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FC-handset-spec: document the use of 74LVC2G126 for LCD backlight current selection
author Mychaela Falconia <falcon@freecalypso.org>
date Tue, 10 May 2022 06:00:13 +0000
parents 5952e2dfdc26
children
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91:2ac06a49dfbc 92:f5b954172002
282 in our application (see section 1.4.3.1) we need at least two different display 282 in our application (see section 1.4.3.1) we need at least two different display
283 brightness levels, and thus at least two switchable LED currents. At first the 283 brightness levels, and thus at least two switchable LED currents. At first the
284 problem seems difficult, but an elegant solution has been found. 284 problem seems difficult, but an elegant solution has been found.
285 285
286 LCD backlight LED current will be selected by way of two Calypso GPIO pins 286 LCD backlight LED current will be selected by way of two Calypso GPIO pins
287 configured as outputs, and a 74LVC2G125 dual tristate buffer. Each tristate 287 configured as outputs, and a 74LVC2G126 dual tristate buffer. Each tristate
288 buffer's A input will be tied high, and the two Calypso GPIO outputs will be 288 buffer's A input will be tied high, and the two Calypso GPIO outputs will be
289 connected to buffer output enable inputs. There will be two resistors with 289 connected to buffer output enable inputs. There will be two resistors with
290 different carefully computed values, each connected between one of the two 290 different carefully computed values, each connected between one of the two
291 tristate buffer outputs and MAX1916 SET pin. There will also be a third 291 tristate buffer outputs and MAX1916 SET pin. There will also be a third
292 resistor connected between MAX1916 SET pin and the V-IO rail. 292 resistor connected between MAX1916 SET pin and the V-IO rail.
297 corresponding to the long-call dimmed mode, whereas the sum of all 3 297 corresponding to the long-call dimmed mode, whereas the sum of all 3
298 contribution currents will equal the 15 mA limit - the current is reckoned per 298 contribution currents will equal the 15 mA limit - the current is reckoned per
299 LED. The difference between the 15 mA maximum and the small dimmed mode current 299 LED. The difference between the 15 mA maximum and the small dimmed mode current
300 will be split unequally between the two switched SET resistors, allowing a total 300 will be split unequally between the two switched SET resistors, allowing a total
301 of 4 different backlight intensity levels to be selected via the two Calypso 301 of 4 different backlight intensity levels to be selected via the two Calypso
302 GPIO outputs going to the 74LVC2G125 buffer's output enable inputs. The actual 302 GPIO outputs going to the 74LVC2G126 buffer's output enable inputs. The actual
303 currents will be determined after some further experimentation with our lunalcd2 303 currents will be determined after some further experimentation with our lunalcd2
304 test setup: this setup features our KWH020ST23-F01 LCD, a MAX1916-based 304 test setup: this setup features our KWH020ST23-F01 LCD, a MAX1916-based
305 backlight circuit, and a DIP switch pack selecting the LED current in 1 mA 305 backlight circuit, and a DIP switch pack selecting the LED current in 1 mA
306 increments. 306 increments.
307
308 74LVC2G126 has been chosen as the dual tristate buffer part instead of more
309 common 74LVC2G125 because the '126 has active-high output enable inputs, making
310 the software interface (bits in the GPIO output register) more intuitive: with
311 a '126 buffer, writing 1 into GPIO11 or GPIO12 will turn on the corresponding
312 current contribution, whereas a '125 buffer would produce an inverted sense.
313 74LVC2G126GT parts were less readily available than the '125 counterpart, but
314 we have now made a lifetime buy of the preferred part.
307 315
308 1.4.5. Slight regression relative to Pirelli DP-L10 316 1.4.5. Slight regression relative to Pirelli DP-L10
309 317
310 The actual LCD backlight LED driving circuit inside the Pirelli phone is not 318 The actual LCD backlight LED driving circuit inside the Pirelli phone is not
311 known, but reverse engineering of Pirelli's firmware followed by experimentation 319 known, but reverse engineering of Pirelli's firmware followed by experimentation