—
—
—
—
R/W
POR
—
—
—
—
—
—
—
0
Bit 7~1
Unimplemented, read as "0"
Bit 0
VBGREN
: Bandgap reference voltage control
0: Disable
1: Enable
This bit is used to enable the internal Bandgap reference circuit. The internal Bandgap
reference circuit should first be enabled before the V
BGREF
voltage is selected to be
used. A specific start-up time is necessary for the Bandgap circuit to become stable and
accurate.
A/D Converter Reference Voltage
The actual reference voltage supply to the A/D Converter can be supplied from the positive power
supply pin, AVDD, an external reference source supplied on pin VREF or an
internal
reference
source
derived
f
rom
the PGA output V
R
. The desired selection is made using the SAVRS
1
~SAVRS0
bits in the SADC2 register. The internal reference voltage is amplified through a programmable gain
amplifier, PGA, which is controlled by the ADPGAEN bit in the SADC2 register.
The PGA gain can
be equal to 1,
1.667
,
2.5 or
3.333 selected using the PGAGS1~PGAGS0 bits in the SADC2 register.
The ADPGAEN bit should be set high to enable the PGA output before the PGA output voltage
V
R
is selected as the A/D converter reference voltage. The PGA input can come from the external
reference input pin, VREFI, or an internal Bandgap reference voltage, V
BGREF
, selected by the PGAIS
bit in the SADC2 register.
As the VREFI and VREF pin both are pin-shared with other functions,
when the VREFI or VREF pin is selected as the reference voltage pin, the VREFI or VREF pin-
shared function selection bits should first be properly configured to disable other pin-shared
functions. However, if the internal reference signal is selected as the reference source, the external
reference input from the VREFI or VREF pin will automatically be switched off by hardware. The
internal Bandgap reference circuit should first be enabled before the V
BGREF
is selected to be used. A
specific start-up time is necessary for the Bandgap circuit to become stable and accurate.
Note that the analog input signal values must not be allowed to exceed the value of the selected A/D
Converter reference voltage.