Hardware Reference
In-Depth Information
B
LINK
O
UTPUT
The blink output (the BLINK pin) indicates the blink phase and is high during the P0 period
and low during the P1 period. Blink phase status can be read back as the P bit in the configura-
tion register. Typical uses for this output are
•
To provide an interrupt to the processor so that segment data can be changed
synchronously to the blinking. For example, a clock application may have
colon segments blinking every second between hour and minute digits, and
the minute display is best changed in step with the colon segments. Also,
if the rising edge of blink is detected, there is half a blink period to change
the P1 data (P0 data drives the displays during this interval). Similarly, if the
falling edge of blink is detected, there is half a blink period to change the
P0 digit data.
•
If OSC is driven with an accurate frequency, blink can be used as a seconds
counter.
The RC oscillator uses an external resistor,
R
SET
, and an external capacitor,
C
SET
, to set the
oscillator frequency,
f
OSC
. The allowed range of
f
OSC
is 1 to 8 MHz.
R
SET
also sets the peak seg-
ment current. The recommended values for
R
SET
and
C
SET
set the oscillator to 4 MHz, which
sets the slow and fast blink frequencies to 0.5 Hz and 1 Hz. The recommended value of
R
SET
also sets the peak current to 40 mA, which makes the segment current adjustable from 2.5 to
37.5 mA in 2.5-mA steps:
I
SEG
5
K
I
/
R
SET
mA
f
OSC
5
K
F
/(
R
SET
×
C
SET
1
C
STRAY
) MHz
where
K
I
5 2144
K
F
5 6000
R
SET
5 external resistor in k
Ω
C
SET
5 external capacitor in pF
C
STRAY
5 stray capacitance from OSC pin to GND in pF, typically 2 pF
The recommended value for
R
SET
is 53.6 k
and the recommended value for
C
SET
is
26 pF. The recommended value for
R
SET
is the minimum allowed value since it sets the dis-
play driver to the maximum allowed segment current.
R
SET
can be set to a higher value to
set the segment current to a lower peak value whenever it is desirable. The effective value of
C
SET
includes not only the actual external capacitor used but also the stray capacitance from
OSC to GND.
Ω
Example 10.9
▼
Daisy-chain two MAX6952s to drive eight of the cathode-row, monocolor, matrix L
ED
displays GMC8975C made by Fairchild, assuming that the PM5 pin is used to drive the CS
input of two MAX6952s. The connection of two MAX6952s with the HCS12 (24-MHz E-clock)
is shown in Figure 10.30. Each MAX6952 is driving four matrix LED displays. Write a program
to configure the SPI module to operate in master mode, and transfer data at 12 MHz, shift data
in/out on the rising edge of the SCK clock. Display
MSU ECET
on the matrix LED displays
without blinking.
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