LCD questions.
- Thread starter Gump
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Hello,
Just a couple of quickies...
1) The supply current states that at Vdd=5V, the pull is at most 1.2mA, but there's nothing that says what the current is that is pulled (is this also known as sunked) by the pins when they are inputting, nor how much current they can give out (is this known as sourced?)?
2) What would you do in this particular situation where there wasn't a figure given for the lowest value of Vdd where it could still work? The absolute minimum is given to be -0.3V, but the value where the screen still did something useful must be over 0V, how would you find this out?
Thanks.
G.
Just a couple of quickies...
1) The supply current states that at Vdd=5V, the pull is at most 1.2mA, but there's nothing that says what the current is that is pulled (is this also known as sunked) by the pins when they are inputting, nor how much current they can give out (is this known as sourced?)?
2) What would you do in this particular situation where there wasn't a figure given for the lowest value of Vdd where it could still work? The absolute minimum is given to be -0.3V, but the value where the screen still did something useful must be over 0V, how would you find this out?
Thanks.
G.
1) Current is usually sunk by a device.
IO pins do sink/source current but it is usually tiny.
2) I guess you would find it out by trying it out. It's like one of my video cameras. It was rated for 9V-12V but I found it would work perfectly down to 6V, and cut out around 5.85V.
IO pins do sink/source current but it is usually tiny.
2) I guess you would find it out by trying it out. It's like one of my video cameras. It was rated for 9V-12V but I found it would work perfectly down to 6V, and cut out around 5.85V.
Absolute max ratings state where the device will be damaged, not where it will operate correctly. The electrical characteristics table spec the normal operating conditions. For Vdd, they state that the device is normally operated on 5v, however it will work anywhere from 2.7 to 5.3v on Vdd.
For the supply voltage in the Abs max table, they list the supply voltage as "Vdd to Vss" and the note specs Vss=0V, Vdd=5v. This implies that when they spec a voltage in the form "Va to Vb", Va is positive with respect to Vb. For the contrast voltage, they list it as "Vdd to Vo" so that implies that the voltage is defined between Vdd (pos) and Vo (neg). Since the power supply that you are designing will be referenced to Vss (neg) instead, Vo may be a negative voltage with respect to ground. Other data sheets define the contrast voltage as "Vdd-Vo" to make this a little clearer.
As for the backlight, they describe two different backlight types (and of course, there's always the option of no backlight, in which case you just ignore those specs).
The backlight may be an LED (actually a series string of LEDs) with a forward voltage of 4.2-4.6v that wants a typical current of 280mA - use Ohm's law to figure out the correct series resistor to run it off 5v. Alternatively, the backlight may be an electroluminescent strip, that needs 110VAC, 400 Hz. at 5mA to light up, so you would need an external inverter to power it.
Yes, this (and many other LCD data sheets) is not the clearest; I guess that there is a lot of copy-and-paste from other data sheets, and translation issues.
In most cases, when running on 5V, you can connect the contrast pin to the wiper of a trimpot. The two end terminals connect to ground and (optionally with a series resistor) to +5. If you are running on 3.3v or need to work in extreme cold, you will need to generate a negative supply for the contrast pin. While not spec'ed, this is a very low current; more is dissipated in the trimpot than in the LCD. Common sources of this negative voltage is a voltage inverter like the ICL7660, the negative charge pump output of a MAX232 or similar serial port level shifter, or a voltage inverter made from an oscillator (or PWM output from your micro), 2 diodes, and 2 caps.
/mike
For the supply voltage in the Abs max table, they list the supply voltage as "Vdd to Vss" and the note specs Vss=0V, Vdd=5v. This implies that when they spec a voltage in the form "Va to Vb", Va is positive with respect to Vb. For the contrast voltage, they list it as "Vdd to Vo" so that implies that the voltage is defined between Vdd (pos) and Vo (neg). Since the power supply that you are designing will be referenced to Vss (neg) instead, Vo may be a negative voltage with respect to ground. Other data sheets define the contrast voltage as "Vdd-Vo" to make this a little clearer.
As for the backlight, they describe two different backlight types (and of course, there's always the option of no backlight, in which case you just ignore those specs).
The backlight may be an LED (actually a series string of LEDs) with a forward voltage of 4.2-4.6v that wants a typical current of 280mA - use Ohm's law to figure out the correct series resistor to run it off 5v. Alternatively, the backlight may be an electroluminescent strip, that needs 110VAC, 400 Hz. at 5mA to light up, so you would need an external inverter to power it.
Yes, this (and many other LCD data sheets) is not the clearest; I guess that there is a lot of copy-and-paste from other data sheets, and translation issues.
In most cases, when running on 5V, you can connect the contrast pin to the wiper of a trimpot. The two end terminals connect to ground and (optionally with a series resistor) to +5. If you are running on 3.3v or need to work in extreme cold, you will need to generate a negative supply for the contrast pin. While not spec'ed, this is a very low current; more is dissipated in the trimpot than in the LCD. Common sources of this negative voltage is a voltage inverter like the ICL7660, the negative charge pump output of a MAX232 or similar serial port level shifter, or a voltage inverter made from an oscillator (or PWM output from your micro), 2 diodes, and 2 caps.
/mike
