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minnie/doc/Design-spec: finished in the first pass
author Mychaela Falconia <falcon@freecalypso.org>
date Sun, 01 Oct 2023 08:17:05 +0000
parents 971c05950675
children
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971c05950675 starpoints in core: commit to using pcb-rnd intnoconn
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1 iWOW's TR-800 module, whose reverse-engineered PCB layout will serve as a
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2 reference for the Calypso core of FC Venus, is nothing but a mass-produced
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3 version of TI's Leonardo reference design. We don't have any original PCB
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4 layout files for Leonardo, but we do have schematics, in 3 different versions:
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5
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6 ftp://ftp.freecalypso.org/pub/GSM/Calypso/Leonardo_plus_RD122.pdf
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7 ftp://ftp.freecalypso.org/pub/GSM/Calypso/Leonardo_plus_quadband_schem.pdf
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8 ftp://ftp.freecalypso.org/pub/GSM/Calypso/Leonardo_rev05.pdf
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9
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10 TR-800 is based on quadband Leonardo+ (that's what makes it interesting), but
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11 the hobbled 2-band version of Leonardo is the only one for which we have TI's
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12 original DSN (OrCAD) schematic file, in addition to the corresponding PDF:
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14 ftp://ftp.freecalypso.org/pub/GSM/Calypso/Leonardo_rev05.dsn
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15
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16 TI's group in Aalborg (Denmark) that designed and built their development boards
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17 used OrCAD for schematics and PADS for PCB layout. We have their PADS PCB
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18 design file for a different board from about the same time frame (E-Sample),
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19 but not for any Leonardo variant.
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20
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21 If you look at the baseband page of Leonardo schematics (this part is the same
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22 in all 3 versions), you will see 3 symbols denoting star routing: HST201 and
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23 HST202 (labeled "STAR_CONFIG") split V-DBB net into VDD-PLL and VDD-CORE
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24 branches, and HST203 (labeled "GND_POINT") creates a special ground for the
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25 32.768 kHz oscillator circuit, connected to the rest of board ground at just
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26 one point.
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28 This star routing in TI's reference version of Calypso/Leonardo core exists not
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29 only in TI's schematics, but also in the physical PCB layout which we have
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30 recovered from TR-800 by reverse engineering. If you look at the Gerber files
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31 which we received from the PCB RE company (published on our FTP site in 2021),
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32 specifically the OKPCBM~1.G1 gerber file corresponding to the inner layer just
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33 below the surface (L2 or L5 depending on which way you count), you can see the
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34 physical features that were once produced, presumably in PADS, to effect the
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35 star routing depicted in the schematics.
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36
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37 The physical PCB star routing feature corresponding to HST201 & HST202 resides
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38 just underneath C214 footprint. There is a microvia (blind via going only one
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39 layer in) inside C214 pad 1 (the left pad in the Gerber view), and on the inner
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40 layer where this microvia goes, one can clearly see the feature that splits the
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41 V-DBB net coming from C214.1 into VDD-PLL and VDD-CORE branches. The lower (in
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42 the Gerber view) branch that goes to two microvias back to the surface is
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43 VDD-PLL, whereas the other branch that goes to an inner buried via (spanning L2
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44 through L5) is VDD-CORE.
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45
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46 The other star routing feature in the TR-800 PCB corresponding to TI's HST203
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47 can be seen in the far lower right corner of the same OKPCBM~1.G1 gerber file:
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48 look for an L shape that connects a short thin trace (going to a microvia to the
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49 surface) to the surrounding GND copper pour. The short thin trace connected to
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50 the GND copper pour by the L shape is what we named GND_32khz in
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51 src/core/dbb_block.v, and the L shape on the inner layer is what eventually
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52 became of HST203.
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53
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54 The present ueda netlist source code for FC Venus is written with the
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55 expectation that the same pseudo-component EDA trick for star routing will be
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56 used on our board, using intnoconn feature of pcb-rnd:
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57
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58 http://repo.hu/cgi-bin/pool.cgi?project=pcb-rnd&cmd=show&node=intnoconn
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59
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60 There is one difference in this regard between TI's original version and ours.
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61 In TI's original version all 3 pseudo-component objects HST201, HST202 and
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62 HST203 were 2-way stars, with HST201 and HST202 being somehow combined in PADS
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63 layout to form the desired 3-way star. In our version 3-way star HST200
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64 explicitly replaces TI's HST201+HST202 pair. The actual pcb-rnd subcircuits
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65 for the 3-way star HST200 and the 2-way star HST203 will need to be created by
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66 the PCB layout engineer.