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ECE243

LEGO Autobalance LAB

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But first: LOGICAL SHIFTS• SLL/SLLI: shift left logical

– like ‘<<’ in C

• SRL/SRLI: shift right logical – like ‘>>’ in C

• NOTE: new bits shifted in are zeros

• Ex: SLLI r8, r9, 0x4

• Ex: SRLI r8, r9, 0x3

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Bit Fields and Masking• Use a word, hword, or byte to hold multiple values

– ex, packing ctrl & data signals into a connector (GPIO)– ex, packing small fields to save memory

• Ex: Course info: – department: (ECE, CIV, etc) 12 total =>– Course code: 0..2000 =>– Fall, spring: F, S =>

• Better than one halfword each: – 3 * 16 bits = 48bits

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Course info Example#put dept in r8: 

 

#Put course code in r8:

 

#Put F/S in r8:

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Next Lab: You will Build a Segway!

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Actually, a LEGO Autobalancer

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LEGO Setup

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LEGO Controller

0F

0xF means low light

0x0 means full light

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LEGO Breakout Box

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Autobalancer with Breakout Box

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Using SensorsSensor

0x0 (full light)

Sensor

0xF (low light)

Sensor

0x7 (some light)

Sensor

0x0 (full light)

Sensor

0xD (low light)

FixedDistance:

VaryingDistance:

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Talking to LEGO Controler via GPIO• Bits0..9

– configure/control 5 motors– enable/disable; clockwise/counter-clockwise

• Bits10..19– configure/control 5 sensors

• Bits27..30– read data value from sensors

• 2 ways to use sensors:– polling mode (default mode, recommended)– state mode (more advanced, see documentation)

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Motor control9 8 7 6 5 4 3 2 1 0

for/

rev

on/

off

for/

rev

on/

off

for/

rev

on/

off

for/

rev

on/

off

for/

rev

on/

off

motor4 motor2 motor1 motor0

• for/rev: 0 = forwards/clockwise, 1 = reverse/ctr-clk-wise• on/off: 0 = on, 1 = off

0(JP1):(DR):

9 8 7 6 5 4 3 2 1 0

1 1 1 1 1 1 1 1 1 1

motor3 motor2 motor1 motor0

4(JP1):(DIR)

• set to all 1’s, since they are all outputs

motor3

motor4

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Sensor control

• on/off: 0 = on, 1 = off• rdy (ready): 0 = ready/valid, 1 = not-ready/invalid

0(JP1):(DR)

4(JP1):(DIR)

• set to 1’s for on/off bits (outputs)• set to 0’s for ready bits (inputs)

19 18 17 16 15 14 13 12 11 10

rdy? on/

off

rdy? on/

off

rdy? on/

off

rdy? on/

off

rdy? on/

off

sensor4 sensor2 sensor1 sensor0sensor3

19 18 17 16 15 14 13 12 11 10

0 1 0 1 0 1 0 1 0 1

sensor3 sensor2 sensor1 sensor0sensor4

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Sensor values30 29 28 27

- - - -

• 4-bit sensor value – can only read one sensor value at a time– first enable one sensor, then await ‘ready/valid’, then read

0(JP1):(DR)

30 29 28 27

0 0 0 04(JP1):(DIR)

• set direction for sensor value bits to 0’s• since they are inputs

Sensor value

Sensor value

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Direction Register DIR• Typical use of DIR:

– set to 1(output): • bits0..9 (motor enables), bits10,12,14,16,18 (sensor enables)

– set to 0 (input): • bits 11,13,15,17,19 (sensor readys), bits27..30 (sensor value)

– Set to 0bxx00 00xx xxxx 0101 0101 0111 1111 1111– X’s must also be set to certain values (see docs)

• Magic number for typical use: (set DIR to this)– 0b0000 0111 1111 0101 0101 0111 1111 1111– 0x07f557ff

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Ex: enable motor1, forward.equ ADDR_JP1, 0x10000060movia r8,ADDR_JP1

movia r9, 0x07f557ff # set DIR to typical-use valuestwio r9, 4(r8)

movia r9, 0xfffffff3 # 0b... 11110011# bit2=0 for enabled, bit3=0 for

forwardstwio r9,0(r8)

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Ex: read sensor2.equ ADDR_JP1, 0x10000060

movia r8,ADDR_JP1

movia r9, 0x07f557ff # set DIR to typical-use value

stwio r9, 4(r8)

movia r9, 0xffffbfff # bit14=0 to enable sensor2

stwio r9,0(r8)

POLL ldwio r9,0(r8) # read DR

srli r10,r9,15 # shift bit-15 to the bit-0 position

andi r10,r10,0x1 # mask it to isolate it

bne r10,r0,POLL # ready if r10==0, try again otherwise

srli r9,r9,27 # shift-right-logical-immed by 27 bits

# sensor value is now in lowest 4 bits of r9

andi r9,r9,0x0000000f # mask it to isolate it

# can now use sensor value in r9

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Some Hints• Do not use delay counters, must use timer

• Not all sensors are equal– sensitivities vary, you must calibrate them

• motors will stall if you switch directions fast

• motors can be too fast/jerky:– if you give it full voltage:– instead give it part voltage: – called Pulse Width Modulation (PWM)

• See DESL for full details!– page on Lego Controller (quick reference)– full Lego manual (pdf)