FreeCalypso > hg > freecalypso-docs
comparison FC-handset-spec @ 44:a415ae467c6d
FC-handset-spec: main keypad and its backlight documented
| author | Mychaela Falconia <falcon@freecalypso.org> |
|---|---|
| date | Thu, 10 Jun 2021 18:10:35 +0000 |
| parents | e1b75668a706 |
| children | 9df8683daec4 |
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| 43:e1b75668a706 | 44:a415ae467c6d |
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| 232 special chip takes the place of "dumb" ballast resistors: connect Vbat (battery | 232 special chip takes the place of "dumb" ballast resistors: connect Vbat (battery |
| 233 positive terminal) directly to the common anode of the 3 LEDs, but instead of | 233 positive terminal) directly to the common anode of the 3 LEDs, but instead of |
| 234 series resistors, connect each cathode to the corresponding LEDn pin of MAX1916 | 234 series resistors, connect each cathode to the corresponding LEDn pin of MAX1916 |
| 235 - *without* any resistors or transistors! FETs inside the MAX1916 take the | 235 - *without* any resistors or transistors! FETs inside the MAX1916 take the |
| 236 place of resistors as current-limiting elements, and the chip's global on/off | 236 place of resistors as current-limiting elements, and the chip's global on/off |
| 237 control takes the place of a separate switching transistor. | 237 control (which will be driven with a Calypso GPIO) takes the place of a separate |
| 238 switching transistor. | |
| 238 | 239 |
| 239 The special quality of MAX1916 is that it produces constant current through each | 240 The special quality of MAX1916 is that it produces constant current through each |
| 240 LED (based on a set reference current and 230x current multiplication circuit | 241 LED (based on a set reference current and 230x current multiplication circuit |
| 241 inside the chip) regardless of variations in both Vbat and Vf! Of course the | 242 inside the chip) regardless of variations in both Vbat and Vf! Of course the |
| 242 requested current can only be sustained as long as Vbat >= Vf + Vds, where Vds | 243 requested current can only be sustained as long as Vbat >= Vf + Vds, where Vds |
| 319 induced reductions, just the natural one. | 320 induced reductions, just the natural one. |
| 320 | 321 |
| 321 Thus based on all of the above considerations, I feel justified in my design | 322 Thus based on all of the above considerations, I feel justified in my design |
| 322 choice of foregoing PWM control of backlight intensity in favor of fixed current | 323 choice of foregoing PWM control of backlight intensity in favor of fixed current |
| 323 switching with much more limited selection. | 324 switching with much more limited selection. |
| 325 | |
| 326 1.5. Main keypad | |
| 327 | |
| 328 The main keypad on our FC Libre Dumbphone handset will have the following | |
| 329 21-button arrangement: | |
| 330 | |
| 331 left soft key ^ right soft key | |
| 332 <O> | |
| 333 green call V red power/hang-up | |
| 334 button button | |
| 335 | |
| 336 1 2 3 | |
| 337 4 5 6 | |
| 338 7 8 9 | |
| 339 * 0 # | |
| 340 | |
| 341 The top section above the traditional numeric dial buttons (12) consists of left | |
| 342 and right soft keys, green and red buttons (classically called SEND/END), and a | |
| 343 5-way navigation button group (left, right, up, down and center), for a total of | |
| 344 9 buttons in this section. The red hang-up button is also the hardware power-on | |
| 345 button; having the same button effect power-off when held down for some time is | |
| 346 a firmware function. | |
| 347 | |
| 348 This 21-button main keypad arrangement is exactly the same as featured on | |
| 349 Motorola C1xx and Pirelli DP-L10 phones, on TI's D-Sample development platform, | |
| 350 and also on many other phones (non-Calypso) from the appropriate era, such as | |
| 351 Samsung E2232. | |
| 352 | |
| 353 1.5.1. Keypad backlight | |
| 354 | |
