COSMAC Quickies
Jess Hillman
Quick, inexpensive solutions to control problems are always desirable, so owners of COSMAC Elf microcomputers may find many interesting ways to use the “quickie” programs in listings one, two and three accompanying this article.
The programs were written specifically for my Quest Super Elf, which has 4.25K RAM, but they should run with very little tweaking on any 1802-based system, if entered beginning at any quarter-K page boundary.
Listing one is an interval timing program that can be set for any delay from a couple of seconds to about ten minutes by varying only the data byte in location 0011. By changing the program beginning at location 0015 to read: “9F FB XX (any value from 00 to FF) CE 30 05 7B 30 00” the program can set intervals up to two days (actually the maximum value for Register F falls a few minutes short of 48 hours). The data bytes in locations 0011 and 17 set the final value selected. Since the 1802 has plenty of registers for such usage, it would be very easy to establish intervals months long.
The program specifically uses Register E, one of the 1802's sixteen, sixteen-bit general purpose registers, as a timer that continually counts down from hex FFFF. When Register E reaches zero, a fact discovered by testing both high and low bytes, Register F is incremented by one. The F register is then tested to see if the predetermined value has been set. If not, the timing loop continues.
Once the proper value for F has been reached, the 1802 sets its Q line, an external flag that can be set or reset depending on various internal conditions of the processor, to a logic “1.” After thus acknowledging it has reached the required time, the Q line is reset to logic zero and the timer resumes its labors. The Q line transition can be latched by connecting it to an integrated circuit such as the 74LS175 or the CMOS 4016, and held for use in driving a transistor, opto-isolator or relay (for high voltage uses) to operate a coffee pot, television, stereo — practically anything controllable with an electronic switch.
Newcomers to the 1802 be warned: when tying to the Q line always buffer it generously with an IC like the 4050 or 4049, either of which can drive two TTL loads. Otherwise, you risk ruining your microprocessing chip.
A variation of this use of the Q line is found in listing two, in which the operator wishing access to the Q line must first enter three predetermined, two-digit hex numbers into memory in the proper sequence. That oughta keep Pop's pet project safe from the kids!
As the data bytes for the “combination” are entered into memory, the 1802 performs a logical exclusive or with each byte in turn, using data stored at addresses 000D, 0017 and 0021 respectively. If the wrong number is entered at any point, the program jumps to the error subroutine beginning at location 0030, which momentarily outputs an “EE” to the data display (I have seven-segment LEDs) while executing a three-second timing delay, then outputs a “00” to the data display and jumps back to the beginning of the program.
In listing two, once the proper number sequence has been entered, the Q line goes high and stays that way until the input key is pressed and released (or external data flag EF4 is otherwise pulled low). Once EF4 goes low and returns to its normal state, Q goes to logic zero and the program loops back to the beginning again.
As written, you would have to enter 05 (at 0D), 17 (at 17) and 98 (at 21) to turn the Q line from logic low to logic high. You can change the data bytes for any combination you wish. The chances of someone solving the combination decrease if you add more numbers to the combination.
Listing three changes this program to utilize an output port and eight data bits to control various devices. When entered as listed and run, the program will: require you to enter the three number combination properly, after which the Q line goes high (on my system this turns on an LED); then you must enter a status byte which will be put in the memory stack and also latched into the output port (1802 output instructions are 6N, where N designates a port from one to seven). I use a 63 instruction because that port is readily available on my Elf’s expansion board. Once the status byte has been latched to the output port, the program loops back to the beginning of memory and starts again.
The status byte can be whatever you want it to be, depending on your interface configuration. Eight data lines are immediately available, so using transistors, relays or a combination of techniques can give you immediate computer control of the major energy consuming devices in your home — air conditioning, hot water heater, and so on.
By expanding the interval timer to include a lookup table of status bytes for dispatch to the output port at various times of the day, automatic control your home's major functions becomes possible. The only question you must answer is how elaborate you want it to be.
Using the upper four bits of the status tied to, say, a 74LS154 four-to-sixteen line decoder, with the lower four bits or-tied to sixteen latches like the 74LS175, it would be possible to control up to sixty-four devices from your micro's output port. Possible expansions and combinations of these programs are virtually endless. As quickie programs go, however, they should give newcomers to the 1802, or people struggling with a system they've had for a time, a feel for register manipulation and control application possibilities of the typical COSMAC system.
Listing 1 — Interval Timer
Address Data Mnemonics Comments 0000 F8 00 LDI 00 Initialize Reg. F for 02 AF BF PLO, PHI use as workspace 04 7A REQ Make sure Q is at logic "0" 05 2E DEC R.2 Decrement timer 06 9E CE GHI, LSZ Check timer, long skip if zero 08 30 05 BR 05 If not zero, continue loop 0A 8E CE GLO, LSZ If high byte zero, check low byte 0C 30 05 BR 05 If not zero, continue loop 0E 1F INC Reg. F If low byte zero, increment workspace register by one 0F 8F GLO R.F Get new value from Register 10 FB 17 XRI 17 Exclusive Or with predetermined value 12 CE LSZ If values match, long skip (PC incremented by 2) 13 30 05 BR 05 If no match, continue loop 15 7B SEQ Set Q line at logic "1" 16 30 00 BR 00 Then start looping again 17 00 IDL End
Listing 2 — Combination Lock
Address Data Mnemonics Comments
0000 F8 00 LDI 00 Set up workspace in memory
02 B4 PHI using Reg. 4 to point to
03 F8 F0 LDI F0 stack beginning at
05 A4 PLO address 00F0
06 E4 SEX
07 3F 07 BN4 07 Loop if EF4 equals zero
09 37 09 B4 09 Loop is EF4 equals one
0B 6C INP 4 Get keyboard byte
0C FB 05 XRI 05 Check if correct combination #
0E CE LSZ Long skip if zero (a match!)
0F 30 30 BR 30 Else go to error subroutine
11 3F 11 BN4 Wait until next byte latched
13 37 13 B4 in From keyboard
15 6C INP 4 get the byte
16 FB 17 XRI 17 If it matches, too, then long skip
18 CE LSZ
19 30 30 BR 30 Go To error subroutine otherwise
1B 3F 1B BN4 If second number matches, wait
1D 37 1D B4 for last combination number
1F 6C INP 4 Get it
20 FB 98 XRI 98 See if it, too, matches
22 CE LSZ Long skip if it does
23 30 30 BR 30 Error subroutine if it doesn't
25 7B SEQ All numbers OK, Q equals "1"
26 3F 26 BN4 Keep Q line on until input
28 37 28 B4 key pressed and released
2A 7A REQ Then turn it off and go back
2B 30 00 BR 00 to beginning
Error subroutine
30 F8 EE LDI EE Load message “EE”
32 54 STR store it in stack, then
33 64 OUT 4 output to LED display port
34 BF PHI Reg. F Also store in Register F
35 2F DEC Reg. F Reg. F decremented by one
36 9F GHI Check byte in Reg. F
37 CE LSZ Skip next two bytes if zero
38 30 35 BR 35 Otherwise loop
3A F8 00 LDI 00 Load "00" and output to
3C 54 STR clear error message from
3D 64 OUT 4 display
3E 30 00 BR 00 Go back and try again
Listing 3 — Controlling Multiple Devices
Address Data Mnemonics Comments 002A 6C INP 4 Get the byte input after correct combination given 2B 54 STR Put status byte in stack 2C 63 OUT 3 Output status byte to device interface 2D 7A REQ Turn Q off 2E 30 00 BR 00 Then go back to start