BD VFDs Application Note APN140

3. INTERFACE

The interface of all BD VFD is "C-MOS" synchronous serial data inputs.

3.1 Interface

Table 2 lists the functions of pins of the interface. Table 3 summarizes their DC characteristics. Figure 12 represents the timing charts of these interfaces.

Table 2 Function Table of Interface Terminals

Terminal	Function
CLK	Shift Register Clock	Data Read and Shift at Rising Edge
GCLK	Shift Register Clock for the Grid Driver 1)	Data Read and Shift at Rising Edge
SI	Serial Data Input	H=ON, L=OFF
GSI	Serial Data Input for the Grid Driver 1)	H=ON, L=OFF
SO, GSO	Serial Data Output	Factory use only. Keep it open.
LAT	Data Latch Control	H=Through, L=Latch
GLAT	Data Latch Control for the Grid Driver 1)	H=Through, L=Latch
BK	Driver Output Blanking Control	H or OPEN=Output OFF, L=Output ON
GBK	Grid Driver Output Blanking Control 1)	H or OPEN=Output OFF, L=Output ON

Note 1) For the multiplex drive grid/anode independent type only.
The function may vary from item to item. Please check the specification for details.

Table 3 Interface DC Characteristics

Symbol Item Condition MIN TYP MAX Unit

VIH H-level Input Voltage
VSS+2.4 - VDD1 V

VIL L-level Input Voltage
VSS - VSS+0.7 V

IIH H-level Input Current VIH=VDD1 See individual spec. uA

IIL L-level Input Current VIL=VSS See individual spec. uA

Fig.12 Interface Timing Chart

3.2 Connection with CPU

The BD VFDs use a synchronous serial interface so they do not require a CPU with high voltage capacity.

3.2.1 Static Drive Type

For static drive, control the BD VFD from Serial I/0 as shown in Figure 13. A standard CPU is sufficient. It is also possible to control the BD VFD by a standard parallel I/0 port.

Fig.13 Application Circuit for Static Drive

3.2.2 Multiplex Drive Type

To achieve sufficient control speed, assign at least one serial I/0 port exclusively for controlling a BD VFD. Figure 14 shows an example of the independent grid/anode type. Figure 13 can be used for the hybrid grid/anode type as well.

In case the CPU is interrupted for some reason with the multiplex drive, the grid scan must stop. This will cause permanent damage to the BD VFDs. To protect the VFD, create a watchdog protection circuit which detects the grid scanning operation and will stop supplying the display supply voltage "VDD2", or send a grid blanking signal if needed. Figures 13 and 14 illustrate examples of protection circuit. When the grid scan serial data output of "GSO" is suspended, "GBK" turns "High" so that all output from drivers will be cut to protect the driver chip and the VFD itself.

Fig.14 Application Circuit for Multiplex Drive Type

Back to APN140 INDEX 4. Control Procedure

APN140