| 355 All traditional phones including Mot C1xx and Pirelli DP-L10 feature keypad | |
| 356 backlights, hence we need to include one as well. The exact structure of this | |
| 357 backlight won't be known until we enter the mechanical design phase for the | |
| 358 actual handset (as opposed to intermediate development boards), which will be | |
| 359 much later in the project, but the Mother's understanding is that keypad | |
| 360 backlights are made up of some number of LEDs (2 on Pirelli DP-L10, unknown | |
| 361 number on Mot C139) and some kind of light diffuser. | |
| 362 | |
| 363 Given the discovery of MAX1916 constant-current-sink LED driver chip (see | |
| 364 section 1.4.4), the optimal electrical design of the keypad backlight becomes | |
| 365 clear: use 3 LEDs, and drive them using another MAX1916 chip, separate from the | |
| 366 one used for the LCD backlight. | |
| 367 | |
| 368 Backlight intensity: neither Mot C139 nor Pirelli DP-L10 provides any way to | |
| 369 vary keypad backlight intensity, and no such variability is deemed necessary. | |
| 370 In the long-call state when the LCD backlight is dimmed, the keypad backlight | |
| 371 is fully off. We shall use a fixed LED current setting for our keypad | |
| 372 backlight, set with a single fixed resistor between the keypad MAX1916 chip's | |
| 373 SET pin and the V-IO rail, and the actual current value will be determined in a | |
| 374 much later phase of the project, when we have the actual keypad backlight LEDs | |
| 375 and a better idea of the mechanical design. | |
| 376 | |
| 377 Backlight color: Mot C139 uses blue LEDs, Pirelli DP-L10 uses white LEDs. | |
| 378 Because blue and white LEDs have very similar electrical characteristics | |
| 379 (current needed for appropriate brightness, Vf at this current), the choice | |
| 380 between the two can be made in a much later project phase, based on input from | |
| 381 other team members who are better at aesthetics. | |
| 382 | |
| 383 1.5.1.1. Comparison with Mot C139 and Pirelli DP-L10 | |
| 384 | |
| 385 Both of these two pre-existing reference phones feature keypad backlights that | |
| 386 are switched on/off via Iota LEDB; the actual circuit design is unknown. | |
| 387 However, in our design we forego Iota LEDB altogether (it won't be used for | |
| 388 anything), and use two MAX1916 chips for our LCD and keypad backlights, with | |
| 389 each chip's on/off control being a Calypso GPIO. | |
| 390 | |
| 391 The actual workings of the LEDB driver or switch inside the Iota chip are a | |
| 392 mystery. On the one hand it appears to be nothing more than a "dumb" transistor | |
| 393 on/off switch, no different from an external "digital transistor" (BJT with bias | |
| 394 resistors) controlled by a Calypso GPIO: a resistor still seems to be required | |
| 395 for current control, and at least on the Pirelli DP-L10 the keypad backlight | |
| 396 intensity visibly varies with Vbat ranging over the Li-ion discharge range. But | |
| 397 on the other hand, LEDB requires the 13 MHz clock to be running, and the light | |
| 398 goes out when this clock is stopped. Why in the world would any kind of clock | |
| 399 be required if the circuit is only a transistor on/off switch controlled by a | |
| 400 static register bit? Other parts of TI's Iota datasheet describe its LEDA, LEDB | |
| 401 and LEDC as "current drivers" - but in the absence of any way to actually set | |
| 402 the desired current without depending on Vbat or Vf variations, whatever the | |
| 403 Iota chip actually provides can't be anything like MAX1916. | |
| 404 | |
| 405 Poorly documented, non-understood mystery hardware is best avoided, hence we are | |
| 406 not going to use Iota LEDB, and shall only use MAX1916 instead. We also gain a | |
| 407 functional improvement over Pirelli DP-L10 by using MAX1916: our keypad | |
| 408 backlight intensity will remain the same over the battery discharge range. |