ViewSonic VG150B Service Manual - Page 31

Service Manual VG150 THEORY OF CIRCUIT OPERATION ViewSonic April 1999 - Version 1.0 HN Frequency Counter MT1/112E can discriminate HSYNCNSYNC frequency and saves the information in XFRs. The 15 bits Hcounter counts the time of 64XHSYNC period, but only 11 upper bits are loaded into the HCNTH/HCNTL latch. The 11 bits output value will be (2/Hfreq) / (1/OSCfreq), updated once per VSYNC/CVSYNC period when VSYNC/CVSYNC is present or continuously updated when VSYNC/CVSYNC is non-present. The 14 bits Vcounter counts the time between two VSYNC pules, but only 9 upper bits are laoded into the VCNTHNCNTL latch. The 9 bits output value will be (1Nfreq) / (512/OSCfreq), updated every VSYNC/CVSYNC period. An extra overflow it indicates the condition of HN counter overflow. The VFchg/HFchg interrupt is active when VCNT/HCNT value changes or overflow. HN Present Check The Hpresent function checks the input HSYNC pules, Hpre flag is set when HSYNC IS OVER 10KHz or cleared when HSYNC is under 10Hz. The Vpresent function checks the input VSYNC pules, the Vpre flag is set when VSYNC is over 40Hz or cleared when VSYNC is under 10Hz. Acontrol bit "PREFS" selects the time base for these functions. The HPRchg interrupt is set when the Hpre value changes. The VPRchg interrupt is set when the Vpre/Cvpre value change. However, the Cvpre flag interrupt may be disabled when S/W disable the composite function. Output HBLANKNBLANK Control and Polarity Adjust The HBLANK is the mux output of HSYNC and self-test horizontal pattern. The VBLank is thw mux output of VSYNC, CVSYNC and self-test vertical pattern. The mux selection and output polarity are S/W controllable. The VBLANK output is cut off when VSYNC ferquency is over 200Hz or 133Hz depends on 8MHz/12MHz OSC selection. The HBLANKNBLANK shares the output pin with P4.1/P4.0. Page 28