add FAQ

README.md

@@ -1,7 +1,7 @@
 motoserial
 ==========
 
-the motoserial is a phone controller
+the motoserial (motorola automatic serial) is a phone controller
 it is used to power, flash, and communicate with Motorola C1XX phones
 the purpose is to use it for osmocomBB
 
@@ -17,24 +17,24 @@ technology
 ----------
 
 instead of the ON button, a pin on the back of the phone can be used
-but I noticed that plugin the charger had the same effect
+but I noticed that pluging the charger had the same effect
 and instead of the battery, a 5V power source can also be used
 so the trick is to use 2 transistors: 1 to simulate battery re-plug, and 1 to simulate charger replug; and switch off and on both power sources
 since they share the ground pin, p-MOSFET on the positive side had to be used
 actually the order and switching timing is not too important
 thus one transistor, switching both the battery and charger power sources, is enough
-this way a more common n-channel MOSFET can be used, but the groung pin of serial should then also ne switched
+this way a more common n-channel MOSFET can be used, but the ground pin of serial should then also be switched
 
 the battery can be replaced by a regular 5 V power source, the same as the one used for charging
 if the voltage is above 4.2 V, the battery should not be charged
-to prevent power comming back on the power source if the phone charges the battery, a diode should be use on the positive side on the battery power source
-this would also decrease the voltage (due to the forward voltade drop of the diode) and bring it closer to the usual 3.7-4.2 V voltage level
+to prevent power coming back on the power source if the phone charges the battery, a diode should be use on the positive side on the battery power source
+this would also decrease the voltage (due to the forward voltage drop of the diode) and bring it closer to the usual 3.7-4.2 V voltage level
 
-the phone cosumes at 3.7 V on average:
+the phone draws at 3.7 V on average:
 - ~ 90 mA when idling
 - ~ 120 mA when receiving
-- ~ 220 mA when transmiting
-when transmiting, peaks of 2 A can be reached
+- ~ 220 mA when transmitting
+when transmitting, peaks of 2 A can be reached
 this can be lowered to 1 A when an 470 µF capacitor is used
 
 hardware
@@ -59,7 +59,7 @@ FT2232R can only reset RTS once the option is set in EEPROM, but DTR goes and st
 CP2102 also includes a 700 mA resetable switch on the 5V pin which will prevent draining too much power on the USB port
 connect the UART port to a 2.5mm TRS stereo jack and plug it in the headphone to be able to flash and communicate with the phone
 connect the CHG port to a jack barrel (1.1mm inner diameter and 3.0 mm outer diameter) and plug it in the charger port to able to start the phone
-some phones also provides these pind on the back, but not all
+some phones also provides these pins on the back, but not all
 only though hole components have been used because it could not be all SMD (large capacitors are very expensive, and header connections to weak)
 this allows to have a single sided PCB which can also be produced in amateur labs
 
@@ -101,5 +101,45 @@ bugs:
 - RTS can be reset to high on close, but that need to be programmed in the FT4232H's EEPROM
 - the 3.3 V on DTR can not drive the pMOS because it's switching 5.0 V, but V_GS < 1.1 V
 - decoupling capacitors on the serial lines should improve the quality
-- try if with less feritte beads and decoupling capacaitors PLL still works
+- try if with less ferrite beads and decoupling capacitors PLL still works
 - Silabs CP2108 would be cheaper and easier to use than FT4232H
+
+FAQ
+===
+
+Q: What is the design goal?
+A: Provide a cheap way for everyone to remotely control motorola C1XX phones.
+By controlling the battery and charger power, it is possible to trigger the flashing procedure. This is done through the DTR signal of the USB to UART converter.
+The PCB is one layer, and only used through hole components, making it easy to produce and solder.
+The components are basic: diode, transistor, capacitor, resistor. This makes it cheap.
+
+Q: Don't have electrolytic capacitors high ESR?
+Ceramic capacitors don't come in such high values like 470µF.
+Tantalum capacitors rarely come in through hole packages.
+Either way, they would cost far too much.
+Electrolytic capacitors can have high values, and are cheap, particularly in through hole packages.
+The three capacitors are also in parallel, which lowers the global ESR.
+You can still used low ESR electrolytic capacitors instead.
+Since they are in free air, they should not heat up and dry out so fast.
+
+Q: Can capacitors replace a battery?
+I tested calling with them, and it works.
+During transmission bursts I measured voltage drops less than 0.3V, which is acceptable.
+It also depends on the power source and the USB to UART convert you use. They might have additional capacitors, or current limiters/fuses.
+
+Q: Why so many cables and connections?
+The inputs (from the USB to UART converter) and outputs (to the power/charging and serial/headphone sockets) are clearly labeled.
+Some phones provide pins behind the battery to control power and UART. That would require using pogo pins, which would be more complicated and expensive.
+External cables are simpler and it works on all phones.
+Since the goal is to be able to restart the phone automatically in a monitoring setup, I only attach the cables once and let it run the rest of the time.
+
+Q: Why not integrate the USB to UART converter on the board?
+I would have to make a double sided board to put the surface mount converter on.
+And the chip would cost more than a complete USB dongle from china.
+This contradicts the design goal.
+And you can pick any USB to UART converter, as long as it provides 5V and DTR.
+
+Q: Why not control multiple phones?
+The first designed included a USB device controlling 4 phones based on a FT4232 or CP2108.
+But you still need the boards in the phone.
+And USB to UART from china are far less expensive, even with an additional USB hub.