Stache, technical Information and circuit description

(c) 2000 EME Systems
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The Stache is built around a Scenix SX18AC microcontroller running at 4mhz in the turbo mode.

Refer to the schematic below.

The pushbutton is connected to pin ra.3 with a pullup resistor to Vdd. Pushing the button brings ra.3 low. A capacitor debounces the switch. A second capacitor couples the pushbutton into the reset circuit, so that processor reset occurs when the pushbutton is pressed. The processor may also be awakened from the sleep state by activity on the attention pin, rb.0.

An led is connected between pin rb.6 and rb.7. to signal the operational status of the Stache to the user. This is a bicolor led with red and green in the same package. Green indicates successful operations, and red signals error codes.

Application data is stored in a 32kbyte 24LC256 I2C memory chip. It has two control lines, SDA (serial data) and SCL (serial clock), with pullup resistors as required by the I2C protocol. The memory is organized by the program into 15 banks of data, 2kbytes each, to store BASIC Stamp program data, and one 2k bank reserved for an allocation table.

RS232 serial data is received directly on the SX18 input pins, via 22kŻ resistor for protection, while data for transmission is actively asserted high via the transistors Q1, Q2 and Q3. The DB-9 can function as either DCE or DTE. The operation as DCE or as DTE is determined by the conditions that prevail on the pushbutton (ra.3) and on the ATN (rb.0) and TX (rb.2) pins at reset.

Pins rb.0, rb.2 and rb.4 are always inputs from the DB9, and rb.1, rb.3 and rb.5 are always outputs to drive the PNP transistorsQ1, Q2 and Q3. When a transistor is off, the signal from the DB-9 passes through directly to the input pin. When the transistor is on, it drives its output line to a high level. The signal lines have pulldown resistors that pull low when the transistor is off, or they may be pulled below ground by the rx line (through the 3kŻ resistor) when receiving a program from a PC. When downloading data into the Stache, the DCE connections on the DB9 are: pin 2) rx output from rb.5, via Q1; pin )3 tx input to rb.2, pin 4) atn input to rb.0. When uploading data from the Stache to a Stamp, the DTE connections on the DB9 are: pin 2) rx input to rb.4, pin 3) tx output from rb.3 via Q2, pin 4) atn output from rb.1 via Q3.

The pushbutton coupled with feedback from the l.e.d. allow the user to select advanced features. The simplest operation is the single press of the button, which will send the "default" program from the Stache to the target BS2 or BS2SX or BS2e. Holding the pushbutton down for a longer interval starts the l.e.d. flashing at one second intervals, to allow the user to select alternate programs for upload. Pin ra.2 remains available for additional functions.

The details of the programming algorithm for the Basic stamp family can be found here.

Power is supplied from an external 9 volt battery. Optionally, for OEMs, power can be supplied from from pin 9 of the RS232 connector, if the diode shown shaded is soldered into place. The active current is 8&endash;16 milliamps . Sleep current is 30 microamps.

The Stache is a joint effort of Tracy Allen at EME Systems (hardware/production)and Steve Parkis (firmware).


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