L03 - AVR General Purpose I/Os, Pull-Up Resistors, Programming IOs

ATMega169P Chip

I/O Pin Buffer Hardware

Input Buffer
- used to read external signals, "input"
- reduces input load seen by external drivers
  - allows for weaker drivers
- perform signal amplification, to ensure that internal signal is Valid High or Valid Low
- ensure timing properties such as signal transition time (edge sharpening)
Output Buffer
- used to transmit internal signals externally signals, "output"
- drive large external loads, isolating internal circuitry
Tri-State Output Buffer
- like output buffer, but can be deactivated dynamically
- used in a bidirectional port alongside a input buffer
  - enabled for output mode, disabled for input mode
  - dynamic activation allows for run-time toggling of input vs output mode
  - can be used for external, tristate buses
    - external bus should only have one IC driving at a time
Pull-Up with Enable
- Used to weakly drive pins high, easily override by other activated drivers
- Commonly used for inputs from pull-down-only devices
- supports external tri-stable busses, preventing the need for a board-level pull-up resister

General Purpose I/O Port Registers

All AVR Ports have true Read-Modify-Write functionality
- Each pin on a port can be independently modified, without modifying any other pin
Three I/O memory address locations allocated for each port
- Data Register – PORTx (Read/Write)
- Data Direction Register – DDRx (Read/Write)
- Port Input Pins – PINx (Read)

General Purpose Digital I/O

Circuit Signals
- PUD: PULLUP DISABLE
- SLEEP: SLEEP CONTROL
- $\rm clk_{i/o}$ : I/O CLOCK
- Bus Requests
  - WDx: WRITE DDRx
  - RDx: READ DDRx
  - WRx: WRITE PORTx REGISTER
  - RRx: READ PORTx REGISTER
  - WPx: WRITE PINx REGISTER
ATMega 169P has 7 ports
- A,B,C,D,E,F,G
Pxn represents nth bit in Port x
- E.g. PA6 == 6th bit of Port A
If DDxn is a:
- 1 – Pxn is configured to be an output pin
- 0 – Pxn is configured to be an input pin
If DDxn is configured as output and PORTxn is:
- 1 – Pxn is driven high (1)
- 0 – Pxn is driven low (0)
If PORTxn is written logic one (high) when the pin is configured as an input, the pull-up resistor is activated.
Special Feature note: writing a logic one to a bit in the PINx Register, will result in a toggle in the corresponding bit in the Data Register.

Exerpt from "m169Pdef.inc"

.equ    PINB = 0x03  
.equ    DDRB  = 0x04  
.equ    PORTB = 0x05

Pull-Up Resistor for Reading Pull-Down Switch

With a pull-up resistor, the input pin reads high state when the grounding switch is open, low when switch is closed
AVR has internal pull-up
- No need to add resister at board level
- Can also be used to toggle the pin’s output with a weak drive

Tristate

Toggling the output driver can support tristate bus communication
Enable the output to take control of the bus signal for writing

Bit masking for Output

Masking is important for Controlling Port I/O
- Allows control of subset of pins without affecting others
Ex using OR as a masking bring-up of only 2 output pins
- ```
in  r16, PORTA
ori r16, 0b00000110
out  PORTA, r16
```
- Only makes the bit 2 and 3 high, leaves others unaffected
Ex using AND as a masking bring-down of only 2 output pins
- ```
in  r16, PORTA
andi r16, 0b11111001
out  PORTA, r16
```
- Only makes the bit 2 and 3 low, leaves others unaffected

Bit masking for Input Checking

Ex using AND to check if both of two 2 input pins are low
- ```
in  r16, PINA
andi r16, 0b00000110
breq botharelow
```

Ex using AND to check if both of two 2 input pins are high

in  r16, PINA
andi r16, 0b00000110
cpi r16,  0b00000110
breq botharehigh

Rapid Single-Cycle Single-bit Output Manipulation

Single-cycle manipulation of individual output port pins is a specialty of AVR

Ex: Thermo up/down

sbi PORTA, 0   ; .......*   
sbi PORTA, 1   ; ......** 
sbi PORTA, 2   ; .....*** 
sbi PORTA, 3   ; ....**** 
sbi PORTA, 4   ; ...***** 
sbi PORTA, 5   ; ..****** 
sbi PORTA, 6   ; .******* 
sbi PORTA, 7   ; ******** 
cbi PORTA, 7   ; .******* 
cbi PORTA, 6   ; ..****** 
cbi PORTA, 5   ; ...***** 
cbi PORTA, 4   ; ....**** 
cbi PORTA, 3   ; .....*** 
cbi PORTA, 2   ; ......** 
cbi PORTA, 1   ; .......* 
cbi PORTA, 0   ; ........

