Vax 613 0S Instruction Manual download pdf (Page 96)

6.4.9 START, BCONT, VIDEO_VALID, ENABLE_INTERRUPT,

CLEAR_INTERRUPT Registers

When written to, these registers change the ASIC:

• START causes the address in the ADDRESS register to be used for the next

write operation.

• BCONT uses a pre-generated address from the former instruction as start

address for the next instruction.

• VIDEO_VALID signals that the TURBOchannel writes to the video registers

have completed and video operations may begin. VIDEO_VALID can also be

turned off to update video registers.

• ENABLE_INTERRUPT enables or disables interrupt checking, depending on

the low order bit of the data ﬁeld.

• CLEAR_INTERRUPT, if written to, clears an interrupt once it is posted.

6.4.10 Video Timing Registers

There are two clocks in the SFB ASIC.

• The TURBOchannel clock

This clock controls the TURBOchannel interface, the SFB datapath logic, and

the VRAM read/write/read modify write logic.

• The video clock

A large part of the video subsystem involves displaying data in the VRAM to

the monitor. This display is controlled by the monitor frequency, which is a

combination of the refresh rate and the resolution.

The RAMDAC receives 4 pixels/read from the VRAM interface. Therefore, the

clock that the video subsystem runs on is 1/4 the frequency of the monitor’s

oscillator. The SFB ASIC uses this clock to count active and blank video

cycles, and provide the synch pulse to the RAMDAC for the correct number of

cycles. These registers are programmable through software, and the values

for different resolution/refresh rates can be found in the manufacturer’s

CXTurbo Design Speciﬁcation.

At initialization, the BLANK signal is sent to the RAMDAC, since the video

registers do not contain valid data. After values for these counters are written

through the TURBOchannel interface, the VIDEO_VALID register must be

written to start the video logic and serial VRAM output to the RAMDAC.

This VIDEO_VALID register is synchronized to the video clock from the

TURBOchannel clock.

6–16 CXTurbo Graphics Subsystem: 300/500 Models

1 2 ... 91 92 93 94 95 96 97 98 99 100 101 ... 343 344

No comments

Vax 613 0S Instruction Manual Page 96