Main Page3. D/A conversion
It's a pitty that you may only interface RCX inputs in an analog way. Deadly for digital devices. But here's the new idea. The PIC should measure the time-delay between the impact of the IR and ultrasonic pulses. The value should be one byte large! The resistor-net combined with the opto-couplers is the easiest way (not the best) to get a D to A conversion AND the desired galvanic separation of the RCX input from the rest of the device. We first tried an eight-resistor-net, but the result was not convincing, because of the difficulty to distinguish the small raw-values from each other. The program-trick here is to send first the low-byte then the high-byte of the time-delay.
At 4MHz the PIC's internal TMR0-timer is incremented every microsecond (at 1:1 prescale)
at a prescale of 1:128 it is incremented only every 0.128 millisecond
x = c * t, where c is the sound-velocity =343 m/sec
so we get a 4.39cm resolution
a TMR0-overflow occurs after 0.128 * 256 = 32.77 msec, which corresponds to a sound-distance x = 343 * 0.03277 = 11.24 m, which is above our maximum distance of a 5 x 5m room: sqrt(2)*5=7m
Here once again the value list:
RAW = 1023 * R / (10000 + R)
| 8,2k | 15k | 33k | 56k | ||
| B3 | B2 | B1 | B0 | R | RAW |
| 0 | 0 | 0 | 0 | - | 1023 |
| 0 | 0 | 0 | 1 | 56k | 868 |
| 0 | 0 | 1 | 0 | 33k | 785 |
| 0 | 0 | 1 | 1 | 20,8k | 690 |
| 0 | 1 | 0 | 0 | 15k | 614 |
| 0 | 1 | 0 | 1 | 11,8k | 554 |
| 0 | 1 | 1 | 0 | 10,3k | 519 |
| 0 | 1 | 1 | 1 | 8,7k | 476 |
| 1 | 0 | 0 | 0 | 8,2k | 461 |
| 1 | 0 | 0 | 1 | 7,2k | 427 |
| 1 | 0 | 1 | 0 | 6,6k | 406 |
| 1 | 0 | 1 | 1 | 5,9k | 379 |
| 1 | 1 | 0 | 0 | 5,3k | 354 |
| 1 | 1 | 0 | 1 | 4,8k | 334 |
| 1 | 1 | 1 | 0 | 4,6k | 321 |
| 1 | 1 | 1 | 1 | 4,2k | 304 |
The special messages:
| 1 | 1 | 1 | 1 | 1 | 809 | 77 | START,PAUSE,STOP |
| 1 | 0 | 0 | 0 | 0 | 1k | 93 | ERROR |
These values may differ from one RCX and one receiver to some other due to part tolerance and probably battery level.