Single-Cycle Single-Bit Input

Preliminaries: Jumping

Conditional Jumps
- SBIC – Skip if bit in I/O register cleared
- SBIS – Skip if bit in I/O register set
- SBRC – Skip if bit in register cleared
- SBRS – Skip if bit in register set
Unconditional Jumps
- RJMP k:
  - Program execution continues at PC+k+1
  - 2 clock cycles, smaller address range
- JMP k:
  - Program execution continues at k
  - 3clock cycles, full address range

Combining skip and jump

.def=JOYSTICK_INPORT = PINB
.equ UP_BUTTON_BIT = 5;
.equ UP_BUTTON_MASK=(1<<UP_BUTTON_BIT);
.equ UP_BUTTON_MASK_CMP=(0xFF-UP_BUTTON_MASK);
.def RTEMP = r16;

;skip if bit in registger set followed by branch
;  branch occurs if button was pressed
sbis JOYSTICK_INPORT, UP_BUTTON_BIT
rjump LABEL_BUTTON_RESPONSE

longer/slower alternative

in RETEMP, JOYSTICK_INPORT
andi RTEMP,UP_BUTTON_MASK
brne LABEL_BUTTON_RESPONSE

Programming I/O Ports - Assembly

Individual Ouput Bit Manipulation

;Using CBI and SBI to write to ports
SBI DDRB, 1 ;make bit 1 as output bit on PORTB
CBI PORTB, 1 ;make PORTB bit 1 as "0"
SBI PORTB, 1 ;make PORTB bit 1 as "1"

Slow Alternative:

;Using OUT instruction to write to ports
LDI R18, 0b00010000
OUT DDRB, R18 ;make bit 1 as output bit on PORTB
LDI R18, 0b00000000
OUT PORTB, R18 ;make PORTB bit 1 as "0"
LDI R18, 0b00010000
OUT PORTB, R18 ;make PORTB bit 1 as "1“

Novice Note: A common error is that OUT DDRB, R18 doesn't just set bit 1 to a "1", it also sets all of the other bits to 0, so please favor the following methods to set bits

Input

;simple input
IN R18,PINB

Output with Masking

;set pin 5 of B port as output
; without affecting other bits
IN R18,DDRB
ORI R18, 0b00010000
OUT DDRB, R18

;set pin 5 of B port to 1
; without affecting other bits
IN R18,PORTB
ORI R18, 0b00100000
OUT PORTB, R18

;clear pin 5 of B port to 0
; without affecting other bits
IN R18,PORTB
ANDI R18, 0b11101111
OUT PORTB, R18

Changing Multiple Bits Simultaneously

sbi and cbi are more convenient and allowed if only changing one bit at a time. If multiple bits need to be set at the same you can't use sbi/cbi

;set pin 7,3 of B port to 1 at same time
; without affecting other bits
IN R18,PORTB
ORI R18, 0b10001000
OUT DDRB, R18

Toogling

Ex Cycle output pin 1 quickly:

sbi PORTB, 1
cbi PORTB, 1
sbi PORTB, 1

;toggle pin 1 of B using PINB "write-to-input trick"
OUT PINB, 0b00000010

Slow Alternative

;toggle pin 1 of B (no eori available)
; without affecting other bits
IN R18,PORTB
LDI R19,0b00000010
EOR R18, R19
OUT PORTB, R18

General Purpose I/O Ports on Butterfly Board

Butterfly Board Joystick

The common line of all directions is GND on the board. This means internal pull-up must be enabled on ATmega169P to detect the input.

