PIC18F2331 Datasheet, PDF (255/396 Pages) Microchip Technology – 28/40/44-Pin Enhanced Flash Microcontrollers with nanoWatt Technology, High Performance PWM and A/D

PIC18F2331/2431/4331/4431
20.2 A/D Result Buffer
The A/D module has a 4-level result buffer with an
address range of 0 to 3, enabled by setting the FIFOEN
bit in the ADCON1 register. This buffer is implemented
in a circular fashion where the A/D result is stored in
one location and the address is incremented. If the
address is greater than 3, the pointer is wrapped back
around to 0. The result buffer has a buffer empty flag,
BEMT, indicating when any data is in the buffer. It also
has an overflow flag, BOVFL, which indicates when a
new sample has overwritten a location that was not
previously read.
Associated with the buffer is a pointer to the address for
the next read operation. The ADPNT<1:0> bits
configure the address for the next read operation.
These bits are read-only.
The Result Buffer also has a configurable interrupt
trigger level that is configured by the ADRS<1:0> bits.
The user has three selections: interrupt flag set on
every write to the buffer, interrupt on every second write
to the buffer, or interrupt on every fourth write to the
buffer. ADPNT<1:0> is reset to ‘00’ every time a
conversion sequence is started (either by setting the
GO/DONE bit, or on a trigger).
Note:
When right justified, reading ADRESL
increments ADPNT. When left justified,
reading ADRESH increments ADPNT.
20.3 A/D Acquisition Requirements
For the A/D converter to meet its specified accuracy,
the charge holding capacitor (CHOLD) must be allowed
to fully charge to the input channel voltage level. The
analog input model is shown in Figure 20-2. The
source impedance (RS) and the internal sampling
switch (RSS) impedance directly affect the time
required to charge the capacitor CHOLD. The sampling
switch (RSS) impedance varies over the device voltage
(VDD). The source impedance affects the offset voltage
at the analog input (due to pin leakage current). The
maximum recommended impedance for analog
sources is 2.5 kΩ. After the analog input channel is
selected (changed), the channel must be sampled for
at least the minimum acquisition time before starting a
conversion.
Note:
When the conversion is started, the
holding capacitor is disconnected from the
input pin.
To calculate the minimum acquisition time,
Equation 20-1 may be used. This equation assumes
that 1/2 LSb error is used (1024 steps for the A/D). The
1/2 LSb error is the maximum error allowed for the A/D
to meet its specified resolution.
Example 20-1 shows the calculation of the minimum
required acquisition time TACQ. In this case, the
converter module is fully powered up at the outset and
therefore the amplifier settling time, TAMP, is negligible.
This calculation is based on the following application
system assumptions:
CHOLD
= 9 pF
Rs
= 100 Ω
Conversion Error ≤ 1/2 LSb
VDD
= 5V → Rss = 6 kΩ
Temperature
= 50°C (system max.)
VHOLD
= 0V @ time = 0
EQUATION 20-1: ACQUISITION TIME
TACQ
=
=
Amplifier Settling Time + Holding Capacitor Charging Time + Temperature Coefficient
TAMP + TC + TCOFF
EQUATION 20-2: MINIMUM A/D HOLDING CAPACITOR CHARGING TIME
VHOLD =
or
TC
=
(VREF – (VREF/2048)) • (1 – e^{(-Tc/CHOLD(RIC + RSS + RS))})
-(CHOLD)(RIC + RSS + RS) ln(1/2048)
 2003 Microchip Technology Inc.
Preliminary
DS39616B-page 253