268XA
Service Manual
2-12
Microcontroller
A Texas Instrument MSP430P337A mixed signal microcontroller is the heart of the DIO
PCA. The microcontroller, U4, features bi-directional digital ports, a built in UART and
internal timers for controlling operation. A quartz crystal operating at 32.8 kHz provides
the main clock signal for the microcontroller. This signal is internally multiplied to
3.6925 MHz for clocking of the microcontroller functions and is also divided down to
8 kHz (XBUF U4 Pin 97) providing a control signal for the debounce PLD; U3.
Internal to the microcontroller is a watchdog timer that is set to a 1 second interval.
Should the microcontroller become inoperative for a period of 1 second, the watchdog
timer will activate and perform a Power Up Clear (PUC) operation that resets all outputs
and restarts the microcontroller. C50 controls the duration of the reset signal.
During power up, U16 provides a reset pulse of approximately 2 seconds to insure that
the crystal oscillator (Y1) is given sufficient time to start. U16 also has undervoltage
detection such that if the supply voltage to pin 2 drops below 4.5 V, the output will reset.
Depressing S1 can manually reset the microcontroller. This applies a ground signal to the
reset (RST U4 pin 96) pin of the microcontroller.
U3 is an Atmel ATF750C Programmable Logic Device (PLD) and is configured as a
signal debouncer. The Totalizer signal is applied to U3 Pin 13 from the Totalizer Input
Protection Buffer circuit (U1 Pin 1 and U18. See Totalizer Input section for more
information).
Localized communication to the DIO PCA can be accomplished by connecting a RS-232
terminal to P3 or P4 and moving R35 (a zero ohm resistor) to position R36 (this redirects
the receive signal from the mainframe to the microcontroller to the RS-232 port). U2
(Maxim MAX232) accomplishes the bi-directional RS-232 communication. It uses a
single supply (+5 Vcc) and internally level shifts the signals to standard RS-232 levels.
Communication is controlled by the built in UART in the microcontroller (URXD U4 Pin
51 receive and UTXD U4 Pin 50 transmit) at a rate of 120k bps. Communication is
performed in 6 byte packets (bytes 0-4 contain command data while byte 5 is a
checksum) and is done in hexadecimal format. You will need a special software program
(Perl script) to communicate via P3 or P4 for debug purposes. Using “terminal.exe” or
“hyperterminal.exe” in windows will not work as the communication is in ASCII format.
U17 is simply an output buffer for driving the opto-isolated A/D power supply.
The MSP430P337A is a One Time Programmable (OTP) device and will be pre-
programmed by the supplier however should the need arise, it is possible to program a
blank device on board through the JTAG Programming Port P2. A serial-programming
adapter supplied by Texas Instruments must be used in order to program the device. R34
must be replaced with a zero ohm resistor so that the programming adapter can control
the clock of the microcontroller. After programming is complete, R34 is removed.
Data Input Threshold
The Digital Input Threshold circuit sets the input threshold level for the Digital Input
Buffers and the Totalizer Input. A fixed value voltage divider (R5, R6) and a unity gain
buffer amplifier (U1) are the main components in the circuit. The voltage from the
divider (approxi1.4 V dc) is then buffered by U1, which sets the input threshold.
Capacitor C8 filters the divider voltage at the input of U1.
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