VP2611

8

CBUS Control Port

The CBUS control port is used to input control and setup

information and also to output status information. In order to

save on pin count, a microprocessor driving this port is

required to execute two I/O instructions in order to transfer a

single byte of information to or from the device. The first

transfer is always a write operation, with a low level on the

single address line which is used by the interface. Data on the

bus then defines the instructions listed in Table 3. The second

transfer can be a read or write operation as necessary, but the

address line must then be high with the set up time given in

Figure 10.

In addition to the single addresss line (CADR), data

transfers use a control strobe (CSTR) which is only effective

when a chip enable is present (CEN). Detailed timing informa-

tion is given in Figure 10, and when writing data or instructions

to the VP2615 the set up and hold times which are referenced

to the rising edge of CSTR must be maintained.

When a write instruction has been defined CADR should

be pulled high, valid data presented to CBUS7:0 and then

strobed in using CSTR. Other system I/O transfers can occur

between defining a write operation and supplying the data to

be written, assuming CEN is not active during those other

transfers. If CSTR does not go active because of I/O transfers

to other devices, then CEN can remain active low between the

instruction and data.

When a read instruction has been specified the requested

data will then be output on CBUS7:0 after the access time

specified from CEN going low, assuming that CADR was

already high. Otherwise the data will become valid after the

access time specified from CADR going high after CEN was

Fig 10 : Use of the Control Port

*

*

20ns

*

20ns

GO

O

C US

WRITING DATA FROM THE CBUS:

This diagram shows a typical instruction and associated data field being written to the device.

READING INFORMATION ON CBUS :

This diagram shows a typical instruction and associated data field being read from the device.

If Th is less than 5 ns then CBUS may be driven by the VP2615until CEN going high eventually turns off

the drivers. It will not prevent correct data being read when CEN again goes active

N.B. All timings shown are minimum values except those marked * which are maximums.

DATA IN

CEN

CADR

CSTR

CBUS

I/P

INSTRUCTION

20ns

20ns

20ns

DATA OUT

CEN

CADR

CSTR

CBUS

INSTRUCTION

10ns

10ns

20ns

50ns

20ns

Th

20ns

20ns

20ns

10ns

10ns

10ns

20ns

20ns

20ns

20ns

10ns

10ns

10ns

20ns

20ns

20ns

20ns

10ns

10ns

10ns

20ns

20ns

20ns

20ns

10ns

10ns

10ns

10ns

20ns