OE is independent of the operation of the serial inter-

face. Data can be shifted into the serial-interface shift

register and latched regardless of the state of OE.

DOUT is the serial-data output, which shifts data out

from the MAX6978’s 8-bit shift register on the rising edge

of CLK. Data at DIN is propagated through the shift reg-

ister and appears at DOUT eight clock cycles later.

Watchdog

The MAX6978 includes a watchdog circuit that monitors

the CLK, DIN, and LE inputs. If there is no transition on

any one of these inputs for nominally 1s, then the output

latches are cleared and outputs OUT0–OUT7 go high

impedance like the initial power-up condition. This turns

off all LEDs connected to the outputs. The shift-register

data does not change, just the output-latch data.

The watchdog is intended to be used as a fail-safe fea-

ture for applications, which prefer a blank display to an

incorrect display if the serial interface fails. When the

watchdog triggers, the outputs remain off until the dri-

ver output latches are updated with data turning them

on. Recovery is therefore automatic if the transmission

failure is temporary, because the MAX6978 does not

lock up in the watchdog timeout state. The MAX6978

operates correctly when the serial interface is next acti-

vated, and the watchdog circuit is reset and starts

monitoring the serial interface again. The watchdog

function requires no software change to the application

driving the MAX6978.

LED Fault Detection

The MAX6978 includes circuitry that detects open-cir-

cuit LEDs automatically. An open-circuit fault occurs

when an output is programmed to sink current but less

than about 50% of the programmed current flows.

Open circuits are checked just after the rising edge of

LE, when new LED data is loaded into the output latch-

es from the shift register(s). If one or more output port is

detected with an open-circuit fault, then the 2 bits, D6

and D5, in the serial-interface shift register are both set

high. If no open-circuit faults are detected, then D6 and

D5 are both cleared low. The data in other 6-bit posi-

tions in the serial-interface shift register is not altered.

Fault status is automatically shifted out on DOUT when

the next data transmission is shifted in. LE is normally

taken high after all 8 bits of new LED data have been

clocked into the shift register, and so at that time,

DOUT is outputting data bit D7. On the next two rising

edges of CLK, the 2 fault status bits, D6 and D5, are

clocked out in that order, followed by the remaining 5

unchanged data bits D4 to D0.

A typical fault-detecting application tests all the shifted

out data. Bits D0–D4 and D7 are checked against the

originally transmitted data to check data link integrity.

Bits D5 and D6 are checked first to see that they con-

8-Port, 5.5V Constant-Current LED Driver with

LED Fault Detection and Watchdog

8

_______________________________________________________________________________________

.

D7

D6

D1

D0

D7

LE

CLK

DIN

DOUT

OE

OUT_

80%

20%

Figure 2. 4-Wire Serial-Interface Timing Diagram