![](http://datasheet.mmic.net.cn/390000/TLC34076M-135_datasheet_16838079/TLC34076M-135_19.png)
2–3
video rates up to 135 MHz. The dual-mode clock input (ECL/TTL) is primarily an ECL input but can be used
as a TTL-compatible input if the input-clock selection register is so programmed. The clock source used at
power up is CLK0; an alternative source can be selected by software during normal operation. This chosen
clock input is used unmodified as the dot clock (representing the pixel rate to the monitor). The device does
allow for user programming of the SCLK and VCLK outputs (shift and video clocks) via the output-
clock-selection register. The input-/output-clock-selection registers are shown in Tables 2–3, 2–4, and 2–5.
The ECL input can be used as a differential or single-ended input. If the CLK3 input is used as a single-ended
ECL input, the CLK3 input must be externally terminated to set the input common-mode signal level. This
can be done with a simple resistor divider, as is the case with fully differential ECL.
SCLK is designed to drive the VRAMs directly, and VCLK is designed to work with video control signals like
BLANK and SYNC. While SCLK and VCLK are designed as general-purpose shift clock and video clock,
respectively, they also interface directly with the TMS340x0 GSP family directly. Even though SCLK and
VCLK can be selected independently, there is still a relationship between the two. Many system
considerations have been carefully covered in the design, allowing maximum user freedom.
Internally, both SCLK and VCLK are generated from a common clock counter that increments on the rising
edge of the dot clock. Therefore, when VCLK is enabled, it is in phase with SCLK (see Figure 2–1).
The internal clock counter is initialized to value 0 any time the output-clock-selection register (bits 5, 4, 2,
1) are all set to 1s. This provides a simple mechanism to synchronize multiple video interface palettes by
providing a known phase relationship for the various system clocks. One can write directly to the output-
clock-selection register to cause this to occur or any of the various resets (POR, hardware, software–see
section 1.5) also causes the appropriate bits to be written and the counters to reset. It is up to the user to
provide some means of disabling the dot-clock input to the part while this reset is occurring if multiple parts
are to be synchronized.
Appendix A discusses the SCLK/VCLK relationship specific to the TMS340x0 GSP.
as an example)
(dot clock/2
SCLK
as an example)
(dot clock/4
VCLK
Dot Clock
Figure 2–1. Dot Clock/VCLK/SCLK Relationship
Table 2–3. Input-Clock-Selection Register Format
BITS
2
FUNCTION
3
1
0
0
0
0
0
Select CLK0 as clock source§
0
0
0
1
Select CLK1 as clock source
0
0
1
0
Select CLK2 as clock source
0
0
1
1
Select CLK3 as TTL clock source
0
1
0
0
Select CLK3 as TTL clock source
1
0
0
0
Select CLK3 and CLK3 as ECL clock sources
Register bits 4, 5, 6, and 7 are don’t care bits.
When the clock selection is altered, a minimum 30-ns delay is incurred before the
new clocks are stabilized and running.
§CLK0 is chosen at power up to support the VGA pass-through mode.