Transcript Interfacing

Interfacing
• CPU vs. I/O Bound
• Buffered vs. Unbuffered
• Synchronization
– Blind cycle
– Busy polling
– Interrupts
• vectored
– Direct Memory Access (DMA)
• I/O Modules/Ports
– isolated
– memory mapped
CMOS I/O structures
see Wakerly 3.3 - 3.7, pp 86 - 135
• Schmitt Trigger Inputs
• Tri-State Outputs
– Hi-Z(impedance) state:
effectively disconnection from
circuit.
• Open Drain Outputs
– floating output requires pull up
resistor(small) connection to
positive voltage.
I/O Port Pin
shared: Tri-State Output
D- Latch Input
•
•
Two control registers TRISx and PORTx
bsf/bcf use a read/write back cycle
– Be careful when changing TRISx mid-operation
Timing/Delay
• Count the instructions (know
the clock frequency)
delay_1ms
usec4
movlw 0xF9
; 0xF9 = 249
nop
addlw 0xFF
; add -1
btfss STATUS,Z
goto usec4
return
• Use the timers
– Watchdog
• nominal time-out period of 18 ms.
Period varies with temperature,VDD
• Scale up to 1:128 (max. 2.3 seconds)
• reset/wake-up on timeout
– Timer0/1/2
• period determined by external
oscillators or the instruction cycle
• pre/post scaling
• poll for timeout/interrupt on timeout
Timer 0
Loop
Delay
Init
bsf STATUS,RP0
movlw B'00100110'
movwf OPTION_REG
bcf STATUS,RP0
bcf INTCON,T0IF
.
.
call Delay
.
.
movlw 106
movwf TMR0
bsf STATUS,RP0
bcf OPTION_REG,T0CS
bcf STATUS,RP0
btfss INTCON,T0IF
goto Loop
bcf INTCON,T0IF
return
; clear flag
; keep checking for overflow
; switch to bank 1 to access OPTION_REG
; start timing
; time for 256 - 106 = 250 cycles
; back to bank 0
; clear to be sure
; bank 1
; set in counter mode, prescaler = 128
Polling
Interrupts
• Global Interrupt Enable Bit
• Individual interrupts
– Interrupt Enable Bit
– Interrupt Flag Bit
• Registers
– INTCON,
– PIR1, PIR2, PIE1, PIE2
status_temp,W
STATUS
w_temp,F
w_temp,W
movf
movwf
swapf
swapf
retfie
;
;
;
;
;
restore pre-isr W register contents
... without affecting the zero flag
return from interrupt
restore pre-isr STATUS register
test if TMR0 overflow occurred
call handler for TMR0
test if external interrupt occurred
call handler for external interrupt
test if PORTB change interrupt occurred
...etc
Interrupt
sample code
Timer_hndler
; clear the overflow flag
INTCON,TOIF
bcf
; do processing
return
Extern_hndlr
; clear appropriate flag etc
return
Change_hndlr
; clear appropriate flag etc
return
;
;
;
;
;
;
INTCON,TOIF
Timer_hndlr
INTCON,INTF
Extern_hndlr
INTCON,RBIF
Change_hndlr
btfsc
call
btfsc
call
btfsc
call
Poll
; save W and STATUS
;
;
w_temp
STATUS,W
status_temp
movwf
movf
movwf
ISR
Simple I/O
• LEDs
– source/sink (check current!)
• 7 segment displays
– common anode (sink)
– common cathode (source)
– multiple digits: cycle
• Switches
– software debounce
– hardware debounce (Schmitt
Trigger)
• Shared pin for I/O
– DO NOT enable the external I/O
device inputs when reading the
external outputs.
External Parallel Bus
• Bus lines
– selection/address lines
– data lines
– read, write lines
• Master
– PIC controls access to I/O
devices/modules
– any I/O port may be used
– PIC must adhere to timing
• Slave
– Another microprocessor accesses
data on the PIC
– PSP port must be used.
– Master must adhere to timing
– Interrupt on read/write
Hitachi 44780 LCD
• Synchronous Parallel bus
– read/write line; data lines
– instruction/register select
– clock/enable
• Alternative: Serial interface chips
– specific to 44780
– general serial to parallel converters
PSP
• Operation performed in Q2
• Flags set at start Q4 (except OBF which
follows RD)
• External Master holds signals for 5*Q
A/D conversion techniques
• Parallel encoding
– Compare the input voltage to a series of equally
spaced reference voltages
– Each comparator outputs a single bit of the
digital value
– FAST (all bit simultaneous) but large circuit
• Successive approximation
– Use a DAC, and binary search until the DAC
output matches the input voltage.
