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We will use the [FabFTDI](https://ppatil.pages.cba.mit.edu/FabFTDI/) for serial communication and to power through the "FTDI" header.
- ###Eagle Schematic is shown below.
[Eagle schematic file](./test_simplified.sch)
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- ###Eagle board on the left. ATtiny45 pinout on the right.
[Eagle board file](./test_simplified.brd)
[traces](test_simplified_traces.png)
[outline](test_simplified_outline.png)
[ATtiny45 datasheet](http://ww1.microchip.com/downloads/en/DeviceDoc/Atmel-2586-AVR-8-bit-Microcontroller-ATtiny25-ATtiny45-ATtiny85_Datasheet.pdf)
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- ###The board and the microcontroller program:
- The [program](./test_board.c) and the [makefile](./t45.make).
- The board is powered through the "FTDI" header on the right side. Pin 1 is ground, pin 3 is power (5V). Pin 4 is connected to a microcontroller pin (PB2).
Pin 5 is for sending data (from microcontroller pin PB2).
- One LED (red) is wired to be always on when power is applied to the board.
- A button is attached to PB4. PB4 is set as an input with
["<u>input pullup</u>"](https://tourlomousis.pages.cba.mit.edu/fabclass-recitation-electronics/microcontroller/index.html)
activated. (see programming week for details.)
- When the button is pushed, the second LED (blue) toggles on and off. LED is driven by a 50% PWM waveform from PB3.
- Every 50 mS, one byte, corresponding to the ASCII character "M", is sent from pin 6 (PB1) at 9600 baud (bits per second). Each time a byte is
sent, the pin PB2 (physically pin 7) is toggled between 5V and 0V, just to give us another signal on a pin that is otherwise not used.
- [ASCII](https://en.wikipedia.org/wiki/ASCII) "M" is coded by the decimal number 77.
- Decimal 77 = 4x16 + 13 = Hexadecimal 4D = binary 0100 1101.
- 9600 bits per second corresponds to 104 microseconds per bit.
[Information on RS232 serial protocol.](https://wcscnet.com/tutorials/introduction-to-rs232-serial-communication/)
##Things to measure.
- ###Digital Multimeter (DMM)
- With the board not connected to power, check continuity of traces and solder joints, using the DMM audible setting in resistance mode.
- Still in resistance mode, measure a resistor value.
- Use the DMM to check for correct orientation of the LEDs.
- Plug the board in to a power supply using the "FTDI" header. Set the multimeter on DC voltage mode, using proper scale.
- Connect the ground lead to the board ground, using a clip lead. [Picture](./ground_dmm.jpg)
- Check VCC.
- Check pullup resistor on reset pin.
- Measure voltages in the "always-on" LED circuit.
- What is the voltage drop across the resistor? (Two ways to get this.)
- The resistor is 500 Ohms. Calculate the LED current.
- Measure the voltage at pin PB4 with button not-pressed and pressed.
- Measure the voltage at pin PB3 (LED pin) when LED is on and when it is off. What's going on?
- ###Oscilloscope.
- [Scope picture.](./scope.jpg) [Probe picture.](./probe_pic.png)
- Connect the ground lead to the board ground. [Picture](./ground_scope.jpg)
- Using the Auto-trigger mode, look at VCC and other voltage levels on the board.
- (optional) Unsolder the filter capacitor and see the difference in the 5V waveform.
- Attach the scope probe to the 4th pin of the FTDI header (connected to PB2), to observe the toggling of that signal every 50 mS. [Picture](./signal_pin4.jpg)
- Push the button to turn the blue LED on.
- Use the scope probe to observe the PWM waveform at pin PB3 (physically the second pin) of the t45.
- Set the triggering mode and the time base to get a stable display.
- What are the voltages?
- How long is the waveform "off" and how long "on"?
- What is the PWM frequency?
- Is the LED flashing on and off? Why does it look continuous?
- Using the same triggering method, capture the (serial) waveform being sent from the microcontroller at pin PB1 (5th pin of the FTDI header).
- What is the duration of one bit?
- What does the pattern of high and low voltage mean? You should be able to see:
- a start bit.
- eight bits of data
- a stop bit.
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