LOW POWER OPERATION

The TDS9092 takes about 15mA. This can be reduced in several ways. If the serial ports are not being used in any application they can be turned off with 232OFF. If a serial port is used only occasionally, for transmitting data to a second computer for example, it can be turned on and off as required:

: LOWTYPE ( like TYPE but turns RS232C power on/off

     232ON  100MS  TYPE   232OFF ;

A delay of at least 100ms is advisable to allow the �9V generator to reach full voltage. This technique reduces the power consumption to about 8mA. Disconnection of the terminal when not in use will also save power.

When the negative voltage generator is switched off in this way TDS-PC will give a 'serial data error' message on the PC. For this reason it will probably be necessary to use a dumb terminal and the on-board editor when generating the primitives for low power operation. Once they are working, the code can be uploaded to the PC with F3 and the low power words left out until the application is complete. They can then be restored.

Further power can sometimes be saved by switching off any external ICs. The TDS9092 provides a switched +5V supply for them on pin c15. This is the same as the supply to the serial port voltage generator, and is controlled by 232ON and 232OFF . Use it to save power in external peripherals but take care, some chips will take current at their inputs if they are powered down.

Another idea for saving power is to run the board from below +5V. This takes it out of specification but most boards will work below +4V. The output of the voltage regulator is nominally +5V only as long as link D is made. To reduce the voltage unmake this solder link and add two resistors in the empty positions alongside the existing 2M7 resistor. In the circuit diagram these are shown as 100K (against the 2M7) and 270K (the next one along). These give about +5V at the output. Reduce the 270K to get lower voltages.

To achieve the lowest possible power of about 2 to 3mA it is necessary to put the microprocessor into standby for most of the time. When this is done, pull-down resistors on the address bus ensure that the application PROM and the on-board RAM are also not selected. The only chip drawing any appreciable power is the gate array, which is now in control.

After STANDBY puts the microprocessor into its lowest power state, the watchdog on the gate array will time-out after 106ms. It will reset the microprocessor which will then enter its application program. If you have written the application program correctly the microprocessor can now do a job; a scan of a matrix keyboard, for example. If a new key has not been pressed it can again go into standby. In this example of low power operation, off to on time for the microprocessor is about 100 to 1. An average current consumption of only about 2 to 3mA results, yet the system is alive and working. If a key is pressed, the microprocessor can be left powered up so that the data can be processed before it is put back into standby.

This example gives a model which can be tried on a dumb terminal, and then changed and expanded to meet the requirements of your application:

: WORK ( demonstration of 2 to 3mA operational mode

    NEWKEY -DUP

    IF                    ( new key has been pressed

      4 U.R 10 MS         ( print key number

      ?TERMINAL IF ABORT THEN ( get out on Ctrl+C

    THEN

    STANDBY ;             ( go back to low power

SET WORK ( make WORK the word to execute on reset

You can do anything you want once the microprocessor comes back on line, collect data from an A to D converter and put it into RAM, for example. Or you may stay out of standby until there is no input for ten minutes, say. Note that the only drawback is a delay of up to 106ms in responding to an input. This is acceptable in many cases. For example it is not noticed when using a matrix keyboard with the low power mode.