– Digital value is the final DAC input
– Time proportional to number of bits
• Dual slope integration
– Allow capacitor to charge for a fixed time
– Digitally time the discharge for a constant
current output
– Discharge time is proportional to input voltage
See “Art of Electronics” Horowitz pp.622-624
From: http://niuhep.physics.niu.edu/~labelec/lect/p475_lect242.pdf
•
Joystick
port
8 bit I/0 card
• ISA bus I/O address 201h
• Write: starts position measurement.
• Read: The bits are mapped in the value you get, as:
_______________________________________________________
|
7 |
6 |
5 |
4 |
3 |
2 |
1 |
0 |
| but4 | but3 | but2 | but1 | stk4 | stk3 | stk2 | stk1 |
|______|______|______|______|______|______|______|______|
• Buttons: 0=closed, 1=open (default)
• Monostables: 1=timing, 0=timed-out (default)
http://www.epanorama.net/documents/joystick/pc_joystick.html
From: http://niuhep.physics.niu.edu/~labelec/lect/p475_lect242.pdf
PIC 16F877
A/D Block diagram
8 channels; 10 bit resolution; left/right justification;
reference voltage configuration ADCON1
16F877 A/D Timing
Sampling Time
=Acquisition Time+ Conversion Time
• For correct A/D conversions, the A/D
conversion clock (TAD) must be selected to
ensure a minimum TAD time of 1.6 ms.
• Between A/D conversions or after switching
channels, the minimum sampling time must be
observed (to avoid bad readings).
ICD
tutorial
D/A conversion techniques
• Scaled resistors
– connect all bits via different resistor
values to an op-amp
– each resistor should be twice the value
of the resistor for the next most
significant bit.
• R-2R ladder
– instead of inputting scaled 0/1 voltages,
voltage inputs are switched from a
“ladder” made up of 2 resistor values
• Pulse Width Modulation
– a train of pulses is generated, whose onwidth per period is determined by the
digital value
– the average voltage value of the pulse
train is proportional to the digital value.
– A low pass filter (RC) can be used to
produce an averaged signal.
See “Art of Electronics” Horowitz pp.612-621
PWM Theory
• Duty Cycle often expressed as a
percentage of the period.
• Average output voltage will be
approximately the same percentage
of the “on” voltage.
• Typical uses:
– ideal for loads with slow response (no
capacitive filtering necessary)
– Intensity control
– Motor control
– Temperature control
PWM in PIC16F877
• Software: Timer interrupt
• Hardware: PWM module
• Calculate of period and duty cycle times:
–
–
–
–
–
–
–
instruction frequency 1MHz (4MHz clock)
instruction cycle time TCY=1ms
pre-scale ratio 1:8 (LAB2 formula)
For scale: 1:64; timer overflow cycle time 64 ms
Desired frequency 60Hz, 16.6ms, 260 ticks
Duty Cycle: Period (ticks) by %on-time
For an A/D reading 0-255 we can presume that
the duty cycle is equal to the reading for a 60Hz
PWM.
• Points to note:
– minimum/maximum frequencies:eye/ear
– frequency granularity: # steps
– signal noise -- the CCP/PWM outputs are near
to oscillator input lines!
– current switching: isolate through transistor.
Predko Ch. 6, page283
PIC16F877 PWM
Example
• 16 MHz oscillator
• Required PWM:
– Period -- 1ms
– On-time resolution -- 1ms
• Two possible solutions:
– use one of the two CCPx pins
with the built in PWM function
and Timer2.
– use any generic port pin Rxn with
software interrupts from Timer0,
1 or 2
Built-In PWM
• Find the instruction cycle time:
• FOSC =16MHhz
• FCYC =16/4=4MHz
• TCYC =0.25ms
• Period 1ms
= 1ms/0.25 ms
= 4000 instruction cycles
• Choose TMR2 (8 bit) pre-scale:
• 1:1 -- max. 256 instruction cycles
• 1:4 -- max. 1024 instruction cycles
• 1:16 -- max. 4096 instruction cycles
• PR2 Value = 4000/16 -1=249
• Resolution
– TMR2 will count in increments of
16*0.25 ms = 4 ms
– CCPR1L Value = OnTime (ms)/4
– To get 1 ms resolution we need all ten
bits e.g. 963 ms
963/4 = 240.75
11110000
CCPR1L
11
CCPCON<5:4>
Software -- PWM by force
• Find the instruction cycle time:
• FOSC =16MHhz
• FCYC =16/4=4MHz
• TCYC =0.25ms
• Period 1ms
= 1ms/0.25 ms
= 4000 instruction cycles
• Choose which timer and what prescale value:
• Timer0: 8 bits; pre-scaler
1:1/2/4/8/16/32/64/128/256
• Timer1: 16 bits; pre-scaler 1:1/2/4/8
• Timer2: 8 bits; pre-scaler 1:1/4/16
• Timers 0,2 will give a 4 ms increment; to get
1 ms resolution we will need to implement a
software fraction
• Timer 1 can give use a 0.25 ms increment;
with a pre-scaler of 4 this will automatically
be a 1 ms increment.
