Oscilloscope Watch
Teardown
Agenda
• History and General overview
• Hardware design:
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Block diagram and general overview
Choice of the microcontroller
Design of the analog frontend
Design of the waveform generator
Design of the power stage
• Firmware design:
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Maximize use of the peripherals
High speed sampling using the DMA
Low power techniques to maximize the battery life
Using the XMEGA event system to offload the CPU
• Questions and answers
Oscilloscope Watch history
Xprotolab: World’s
Smallest Oscilloscope
Xprotolab Watch?
Concept image
Oscilloscope Watch Features
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Mixed Signal Oscilloscope: 2 analog and 8 digital channels
Advanced Trigger
Meter Mode: Average, Peak to peak and Frequency
XY Mode
FFT Mode (Spectrum Analyzer)
Horizontal and Vertical Cursors
Arbitrary Waveform Generator with Sweep
Curve tracer function
General Overview
Block diagram and general overview
Choice of the microcontroller
8bit vs 32bit ?
Choice of the microcontroller
• Critical parameters:
– The speed of the ADC
– DAC included
– The size of the package
• Advantages of keeping the 8bit AVR:
– Proven design
– No porting of code
• Disadvantages:
– Limited RAM
– Expansion limited
Design of the Front End
Analog Frontend
𝑉𝑜𝑢𝑡 =
𝑉𝑖𝑛
+ 1.024𝑉
10
Additional gain is done in the micro’s ADC
Design of the Front End
Front end bandwidth
Approximately 320kHz, but the micro’s capacitance and
ADC characteristics lowers it to about 200kHz
Design of the waveform generator
AWG Amplifier
𝑉𝑂𝑈𝑇 = −4 ∙ 𝑉𝐷𝐴𝐶 + 4.096𝑉
Design of the waveform generator
AWG Bandwidth
Bandwith is 50kHz, determined by the RC feedback
Design of the power stage
AN1149 description of the circuit
Battery supplies system load
when power source is absent
Load sharing
Design of the power stage
- Very low quiescent
current
- Switch to 1.8V when
display is off
Digital Power
Battery Monitor
- Battery monitor can
be disabled, so it
doesn’t draw current
- Output impedance of
10kΩ
Design of the power stage
Analog Power
- This section generates +5V and -5V from the input voltage
- The analog power section is disabled when in watch mode
Maximize use of the peripherals
Architecture block diagram
High speed sampling using the DMA
- “High Speed” for an 8bit microcontroller running at 32MHz…
- Instead of polling or using interrupt handlers to read and
process the result registers, the XMEGA’s DMA is used to move
data from the result registers to memory buffers.
- This moving of data is done without CPU intervention.
Low Power techniques
- Maximize the time spent in Sleep mode
- Use the highest CPU clock speed, unless the task requires a
specific amount of time (e.g. serial communication), in which case,
avoid using a higher CPU speed than needed
- Turn off unused peripherals
Microcontroller's Power Budget
Low Power techniques
• Use the RTC with the 1024Hz from the 32768Hz external crystal
• Set I/Os at a known state
• Disable the digital input buffer on pins that are connected to
analog sources
• Disable the BOD - or better, disable it while in sleep - to reduce
power consumption. Use sampled mode if only slow changes in
operating voltage are likely.
• Disable the On Chip Debugging and the JTAG interface
• Enable power reduction mode for EEPROM and Flash to reduce
power consumption in ACTIVE mode
• Use page-wise writing to EEPROM rather than byte-wise
Low Power techniques - LCD
• The DMA is used to send data to the SPI.
• Redundant transfers are removed in the SPI.
• The next frame is rendered during the SPI transfer, this is
performed using the DMA’s double buffering.
• Graphics are pre-rendered and stored in RAM.
XMEGA event system to offload the CPU
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CH0: TCE0 overflow used for ADC
CH1: ADCA CH0 conversion complete
CH2: EXT Trigger or logic pin for freq. measuring
CH3: TCD1 overflow used for DAC
CH4: TCC0 overflow used for freq. measuring
CH5: TCC1 overflow used for freq. measuring
CH6: CLKPER / 32768 -> every 1.024ms
CH7: TCD0L underflow: 40.96mS period - 24.4140625 Hz
Questions?