Notes on ADC and PWM (PIC16F Family)

35 Pages·2010·2.56 MB·English

by  Khan Wahid

EE331 Microprocessor
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KHAN WAHID
2009-10 (Term 1)
EE 331: Microprocessor 2
16F886 Architecture
Exactly same architecture
(cid:134)
as 16F84A
Same 35 instructions
(cid:132)
All single cycle except
(cid:132)
program branches (2-
cycles)
Brown-out Reset
(cid:134)
In-Circuit Debugger
(cid:134)
Low-volt Programming
(cid:134)
Dr. KHAN WAHID
2009-10 (Term 1)
EE 331: Microprocessor 3
16F886 Status Register
The data memory is partitioned into
(cid:134)
four banks which contain the GPR and
the SFR
Each bank extends up to 7Fh (128
(cid:134)
bytes)
The lower locations of each bank are
(cid:132)
reserved for the SFR
The GPR are implemented as static
(cid:132)
RAM
Dr. KHAN WAHID
2009-10 (Term 1)
EE 331: Microprocessor 4
Configuring PORTA
When ports are multiplexed,
(cid:134)
take special caution to
configure
ADCON1
- On reset, all PORTA pins are analog
and read as ‘0’
- To use them as digital, we must
configure ADCON1
Dr. KHAN WAHID
2009-10 (Term 1)
EE 331: Microprocessor 5
Configuring PORTA – for 16F886
However, it may not be always
(cid:134)
trivial from the datasheet
- However, there are still problems with
RA6 and RA7
- Since they are multiplexed with OSC1 and
OSC2 respectively, we must configure the
“Configuration Bits”
- Here, we need to configure ANSEL
“Setting the appropriate ANSEL bit high will cause all
digital reads on the pin to be read as ‘0’ and allow analog
functions on the pin to operate correctly. The state of the
ANSEL bits has no affect on digital output functions. A
pin with TRIS clear and ANSEL set will still operate as a
digital output, but the Input mode will be analog. This can
Dr. KHAN WAHID cause unexpected behavior when executing read-modify-
2009-10 (Term 1)
write instructions (e.g., bsf, bcf) on the affected port.
EE 331: Microprocessor 6
Configuring PORTA/PORTB – for 16F886
Port B is also multiplexed
(cid:134)
Need to be cautious
(cid:132)
Demo in MPLAB
- Here, we need to configure
ANSELH
Dr. KHAN WAHID
2009-10 (Term 1)
EE 331: Microprocessor 7
Analog-to-Digital Converter
The ADC converts an input voltage (that is infinitely
(cid:134)
variable) to one of a fixed number of output (discrete) values
For an n-bit ADC, the maximum output value will be (2n − 1)
(cid:134)
For an 8-bit ADC, the final value will be (28 − 1), or d’255’
(cid:132)
The more the number of output
(cid:134)
bits, the more will be the number of
output steps and the finer is the
conversion
A measure of the fineness of
(cid:132)
conversion is called the resolution
It has a resolution of V /2n
(cid:132)
max
Dr. KHAN WAHID
2009-10 (Term 1)
EE 331: Microprocessor 8
Sample and Hold
Since, an ADC cannot convert accurately a changing voltage,
(cid:134)
a sample and hold (S&H) circuit is often found
This takes a sample of the voltage, like a snapshot, and holds it steady
(cid:132)
for the duration of the conversion
(cid:132) Composed of a semiconductor switch and a capacitor
Sample: When the switch is closed,
(cid:134)
the capacitor charges up to the input
voltage Vs
Vo = Vc = Vs
(cid:132)
Hold: When the switch opens, the
(cid:134)
charge is left on the capacitor and Vc
(and hence Vo) remains at a fixed
Vc = Vs {1 − exp(−t/RC)}
value
Dr. KHAN WAHID
2009-10 (Term 1)
EE 331: Microprocessor 9
Sample and Hold
Vc = Vs {1 − exp(−t/RC)}
The time that Vc (and hence Vo) takes to reach a value
(cid:134)
deemed to be acceptable is called the acquisition time
Suppose that Vc must rise to 90% of its final value (Vs)
(cid:132)
To ensure good accuracy in data conversion, the error
introduced by this process should be less than the
equivalent of half of 1 LSB
For an 8-bit conversion, this implies that the acquired
voltage value Vc must reach ≥ (511/512)Vs, or
0.9980Vs
For a 10-bit conversion it must be ≥ (2047/2048)Vs, or
0.9995Vs
Dr. KHAN WAHID
2009-10 (Term 1)
EE 331: Microprocessor 10
16F886 ADC
Powerful 10-bit 8-channel ADC
(cid:134)
module
The ADC is controlled by: ADCON0
(cid:134)
and ADCON1
The result of the conversion is placed
(cid:134)
into: ADRESH and ADRESL
Other SFRs also have an important
(cid:134)
impact on the ADC:
TRISA and TRISE (used for analog
(cid:132)
input)
PIR1 and PIE1 contain the ADC
(cid:132)
INT flag and INT enable bits
A set of procedure must be followed to properly
complete one conversion
Dr. KHAN WAHID
2009-10 (Term 1)