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Parts Selection
===============

Where possible, we're selecting parts purchasable from LCSC because that way
we can combine shipping from JLCPCB when we order the prototypes...

Prices, where stated, are correct as of Saturday April 13th 2019

Pricing calculated based on the idea of ordering enough parts to build five
so slop components up to ten, though perhaps not for the CPUs.

Large ICs
---------

### Processor

Processor will be an STM32F103C (LQFP48) and the options are:

C8303 - C6 part, 32K flash  - £1.66 in ones, £11.63 for 7
C9861 - C8 part, 64K flash  - £2.02 in ones, £14.17 for 7
C8304 - CB part, 128K flash - £2.05 in ones, £14.32 for 7

For prototypes, we're going with C8304 for the easiest time of things.
Production may drop to C8303 if we can make the firmware small enough.

### Hex inverter

We used an AHCU part in the original ekey, and an ALS in the original
prototype.  We're trying an HCU part for this variant.

C129521 - Hex inverter - £0.084 in tens, £0.84 for 10

https://datasheet.lcsc.com/szlcsc/NXP-Semicon-74HCU04PW_C129521.pdf

Power supplies
--------------

### 3.3v supply

We don't need a huge amount of power here, so 150mA or so of LDO should
be plenty, let's get the most efficient we can though.

C83932 - 3v3 250mA - £0.0171 in 20s, £0.342 for 20 - MOQ



### HV DC-DC

C181783 - MT3540-F23 - £0.0511 in fives - £0.511 for 10

https://datasheet.lcsc.com/szlcsc/XI-AN-Aerosemi-Tech-MT3540-F23_C181783.pdf

The -F23 has a 1.23v reference so a 150k/10k divider ought to give us approx.
20 volts on the HV line.

### Inductor for HV DC-DC

We need a wire-wound inductor with shielding.

C218837 - 10uH 3x3x1.2 inductor - £0.0888 in 10s - £0.888 for 10

https://datasheet.lcsc.com/szlcsc/Cyntec-PST031B-100MS_C218837.pdf

### Diode for DC-DC

Switching diode makes sense, min reverse voltage support 30v since we're
switching to 20v, most switching diodes are 100v reverse capable.

Our chosen diode pack (MMBD4148CA) is two diodes in one package, with the
third pin the common -ve.

C181116 - SOT-23-3 diode, £0.0064 in 50s - £0.32 for 50

Noise Generators
----------------

Noise generation needs moderately high hFE transistors.  We used BC817-25-N
in the original ekey (though I think I used random NPN things when I built the
prototype.  We need four per noise generator, so 40 for building 10 units.

C181168 - SOT-23-3 BEC layout - 0.0046 in 50s - £0.23 for 50

Misc parts
----------

### USB protection chip

While this isn't large, it's also not generic cress, so… we need a USBUF01W6
to protect a device.  This also includes the presence pullup so we're able
to save space by not needing additional transistors and resistors.

C123726 - USBUF01W6 - £0.3664 in 10s, £3.664 for 10

### Crystal

Driving the STM32 HSE can be done with anything from 8 to 16 MHz.  To be
effective we need it to be able to generate the 48MHz USB clock which means
we want 8 or 12 really.  8 is most common on developer boards, so let's stick
with that…

C133333 - SMD-5032_4P - £0.1314 in 10s, £1.314 for 10
XTAL across pins 1 and 3

https://datasheet.lcsc.com/szlcsc/Zhejiang-Abel-Elec-SMD-5032-4P8M20pf20ppm_C133333.pdf

### USB plug/socket

For the production units we'll need an A-plug, but for prototype development
we'll fit B-sockets of some kind.  Full-on chunky B connectors will be very
physically stable which is good for a prototype.

C46392 - B socket - £0.0591 in 5s - £0.591 for 10

https://datasheet.lcsc.com/szlcsc/Jing-Extension-of-the-Electronic-Co-LCSC-USB-BFemale90Degree-of-iron-feet-White-plastic-Not-high-temperature_C46392.pdf

### LED(s)

For the prototype we should fit a bunch'o'LEDs so that we can develop usefully
when debugging gets tough.

C205445 - Orange-red 0603 - £0.0044 in 50s - £0.22 for 50

C205450 - Green-Yellow 0603 - £0.0049 in 50s - £0.245 for 50

2v

Goal 5-10mA

C193191 - Blue 0603 - £0.0049 in 50s - £0.245 for 50

2.6v

0.6v drop 5-10mA

Probably fit one of each to each board for convenience.

### Jumpers

We'll need jumpers in order to fit the boot signals etc

C100114 - Basic jumper (Black) - £0.0041 in 50s - £0.205 for 50

### Buttons

We want to fit a reset button to the board for ease of pushing reset.

C294505 - Basic N/O SPST tactile - £0.0148 in 20s - £0.296 for 20

https://datasheet.lcsc.com/szlcsc/BRIGHT-TSA061G90-250_C294505.pdf