MAX6850 MAXIM [Maxim Integrated Products], MAX6850 Datasheet - Page 27

• Manufacturer Part Number: MAX6850
• Description: 4-Wire Interfaced, 7-, 14-, and 16-Segment Alphanumeric Vacuum-Fluorescent Display Controller
• Manufacturer: MAXIM [Maxim Integrated Products]

4-Wire Interfaced, 7-, 14-, and 16-Segment Alpha-
Table 31. Output Map RAM Codes (continued)
Table 33 shows how to set the output map address
pointer to a value within the acceptable range. Bit D7 is
set to denote that the user is writing the output map
address pointer. If the user attempts to set the output
map address to one of the out-of-range addresses by
writing data in range 0xD4 to 0xFF, then address 0x00
is set instead.
After the last data location 0x53 has been written, fur-
ther output map data entries are ignored until the out-
put map address pointer is reset.
The output map data can be written to the address set
by the output map address pointer. Bit D7 is clear to
denote that the user is writing actual output map data.
The output map address pointer is autoincremented
after the output map data has been written to the cur-
rent location. If the user writes the output map data in
the RAM order, then the output map address pointer
need only be set once, or even not at all as the address
is set to 0x00 as power-up default (Table 34).
The output map data can be read by reading address
0x86. The 7-bit output map data at the address set by
the output map address pointer is read back, with the
MSB clear. The output map address pointer is autoin-
cremented after the output map data has been read
from the current location, in the same way as for a write
(Table 35).
The VFD filament is typically driven with an AC wave-
form, supplied by a center-tapped 50Hz or 60Hz power
transformer as part of the system power supply.
However, if the system has only DC supplies available,
OUTPUT MAP RAM
numeric Vacuum-Fluorescent Display Controller
CONTENT
88 to 127
82 to 85
86
87
______________________________________________________________________________________
(display mode select bit M = 0)
(display mode select bit M = 1)
Only valid for 48/1 mode
Only valid for 96/2 mode
ADDRESS RANGE
4 annunciators
Unused
Cursor
Cursor
Filament Drive
the filament must be powered by a DC-to-AC or DC-to-
DC converter.
The MAX6850 can generate the waveforms on the
PHASE1 and PHASE2 outputs to drive the VFD filament
using a full bridge (push-pull drive). The PHASE1 and
PHASE2 outputs can be used as general-purpose out-
puts if the filament drive is not required. The bridge
drive transistors are external, but the waveforms are
generated by the MAX6850.
The waveform generation uses PWM to set the effective
RMS voltage across the filament, as a fraction of the
external supply voltage (Figure 14) (Table 36). The fila-
ment switching frequency is synchronized to the multi-
plex scan clock, eliminating beating artifacts due to
differing filament and multiplex frequencies.
The PWM duty cycle is controlled by the filament duty-
cycle register (Table 37). The effective RMS voltage
across the filament is given by the expression:
or, rearranged:
where:
FilOn is the number to store in the filament duty-cycle
register, address 0x09.
V
V
V
bridge driver (V).
V
bridge driver (V).
V
Duty = 200 x V
FIL
RMS
LO-BRIDGE
HI-BRIDGE
RMS
Cursor segment for digits 0 to 47 on 2nd row
Cursor segment for digits 0 to 47 on 1st row
is the supply voltage to the filament driver bridge (V).
is the specified nominal filament supply voltage (V).
= FilOn x (V
Annunciator A1 to annunciator A4
ADDRESSED FUNCTION
is the voltage drop across a high-side
is the voltage drop across a low-side
RMS
FIL
No action
/ (V
- V
FIL
LO-BRIDGE
- V
LO-BRIDGE
- V
HI-BRIDGE
- V
HI-BRIDGE
) / 200
27
)