Ri
(ESICHx)
V
MSP430
C
C
SHC(ESICHx)
V
I
V
I
= Input voltage at pin ESICHx
V
S
= External source voltage
R
S
= External source resistance
Ri
(ESICHx)
= Internal MUX-on input resistance
C
SHC(ESICHx)
= Input capacitance
V
C
= Capacitance-charging voltage
R
V
S
S
ESI Operation
971
SLAU367P – October 2012 – Revised April 2020
Copyright © 2012–2020, Texas Instruments Incorporated
Extended Scan Interface (ESI)
Up to four resistor dividers can be connected to ESICHx and ESICOM. AVCC and ESICOM are the
common positive and negative potentials for all connected resistor dividers. When ESIEX(tsm) = 1,
ESICOM is connected to ESIDVSS and allows current to flow through the dividers. This charges the
capacitors of each sample-and-hold circuit to the divider voltages. All resistor divider channels are
sampled simultaneously. When ESIEX(tsm) = 0, the sample-and-hold capacitor is disconnected from the
resistor divider, and ESICOM is disconnected from ESIDVSS. After sampling, each channel can be
measured sequentially using the channel select logic, the comparator, and the DAC.
The selected ESICHx input can be modeled as an RC low-pass filter during the sampling time, t
sample
, as
shown in
. An internal MUX-on input resistance Ri
(ESICHx)
(3 k
Ω
maximum) in series with
capacitor C
SHC(ESICHx)
(9 pF maximum) is seen by the resistor-divider. The capacitor voltage VC must be
charged to within one-half LSB of the resistor divider voltage for an accurate 12-bit conversion. See the
device-specific data sheet for parameters.
Figure 37-5. Analog Input Equivalent Circuit
The resistance of the source R
S
and Ri
(ESICHx)
affect t
sample
.
can be used to calculate the
minimum sampling time t
sample
for a 12-bit conversion:
t
sample
> (R
S
+ Ri
ESICHx
) × ln(2
13
) × C
SHC(ESICHx)
(18)
Substituting the values for Ri
ESICHx
and C
SHC(ESICHx)
given above, the equation becomes:
t
sample
> (R
S
+ 3k) × 9.011 × 9 pF
(19)
For example, if R
S
is 10 k
Ω
, t
sample
must be greater than 1054 ns.
37.2.1.4 Direct Analog And Digital Inputs
By setting the ESICA1X or ESICA2X bit, external analog or digital signals can be connected directly to the
particular comparator through the ESICIx inputs. This allows measurement capabilities for optical
encoders and other sensors.
Both analog front-ends have own control bits to select either the sensor input (ESICHx) or the direct input
(ESICIx). This allows to use different input settings (selection of ESICIx or ESICHx) for AFE1 and AFE2.
37.2.1.5 Comparator Input Selection And Output Bit Selection
The ESICA1X and ESISH bits within AFE1 select between the ESICIx channels and the ESICHx channels
for the comparator input as described in
The AFE2's ESICA2X bit selects either ESICIx channels (ESICA2X = 1) or the ESICHx channels
(ESICA2X = 0) for the analog front-end AFE2.
Table 37-1. ESICAX and ESISH Input Selection
ESICA1X
ESISH
Operation
0
0
ESICHx and excitation circuitry is selected within AFE1
0
1
ESICHx and sample-and-hold circuitry is selected within AFE1
1
X
ESICIx inputs are selected within AFE1
Note that the test insertion feature is only available for AFE1. The TESTDX signal and ESITESTS1(tsm)
signal select between the ESIOUTx output bits and the ESITCHOUTx output bits for the comparator
output as described in
. TESTDX is controlled by the ESITESTD bit.