Sample ISR -- 21 Cycles
ORG
goto
0x04
ISR
movwf
movf
movwf
_w
STATUS,w
_status
6,7
7,8
btfsc
goto
PWM
PWM_ON
PWMOff
8
9
10
11,12
nop
bsf
movf
goto
PWM
Ontime,W
PWM_Done
1,2
3
4
5
ISR
9
10
11
12
PWMOn
bcf
movf
subwf
nop
PWM
Ontime,W
Period,W
13
14
15
16
17
18
19
PWMDone
sublw
movf
bcf
movf
movwf
swapf
swapf
0
TMR0
INTCON,T0IF
_status,W
STATUS
_w,F
_w,W
20,21
retfie
Sample ISR--21 cycles
Points to Note
• Variable PWM predefined
#define PWM PORTx,n
• no interrupt flag polling, presumes
that TMR0 is the only interrupt
• nop s are used to keep the branches
aligned
– PWM is set and clear at instruction 9 in
both branches
– Both branches hit PWM_Done at
instruction 13
• Two’s complement the desired time,
to get the starting value for TMR0
sublw 0
TMR0 - 1:16 prescaler
Period = (1ms/0.25ms - 2*Interrupt Run time)/16
OnTime = (pulse width/0.25ms - Interrupt Run time)/16
Ideal Interrupt Run Time is a multiple of 16.
Modify the routine using additional nop to meet the
requirements.
Resolution is limited only by the clock period 0.25 ms.
Define Fraction as the number of additional instruction
cycles required -- it will be used to delay the onset of the
OFF time in the ISR.
Fraction =
(pulse width/0.25 ms)/16 *16 - (pulse width/0.25 ms)
OnTime
Fraction
Period
TimerIncrement
A possible revised ISR
PWMOn 1 movf
2 sublw
3 addwf
nop
nop
nop
nop
nop
nop
nop
nop
nop
nop
nop
nop
nop
nop
nop
4 bcf
5 movf
6 addwf
7 nop
8 nop
9 nop
10 nop
11 nop
12 nop
13 nop
14 nop
15 nop
16 nop
17 nop
18 nop
19 nop
20 nop
21 nop
22 movf
23 subwf
Fraction,W
0
PCL, F
PWM
Fraction,W
PCL, F
PWMOff
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22,23
nop
nop
nop
nop
bsf
nop
nop
nop
nop
nop
nop
nop
nop
nop
nop
nop
nop
nop
nop
nop
nop
movf
goto
PWM
OnTime,W
PWMDone
ISR call cycles
Pre-branch cycles
Branch cycles
Post-branch cycles
OnTime,W
Period,W
Nearest 16 multiple
nops required
before retfie
2
8
23
9
42
48
6
A possible revised ISR
Points to note
• Fraction resolution of 0.25 ms
exceeds specification of 1 ms
• OnTime has a lower limit of
48/16=3 Timer0 increments =12 ms
• Period=(4000-2*48)/16=244
• The revised ISR will occupy
significantly more code-space, due
to the use of all these nop s
Serial Communication
• Error Detection Overhead
– Single Dataword
• Parity: Even, Odd, Mark, Space,None
• Start/Stop bits
– Packet/Frame
• ByteCount, Checksum, CRC
• Packet ID, Acknowledge
• SOF/EOF; Header/Trailer
• Single / Multi-user
– Dedicated
• (optional) Handshaking/Signalling
– Network
• Receiver/Sender ID
• Arbitration/Collision detection
Serial Communication
Protocols
• Synchronous
– Microwire
– SPI
– I2C
• Asynchronous
–
–
–
–
–
RS232
RS485/422
Manchester
CAN
Dallas Semiconductor 1-wire interface
• All can be implemented with I/O Ports.
• PIC16F877 has built in support for some.
– MSSP
– USART
Dallas Semiconductor 1-wire interface
MPLAB-SIM
Instruction Level Simulator
Limitations
• No A/D result loaded
• PWM resolution > instruction cycle
• Additional clock pulse inputs >
instruction cycle
• No support for serial I/O simulation:
one alternative is pin stimulus and
stopwatch.
• No compares > 8 bits (CCP)