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annotate doc/How-flash-really-works @ 1012:11391cb6bdc0

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patch from fixeria: doc change from SE K2x0 to K2xx Since their discovery in late 2022, Sony Ericsson K200 and K220 phones were collectively referred to as SE K2x0 in FreeCalypso documentation. However, now that SE K205 has been discovered as yet another member of the same family (same PCBA in different case), it makes more sense to refer to the whole family as SE K2xx.
author Mychaela Falconia <falcon@freecalypso.org>
date Mon, 23 Sep 2024 12:23:20 +0000
parents 39a6090a052a
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39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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1 How NOR flash memory really works
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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2 =================================
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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3
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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4 The type of flash memory used in Calypso GSM devices is formally known as NOR
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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5 flash. Most embedded software programmers and tinkerers know the fundamental
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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6 principle of how NOR flash works: any bit can be transitioned from a '1' to a
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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7 '0' at any time in any combination (an operation called programming), but the
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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8 opposite transition (from '0' to '1' bits, an operation called erasure) can only
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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9 be done on fairly large sectors - you can erase a sector and make it all 1s,
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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10 but changing bits from 0 to 1 individually or in any smaller granularity
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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11 (smaller than a sector) is impossible.
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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12
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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13 What many "software-minded" programmers and tinkerers don't realize, however,
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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14 is that sector erasure is not an elementary or atomic operation that magically
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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15 "makes all bits 1s" in one motion. Instead it is a complex process with two or
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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16 three substeps:
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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17
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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18 1) Before starting the physical process of erasure, one has to go through all
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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19 bits in the to-be-erased sector and make them all 0s. Any bits that are in
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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20 '1' state when the sector erase operation is commanded MUST be programmed to
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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21 '0' state before the actual erasure begins! In the language of flash chip
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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22 industry, this step is called preprogramming. In the case of flash chips
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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23 that are used in Calypso GSM devices (all known ones), this preprogramming
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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24 step is done internally by the chip, so that you as the user or software
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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25 developer are not aware of it - but it is there nonetheless. The chip does
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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26 NOT magically "wave" all bits in the sector into '1' state, instead it first
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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27 makes them all '0' internally, and only then erases.
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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28
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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29 2) Once every bit in the sector is in '0' state, the real physics of erasure
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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30 begins. All bit cells in the sector are physically acted upon at once in
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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31 this step, and because it is a probabilistic process involving a Gaussian
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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32 distribution, all bit cells need to be in the fully programmed state before
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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33 they begin their shared journey toward the erased state.
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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34
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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35 3) The step of preprogramming every bit to 0 prior to erasure prevents the
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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36 absolutely unacceptable condition of gross overerasure - but given the
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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37 Gaussian distribution, some bit cells may still get a little overerased.
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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38 Many (most? all? not sure) flash chips therefore implement a third internal
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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39 step before the software-visible "erase" operation is declared complete:
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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40 they go through all bit cells in the just-erased sector, check for
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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41 overerasure, and "soft-program" (move slightly to the right in the Vt
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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42 distribution) any overerased cells. This step is called post-erase
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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43 conditioning or recovery.
39a6090a052a doc/How-flash-really-works: article written
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44
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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45 The above process was originally explained to me (Mother Mychaela) some years
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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46 ago (around 2008, IIRC) by a Spansion support engineer on a conference call at
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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47 my then day job - it was a project for a customer who was big and powerful
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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48 enough to get top-tier support from chip vendors. More recently, however, some
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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49 other flash vendors have posted public documents that provide the same
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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50 explanation - here is one from Renesas/Adesto:
39a6090a052a doc/How-flash-really-works: article written
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51
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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52 https://www.freecalypso.org/pub/embedded/flash/REN_an500_APN_20210702_1.pdf
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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53
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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54 Even though the above document was written by Renesas (or more precisely, the
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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55 part that was originally Adesto), the theory described therein applies just as
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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56 well to Intel, Spansion and Samsung flash chips that are used in Calypso GSM
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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57 devices. For anyone who wishes to know how NOR flash memory really works, I
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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58 strongly recommend reading that Renesas appnote - it is a good description.
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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59
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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60 Additional note on terminology: describing the two states of a flash memory cell
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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61 as '0' and '1', like I did above, is only a convenience for software-minded
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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62 people. A more proper view is to think in terms of a "programmed state" and an
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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63 "erased state" for each bit cell. History and tradition are such that flash
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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64 chips return '0' on read in the programmed state and '1' in the erased state
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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65 (this tradition probably originates from the fact that the actual NV storage
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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66 element, a transistor, conducts read current in the erased state), at least for
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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67 the main flash array - however, when flash memory elements are used for
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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68 additional purposes such as write protection controls, it is best to think
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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69 natively in terms of programmed and erased states. For the latter kind of
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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70 special applications, an opposite polarity may be applied in read-bit values.
39a6090a052a doc/How-flash-really-works: article written
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71
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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72 One straightforward take-away from this theory is that flash endurance is really
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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73 about program-erase cycles, rather than number of program or number of erase
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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74 operations. Every time you give a sector erase command, every bit in that
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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75 sector cycles through the fully programmed (0) state first before becoming
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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76 erased (1), irrespective of whether or not you programmed into it on your own!
39a6090a052a doc/How-flash-really-works: article written
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77 Hence every bit-cell of the affected sector always goes through a full
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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78 program-erase cycle, and all bits in a given sector are always cycled equally,
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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79 irrespective of whether they get written with mostly-0s or mostly-1s in between
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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80 erase cycles.
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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81
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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82 Another situation where this raw physics gets exposed to the user is the case
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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83 of special-purpose non-volatile bits in flash chips outside of the main flash
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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84 memory array - for example, Persistent Protection Bits (PPBs) in some Spansion
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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85 and Samsung flash chips. While program and erase commands for the main flash
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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86 array invoke chip-internal mechanisms that take care of everything and present
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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87 a sane model of 0s and 1s to software, Spansion PL-J PPB program and erase
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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88 commands expose raw guts: there is a command that applies a raw program pulse
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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89 to a single PPB, and there is a command that applies a raw erase pulse to the
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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90 NV memory element (like a little sector of its own) that holds all PPBs.
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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91 Applying the erase pulse without preprogramming every PPB first would be very
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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92 bad (see Renesas appnote about the badness of overerasure) - hence in a seeming
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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93 paradox, one has to explicitly lock every sector before applying PPB erase
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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94 pulses that will eventually unlock everything!
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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95
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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96 Our flash ppb-erase-all command does implement the preprogramming step before
39a6090a052a doc/How-flash-really-works: article written
Mychaela Falconia <falcon@freecalypso.org>
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97 actual erasure, and the present document (hopefully) explains why.