Instruction manual

6.4.11 TCCLK COUNT, VIDCLK_COUNT Registers
In order to determine which values to load into the video registers, there are
two counters in the SFB ASIC. One counter counts 256 TURBOchannel clock
cycles and the other counts video clock cycles, so that software can determine the
oscillator frequency. Order to do this, software performs these operations:
1. Write the TURBOchannel clock address. This write operation resets the video
clock counter and the TURBOchannel clock counter. The TURBOchannel
clock counter counts 256 TURBOchannel clock cycles and stops itself and the
video clock counter.
2. Read the TURBOchannel clock counter. When it finishes, the register
contains the value FF. If the count is not yet finished, the value is a 0.
3. When the value of the TURBOchannel clock counter is equal to FF, it is legal
to read the video clock counter.
This process can be repeated as necessary, by a write operation to the TCCLK
count register.
CXTurbo Graphics Subsystem: 300/500 Models 6–19