Pins
- 1 = UP
- 2 = Center
- 3 = Left
- 4 = Down
- 5 = GND
- 6 = Right

Part:SKRHABE010

Schematic Drawing:

Mechanical Drawing:

Landing Drawing (Bottom View into mounting face):

Might also find a recommended solder pad and solder-mask drawing

https://tech.alpsalpine.com/prod/e/html/multicontrol/switch/skrh/skrhabe010.html

Board Schematic:

https://www.csee.umbc.edu/~alnel1/cmpe311/assignments/UserGuide.pdf

Example Code for Push Button

Can mention in-class and step through in discussion session
Can draw state machine with reference to numbers/labels from code

Code Was Developed Some Time ago on another plaform, may need syntax and other minor updates

/* test . asm
Created : 9/4/2012 10:50:44 AM
Author : rrobucci
*/
// above is the file header comment created by the tool
// . INCLUDE " m169Pdef.inc " 
// optional , file was included by
// default when device was selected on project creation
// normally here we would define alias for ports , pins , and
// registers used instead of using magic numbers in the code
// LED on PB7 , button on PB6
// LED is " ON " when PB7 is high
// PB6 is low when button is pressed
// Will use R16 as temporary register
.ORG 0x00000
  // * may press F1 with cursor over any instruction to see help
  // * use F10 to step through code , note IO values that change
  // * are highlighted in red
  CBI DDRB , 6 // set direction for button
  SBI PORTB , 6 // enable pull - up
  // * in simulator , use the IOVIEW here and set PINB6 to
  // * high to emulate button not being pressed
  SBI DDRB , 7 // set direction for LED

// * in the simulator this loop should repeat until PINB6 is   
// set low using the IOVIEW                                      ;                     
CHECK_BUTTON :                                                   ; <<<<\ 
  // skips jump back if bit 6 of port B input is set             ;     ^               
  SBIC PINB , 6                                                  ;     ^               
  JMP CHECK_BUTTON                                               ; >>>>/                    
                                                                 ;             
LED_BLINK :                                                      ;  <<<<<<<<<<<<<<\
    // Turn LED ON                                               ;                ^
    SBI PORTB , 7 // sets bit of I / O                           ;                ^
                                                                 ;                ^
    // Pause LOOP                                                ;                ^
    // step through a few iterations using F10 and observe       ;                ^
    // R16 changing in the processor tab ( highlighted with red  ;                ^
    // upon change )                                             ;                ^
    LDI R16 , 255 // load immediate                              ;                ^
                                                                 ;  <<<<\         ^
  BLINK_LOOP_ON :                                                ;      ^         ^
    DEC R16 // decrement R16 , set zero flag if zero reached     ;      ^         ^
    BRNE BLINK_LOOP_ON // * set breakpoint here using F9         ;  >>>>/         ^
    // * and press F5 to avoid stepping through all loop         ;                ^
    // iterations                                                ;                ^
    // Turn LED OFF                                              ;                ^
  CBI PORTB , 7 // clears bit of I / O                           ;                ^
                                                                 ;                ^
  LDI R16 ,127 // Pause                                          ;                ^
  BLINK_LOOP_OFF :                                               ;  <<<<\         ^
    DEC R16                                                      ;      ^         ^
    BRNE BLINK_LOOP_OFF                                          ;  >>>>/         ^
JMP LED_BLINK // * using F9 , set breakpoints on the SBI and     ;  >>>>>>>>>>>>>>/
  // CBI code lines above and repeatedly                       
  // press F5 to see the LED pin toggle

Discussion 1 State Machine

Possible Organization of FSM Code

[Part 1] Capture input and/or wait for trigger (such as user input or time elapsed)
[Part 2] Preprocess, update common or extended state variables
[Part 3] State Decision
case S0: decode actions
decide state & extended state variables update
case S1: · · ·
[Part 4] Action (such as output) . . .
Go to [Part 1]

Additional Concepts:

substates
extended state variables
Triggers (when to perform FSM iterations)
Illusion of parallelism
- Reading Inputs and Output
- Throughout course: develope skillsets to manage multiple tasks

Modify the Provide Code Detect an Input Sequence, while the lights are blinking