MIC50398/50399

Micrel

8

August 1998

8-11

Operations:

Six Decade Counter, Latch

The six decade counter is synchronously incremented or

decremented on the positive edge of the count input signal.

A Schmitt trigger on this input provides hysteresis for protec-

tion against both a noisy environment and double triggering

due to a slow rising edge at the count input.

The count inhibit can be changed in coincidence with

the positive transition of the count input. Count inhibit must

remain high while the count input is high to inhibit counting.

The counter will increment when up/down input is high (V

SS

)

and will decrement when up/down input is low. The up/down

input can be changed 0.75

µs prior to the positive transition

of the count input.

The clear input is asynchronous and will reset all decades to

zero when brought high but does not affect the six digit latch

or the scan counter.

As long as store input is low, data is continuously transferred

from the counter to the display. Data in the counter will be

latched and displayed when store input is high. Store can be

changed in coincidence with the positive transition of the

count input.

The counter is loaded digit by digit corresponding to the digit

strobe outputs. BCD thumb wheel switches with four diodes

per decade connected between the digit strobe outputs and

the BCD inputs is one method to supply BCD data for

loading the counter decades.

The load counter pulse must be at V

SS

2

µs prior to the

positive transition of the digit strobe of the digit to be loaded.

The load counter pulse may be removed after the positive

transition of the digit strobe since the chip internally latches

this signal. The BCD data to be loaded must be valid through

the negative transition of the digit strobe.

Inputs, Outputs

The seven segment outputs are open drain capable of

sourcing 10mA average current per segment over one digit

cycle. Segments are on when at V

SS

. The Carry, Zero, BCD

and digit strobe outputs are push pull and are on when at

V

SS

. All inputs except Counter BCD and SCAN inputs are

high impedance CMOS compatible.

Two basic outputs originate from the counter: zero output,

and carry output. Each output goes high on the positive

(V

SS

) going edge of the count input under the following

conditions:

Zero output goes high for one count period when all

decades contain zero. During a load counter operation the

zero output is inhibited. Zero output is on the MIC50399

only.

The carry output goes high with the leading edge of the

count input at the count of 000000 when counting up or at

999999 when counting down and goes low with the negative

going edge of the same count input. During a load counter

operation the carry output is inhibited.

A count frequency of 1.5 MHz can be achieved if the zero

output and carry output are not used. These outputs do not

respond at this frequency due to their output delay illustrated

on the timing diagram.

BCD & Seven Segment Outputs

BCD or seven segment outputs are available. Digit strobes

are decoded internally by a divide by six Johnson counter.

This counter scans from MSD to LSD. By bringing the SET

input low, this counter will be forced to the MSD decade

count. During this time the segment outputs are blanked to

protect against display burn out.

BCD outputs are valid for MSD when SET is low. Applying

V

SS

to SET allows normal scan to resume. Digit 6 output is

active (V

SS

) until the next scan clock pulse brings up digit 5

output.

The segment outputs and digit strobes are blanked during

the interdigit blanking time. Typically the interdigit blanking

time is 3 to 10 microseconds when using the internal scan

oscillator.

BCD output data changes at the beginning of the interdigit

blanking time. Therefore the BCD output data is valid when

the positive transition of a digit output occurs. BCD outputs

are on MIC50399 only.

Scan Oscillator

The counters have an internal scan oscillator. The

frequency of the scan oscillator is determined by an external

capacitor between V

SS

or V

DD

and scan input. The wave

form present on the scan oscillator input is triangular in the

self oscillate mode. An external oscillator may also be used

to drive the scan input.

In the external drive mode the interdigit blanking time will be

the sum of the negative dwell period of the external oscillator

and the normal self oscillate blanking time. (3

→10 µs). Dis-

play brightness can be controlled by the duty cycle of the

external scan oscillator.

Typically, the scan oscillator will oscillate at the following

frequencies with these nominal capacitor values from V

SS

to

scan input.

C

IN

Min

Max

820 pF

1.4 kHz

4.8 kHz

470 pF

2.0 kHz

6.8 kHz

120 pF

7.0 kHz

20 kHz