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(we're not kidding - we get a lot of e-mails with questions that are already addressed in the Frequently Asked Questions web page)

As you can see from the schematic diagram, this is a very simple circuit built around Motorola’s 68HC705C8 8 bit micro-controller. The 68HC705C8 has the following attributes;

• 8K bytes of internal EPROM
• 304 bytes of internal RAM
• 24 user-definable input/output ports
• 7 Input only ports
• Serial Control Interface (SCI)
• Serial Peripheral Interface (SPI)
• 16 bit capture/compare timer system
• Watch Dog timer

The SCI is connected to a MAX233 (or MAX232) serial interface driver from Maxim. This five-volt only device converts the TTL level serial communications to standard RS232 levels through the use of built-in voltage converters. No external capacitors or additional power supplies are required to obtain the ± 12 volts required for standard RS232 levels.

The SPI is connected to a Fairchild 93C56 128 x 16 bit serial EEPROM. Additional EEPROM’s could be added, however, the 128 words contained within one chip is more than enough to save up to 31 64 bit serial numbers and configuration parameters.

A simple power supply, consisting of U3, D1, C4 and C5, converts 12 volt AC or DC to the 5 volts required for the main-board circuit. A LM78L05 version of voltage regulator was used due to the low power consumption (less than 32mA, worst-case) of the circuit.

Bi-polar LED D2 is used as a heart-beat indicator (I always like to know that the software is running!) and changes from green to red whenever a temperature sample is being taken.

The 1-Wire interface consists of up to 4 input/output ports connected together to provide sufficient +5 volts during temperature conversion, and the pull-up resistor R5. R5 is variously recommended to be 5.1K or 1.5K ohms depending on your reference material. The data sheet specifies the higher value, however, Dallas Semiconductor applications engineers have recommended the lower value as a result of extensive field operation which shows the necessity for the lower value.  Connector J5 provides a means of "boosting" the output drive capability of the 1-Wire port.  Shorting the 2 pins of J5 will connect an additional 3 controller ports to the 1-Wire Port.  Tests have shown this to make some amount of difference in reducing errors on sensors mounted far away from the TEMP05 unit.  Note: J5 is no longer available on Version H or higher PCB's.  The boost current is always available.

D4 and D9 are provided to add some amount of transient surge protection.  This is not fool-proof and will not protect against lightning strikes, but will ensure that small voltages above 5V or below ground, are clamped.

Connectors J2 and J6 provide the external interface to the 1-Wire bus.  J2 is a 3 terminal wiring block and J6 can be a RJ11 or RJ12 connector (4 or 6 pins respectively).  If you wish to use an iButton on-board, there is space to place a DS9094F iButton connector in the J6 area.  NOTE: neither the RJ-12 connector nor the iButton holder are provided with the TEMP05.  They must be purchased separately.

If you consider the left-most pin as pin 1, looking at the connector from the front with the PCB at the bottom,

• pin 1 = no connection
• pin 2 = +5V (for local powering)
• pin 3 = DQ
• pin 4 = Ground
• pin 5 = no connection
• pin 6 = no connection

See also our Sensors page for equivalent RJ12 pin-outs from other devices.

The 68HC705C8 is a One Time Programmable (OTP) part. A programmer was available from Motorola, however I can no longer find it for sale.  The programmer can be constructed. I have included a schematic of the programmer (adapted) in the article "Build a 68HC705C8 Programmer". The PC serial port programming software for the micro-controller programmer is available from the following link. Alternatively, a complete construction article is available from Circuit Cellar Ink Issue 58 May 1995.