ICS82C404 Datasheet, PDF (5/13 Pages) Integrated Circuit Systems – Dual Programmable Graphics Frequency Generator

ICS82C404
Serial Programming Architecture
The pins SEL0 and SEL1 perform the dual functions of select-
ing registers and serial programming. In serial programming
mode, SEL0 acts as a clock pin while SEL1 acts as the data pin.
The ICS82C404-01 may not be serially programmed when in
power-down mode.
Since the VCLK registers are selected by the SEL0 and SEL1
pins, and since any change in their state may affect the output
frequency, new data input on the selection bits is only permitted
to pass through the decode logic after the watchdog timer has
timed out. This delay of SEL0 or SEL1 data permits a serial
program cycle to occur without affecting the current register
selection.
In order to program a particular register, an unlocking sequence
must occur. The unlocking sequence is detailed in the following
timing diagram:
Serial Data Register
The serial data is clocked into the serial data register in the
order described in Figure 1 below (Serial Data Timing).
The serial data is sent as follows: An individual data bit is
sampled on the rising edge of CLK. The complement of the
data bit must be sampled on the previous falling edge of CLK.
The set-up and hold time requirements must be met on both
CLK edges. For specifics on timing, see the timing diagrams
on pages 10, 11 and 12.
The unlock sequence consists of at least five low-to-high
transitions of CLK while data is high, followed immediately
by a single low-to-high transition while data is low. Following
this unlock sequence, data can be loaded into the serial data
register.
Following any transition of CLK or DATA, the watchdog timer
is reset and begins counting. The watchdog timer ensures that
successive rising edges of CLK and DATA do not violate the
time-out specification of 2ms. If a time-out occurs, the lock
mechanism is reset and the data in the serial data register is
ignored.
The bits are shifted in this order: a start bit, 21 data bits, 3
address bits (which designate the desired register), and a stop
bit. A total of 24 bits must always be loaded into the serial data
register or an error is issued. Following the entry of the last
data bit, a stop bit or load command is issued by bringing DATA
high and toggling CLK high-to-low and low-to-high. The
unlocking mechanism then resets itself following the load.
Only after a time-out period are the SEL0 and SEL1 pins
allowed to return to a register selection function.
Figure 1: Serial Data Timing
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