Assembling the Handy Board, Part 1

Power, Power Monitor, CPU, and Serial Line Circuitry


This is the start of the assembly sequence for the main Handy Board unit. In this assembly sequence, the 6811 power supply and power monitoring circuitry, the 6811 itself, and the serial communications circuitry will be installed.

For this assembly sequence, collect the following parts:


Microprocessor Power Circuitry

The first order of business is to install the microprocessor power supply and battery charge circuit. The Handy Board battery will not be connected until the end of the overall assembly job, however; instead, power will be supplied to the board through the battery charge circuit.

Assembly

Install C3*, C7, C8*, C9, LED15Y*, D3*, R13, J12, SW1.

Mount R14, raising it about 1/8'' above the surface of the board; it will need to be pressed a bit to the left when J6 is installed later.

Mount U13*, the main power regulator, so that its tabbed side is closer to SW1, as indicated in the board photograph.

Testing

Apply power to the board from the adapter jack and switch the board on using SW1. For a brief instant each time the board is switched on, the CHARGE LED (LED15Y) should flash. This indicates power flowing into the board, which in this case is used to charge C3, the main system capacitor.

Turn the board on, and measure the system power voltage. A convenient place is the terminal marked +5v and GND on the connector for U4 (the LCD display). The voltage should read between 4.9v and 5.1v.

Debugging

If the CHARGE LED does not light, first check the polarity on the DC adapter. Set the voltmeter to a 20 volt range and measure the output of the adapter. The center opening should be the negative terminal and the outer shield should be the positive terminal. If these are reversed, the adapter cannot be used to supply power to the Handy Board via its charge jack. The adapter still may be used with the Interface/Charger, however.

If the adapter polarity is correct, check that LED15Y is installed with the correct orientation.

If the system voltage does not read between 4.9v and 5.1v, check the orientation of U13. Touch the tab of U13, if it is getting hot, remove power immediately. Either U13 or D3 is probably installed backward.


Voltage Monitor Chip

U12, the Dallas Semiconductor DS1233-10, is a voltage monitor chip that measures the system voltage and asserts a reset signal to the microprocessor whenever the voltage is invalid (i.e., below 4.5v). This ensures that the microprocessor is idle during power-on, when system voltage is ramping up from zero volts to its normal operating level, and during power-off, when system voltage falls from normal operating level to zero volts.

The output of the voltage monitor chip is connected to an LED (LED11R, marked BATT). When the chip asserts the ``invalid'' signal, the BATT LED will light. This happens for a brief interval when power is switched off, and for 1/3 of a second when it is switched on. The BATT LED also serves as a low-battery indicator, since if the battery is not charged enough to raise the system voltage to its normal operating level, the DS1233 chip will continuously assert the signal that lights the BATT LED.

Assembly

Unplug the DC adapter, if it is still plugged into the board. Install U12*, R2, and LED11R*.

Testing

Plug the DC adapter back into the board, and switch it on. The BATT LED should light for about 1/3 of a second, and then go off. Switch the board off; the BATT LED should light and then fade as board power dissipates.

Debugging

If the BATT LED does not light as it should, check its orientation, as well as the orientation of U12.

Microprocessor and Oscillator Circuit

In this assembly step, the microprocessor and its oscillator circuit are installed. The testing step assures that the microprocessor is correctly driving the oscillator circuit, which is the first indication of its proper operation.

Assembly

Unplug the board, if it is still plugged in. Install PLCC*, the square, 52--pin socket for U1 (the 6811 microprocessor). Make absolutely sure to get the orientation correct: the chamfered corner (i.e., the diagonally truncated corner) of the socket must align with the chamfer in the socket outline printed on the board. The chamfered corner installs in the upper left when viewing the board from the normal reading orientation indicated by the photographs.
The socket must be installed correctly because the 6811 can only be placed into the socket in one orientation, so the socket orientation must be correct.

Solder all 52 pins of the socket.

Install U1, the 6811 microprocessor, into its socket, aligning the chamfered corner with the one on the socket. Press firmly and evenly until the 6811 is fully seated in the socket. It should click into place.

