www.ams.com/LED-Driver-ICs/AS1105 Revision 1.36 7 - 15
AS1105
Datasheet - Detailed Description
8.4 Initial Power-up
After powering up the system, all register are reset, so that the display is blank. The AS1105 starts the shutdown mode. All registers should be
programmed for normal operation. The default settings enable only scan of one digit, the internal decoder is disabled, data register and intensity
register are set to the minimum value.
8.5 Decode-Mode Register
In the AS1105, a BCD decoder is included. Every digit can be selected via register 09h to be decoded. The BCD code consists of the numbers 0-
9, E,H, L,P and -. In register 09h, a logic high enables the decoder for the appropriate digit. In case that the decoder is bypassed (logic low), the
data Bits D7-D0 correspond to the segment lines of the AS1105. In
Table 7 some possible settings for register 09h are shown. Bit D7, which
corresponds to the decimal point, is not affected by the settings of the decoder. Logic high means that the decimal point is displayed. In Table 8
the font of the Code B decoder is shown. In Table 10 the correspondence of the register to the appropriate segments of a 7 segment display is
shown (see Figure 6).
8.6 Intensity Control and Interdigit Blanking
Brightness of the display can be controlled in an analog way by changing the external resistor (RSET). The current, which flows between VDD
and I
SET
, defines the current that flows through the LEDs. The LED current is 100 times the I
SET
current. The minimum value of RSET should be
9.53k
Ω, which corresponds to 40mA segment current. The brightness of the display can also be controlled digitally via register 0Ah. The
brightness can be programmed in 16 steps and is shown in Table 10. An internal pulse width modulator controls the intensity of the display.
8.7 Scan-Limit Register
The scan limit register 0Bh selects the number of digits displayed. When all 4 digits are displayed the update frequency is typically 800Hz. If the
number of digits displayed is reduced, the update frequency is reduced as well. The frequency can be calculated using 8fOSC/N, where N is the
number of digits. Since the number of displayed digits influences the brightness, the resistor R
SET should be adjusted accordingly. Table 12
shows the maximum allowed current, when fewer than 4 digits are used. To avoid differences in brightness the scan limit register should not be
used to blank portions of the display (leading zeros).
Table 5. Register Address Map
Register
Register
Hex Code
D15-D12 D11 D10 D9 D8
No-Op X 0 0 0 0 0xX0
Digit 0 X 0 0 0 1 0xX1
Digit 1 X 0 0 1 0 0xX2
Digit 2 X 0 0 1 1 0xX3
Digit 3 X 0 1 0 0 0xX4
Decode Mode X 1 0 0 1 0xX9
Intensity X 1 0 1 0 0xXA
Scan Limit X 1 0 1 1 0xXB
Shutdown X 1 1 0 0 0xXC
Not used X 1 1 0 1 0xXD
Reset and ext. Clock X 1 1 1 0 0xXE
Display Test X 1 1 1 1 0xXF
