daver@guille.ece.orst.edu (Dave Rabinowitz) (11/10/89)
In article <8615@microsoft.UUCP> alonzo@microsoft.UUCP (Alonzo Gariepy) writes: >Thanks! Can someone at HP dig this up for us? Since the carrier is much >slower than the clock speed, I would not be surprised if its period is >determined by one of the timers in the slave display controller. In that >case, the 40KHz signal might be achievable. The 32768 Hz carrier comes from a crystal which drives the real time clock. The CPU clock is also derived from this crystal; the output of a VCO (voltage- controlled oscillator) is divided down by a programmable divider to about 32768Hz and this frequency is compared to the crystal frequency and the oscillator frequency is adjusted until the divided down frequency matches the crystal frequency. The CPU clock comes from the VCO. The speed-up programs change the CPU clock speed by reprogramming the frequency divider. There are two bits which control the LED. One turns on the LED modulated by the carrier, producing the 32768Hz signal, but the other turns on the LED independently of the oscillator. A program which used this bit and toggled it fast enough (12uS on, 13uS off) could generate a 40KHz signal. Be very careful! The LED drive circuit assumes the LED will be on with well under a 50% duty cycle, so if you turn on the LED and leave it on accidently you could potentially damage the electronics.
alonzo@microsoft.UUCP (Alonzo Gariepy) (11/14/89)
In article <13672@orstcs.CS.ORST.EDU> daver@guille.ECE.ORST.EDU (Dave Rabinowitz) writes: > There are two bits which control the LED. One turns on the LED modulated by > the carrier, producing the 32768Hz signal, but the other turns on the LED > independently of the oscillator. A program which used this bit and toggled it > fast enough (12uS on, 13uS off) could generate a 40KHz signal. Be very > careful! The LED drive circuit assumes the LED will be on with well under a > 50% duty cycle, so if you turn on the LED and leave it on accidently you could > potentially damage the electronics. Do you have the address of these bits? It would save me a great deal of effort. Thanks for the clear explanation. The warning about duty cycle is very much appreciated; one could do this quite accidentally and lose use of the LED. Alonzo Gariepy alonzo@microsoft