Install R1 (2.2M--red, red, greem).

Install X1, which may be either a ceramic resonator (rectangular plastic package with three leads) or quartz crystal (oval metal package with two leads).

In the photograph, X1 is the ceramic resonator.

Testing

Apply power to the board through the charge jack and turn the board on. By examining the system clock output of the 6811 (known as the E clock), the following test will verify that the 6811 oscillator circuit is operating properly. The E clock signal is at pin 5 of the 6811. It should be a 2 MHz square wave.

In order to test this signal, either an oscilloscope, logic probe, or voltmeter are required, in declining order of preference (i.e., the oscilloscope is best).

  1. With an oscilloscope, measure the E clock signal. It should appear as a 2 MHz square wave over the zero to +5v range.

  2. With a logic probe, examine the E clock signal. The pulse output LED of the logic probe should flash, indicating oscillation. (Make sure that the logic probe's pulse memory feature is turned off.)

  3. With a volt meter, put it in a DC voltage range and measure the E clock signal. It should register as a 2.5v signal.
Note that cases (2) and (3) do not measure the actual frequency of the signal. It will be assumed that if the clock signal is indeed generated, it is of the proper frequency.

The behavior of LED15Y (CHARGE) and LED11R (BATT) should be as before.

Debugging

If the E clock signal is not present, this could indicate faulty soldering between the crystal or resonator and the 6811, or an improper value of the biasing resistor R1.

It is less likely but still possible that the 6811 is defective. First check that the 6811 is receiving power and ground properly (on pins 26 and 1, respectively). To measure the ground, use the logic probe to verify a definite logic low, or measure the potential from the +5v supply.

As a last resort, try another 6811.


Serial Line and Stop Switch

In the next step, circuitry is installed that allows the 6811 to communicate over a serial line to the host computer. This includes a green LED to indicate when the 6811 is transmitting data, and a pushbutton switch that allows the 6811 to be placed into a special ``bootstrap'' mode. This bootstrap mode allows the 6811 to accept an initial operating program that is downloaded by the host computer.

Assembly

Remove power from the board. Install R8, R15, RP4, D1*, LED10G*, SW3, and J5.

Testing

Apply power to the board and turn it on. LED10G, labelled PWR, should light. Now hold down SW3 (STOP) and turn power on. Continue to hold the STOP switch while the BATT LED cycles on and then off. The PWR LED should turn off when the BATT LED turns off. This indicates that the 6811 has entered bootstrap mode, and is waiting for a program to be downloaded over its serial line.

Back at the host computer, connect the Interface/Charger board to a serial port using the appropriate modem cable. Connect the DC adapter power to the Interface/Charger board.

Connect the main board to the Interface/Charger board using the 4--wire telephone cable. Power will be supplied to the main board via the telephone cable. Put the board in download mode by turning it on while holding down the STOP switch. The Handy Board's PWR and BATT LEDs should both light for about 1/3 of a second, and then both LEDs should turn off. This state, in which board power is on but both LEDs have turned off, is the mode required for bootstrap download to function properly.

Now run the appropriate version of the 6811 downloader software. During the first download phase, the green SER LED on the I/C board (transmit from the host computer to the 6811 board) and the green PWR LED on the main board (transmit from the 6811 board to the host computer) should both blink visibly.

The downloader software should reach the second download phase in which it attempts to download the pcode_hb.s19 program. It will then generate an error, because the Handy Board does not have memory to receive this program.

But the point of this test is to see if the downloader reaches the second phase. If it does, congratulations! Serial communication between the host computer and the 6811 board has been established. This is a big step in the process of bringing up a new Handy Board.

Debugging

There are a lot of things to get right in this step. Most likely problems involve software configuration (i.e., choosing the correct serial port) and cabling (using the proper modem cable to connect the Interface/Charger Board to the host computer; having a telephone cable that is wired correctly).
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Fred Martin / MIT Media Laboratory / fredm@media.mit.edu / Tue Jul 16 12:26:48 1996