when reading i2c data, there are performance penalties from reading a few bytes at a time, and performance penalties from reading too much at a time. so probably the best method is to read only the range that you would need and nothing more. from my data it seems that from 32-64 words at a time is the ideal caching.

some cpu designs have cache for i2c, and some have dma access. loading more i2c than needed in many cases will have an overhead of cpu cycles. so you want to have the optimum amount of cache.

https://i2c.info/i2c-bus-specification

All i have is a data comparison for reading the mlx90640 sensor. it is because the sensor in my case requires reading of 768 words of data so 768x2 or 1536 bytes of data. with using an uno or nano processor with only 2k of ram this is not that practical, also there is a lot of overhead, the processor can not do anything else while getting that much data at a time.

i wanted to determine the optimum i2c cache where the benefits of reading data are maximized.

keep in mind that start and stop bits require about a byte cycle as well. (some times it is only 2 bits of clocks)

for example i2c in my case can go up to 1mhz. the protocol for i2c is the following

address of device to talk to start 2 bytes (this gets up to 10 bit address for access)

address of start of memory to read 1 to 2 bytes. in this is also a bit that says write or read

address of last byte of memory to read 2 bytes

there are are also start and stop bits and clocking of serial data. we treat those as bytes as well. 

os for each byte of data sent back (1 byte), but we request 2 at a time minimum for word data

so for our purposes we can assume that reading 2 bytes of 

data (minimum for memory read values for mlx90640 sensor)

READING 1 uint16_t (word) == 9 bytes of data to receive (clock pulses serially)

-------------------startbit---device_ID_---start_address_+end_address_DATA__end bit

 read 2bytes =1byte   +    1byte       +         2bytes         + 2bytes  +   2bytes + 1byte

so to read only 1 word of data from an i2c device in or case takes 9 bytes

READING 2 uint16_t (word)  == 11 bytes of data to receive (clock pulses serially)

to read 2 words of data takes 11 bytes

-------------------startbit---device_ID_---start_address_+end_address_DATA____DATA_end bit

 read 2bytes =1byte   +    1byte       +         2bytes         + 2bytes  +   2bytes +   2bytes + 1byte

the over head of 6 bytes is in every transfer even if we get several at a time

here is the theoretical performance increase based on the amount of bytes read

a 1 would be 100%

1 word is at 22% efficient

16 words  80% efficient or 3.6 times faster than 1 byte

 32 words is 90% efficient. 4.1 times faster than 1 byte

64 words is 94% efficient 4.3 times faster than 1 byte

128 words is 97% efficient 4.4 times faster than 1 byte

the table that shows the efficiency is at the end of this log.

here is the i2c code i use. i just read 32 or 64 at a time, and only when a cache miss occurs do i reload. i make sure that my reads of the data go as linear as possible this prevents over use of the caching.

the cache can be enabled and disabled. and it works without any other coding consideration other than trying to read data within 32 words in order. even if you don't it will still work.

you can see why i would be using a 64 word buffer, and am considering going down to a 32 word buffer so i can use the ram saved to cache other information (serial data caching to burst to screen)

in my case 16 or 32 words of data would be appropriate. i have other issues causing more overhead than the speed of the data transfer of i2c data. but you can see from below what the efficiency would be on size of buffer

this example uses i2c data to read 1 word to as many words as you want at a time.

uint16_t RamGetStoredInLocal(uint16_t value){

#if customSmallCacheForMemReads !=true //we read only 2 bytes at a time with a lot of address overhead
 MLX90640_I2CRead(MLX90640_address, 1024+value, 1,worddata);
return worddata[0];
#endif
#if customSmallCacheForMemReads ==true //we read only 2 bytes at a time with a lot of address overhead
//here is predictive loading. if y is different, we do different things. if y is different, we determin if y is incremental or further away. if further away we cache 64 values
uint8_t valueofy =(value>>6) ;//we are gettint the y line data /64 
if (linecache !=valueofy ){//this means we have a cache miss.
//7 bits wide 65408 //6bits wide 65472 //5bits wide is65504// for masking

//below is i2c data return.

MLX90640_I2CRead(MLX90640_address, 1024+(value&65472),64,SmallMemCache_i2c_efficency);

 linecache =valueofy ;  }//we take the small time to update the cache and change the line buffered

 //this part is alwasy ready on reads and if line is already cached data is instantly available. 
uint8_t valueofx =(value& 63); //32=31//64=63//128=127//0-X //this is a mask
return SmallMemCache_i2c_efficency[valueofx];//we return cached value most if time
#endif  //end of different cached or non cached methods

}

after that it becomes very small improvements

efficiency overhead.  data received at a time.   efficiency level 
7(bytes) overhead 0word(0bytes):::::::::0
7(bytes) overhead 1word(2bytes):::::::::0.2222222222222222
7(bytes) overhead 2word(4bytes):::::::::0.36363636363636365
7(bytes) overhead 3word(6bytes):::::::::0.46153846153846156
7(bytes) overhead 4word(8bytes):::::::::0.5333333333333333
7(bytes) overhead 5word(10bytes):::::::::0.5882352941176471
7(bytes) overhead 6word(12bytes):::::::::0.631578947368421
7(bytes) overhead 7word(14bytes):::::::::0.6666666666666666
7(bytes) overhead 8word(16bytes):::::::::0.6956521739130435
7(bytes) overhead 9word(18bytes):::::::::0.72
7(bytes) overhead 10word(20bytes):::::::::0.7407407407407407
7(bytes) overhead 11word(22bytes):::::::::0.7586206896551724
7(bytes) overhead 12word(24bytes):::::::::0.7741935483870968
7(bytes) overhead 13word(26bytes):::::::::0.7878787878787878
7(bytes) overhead 14word(28bytes):::::::::0.8
7(bytes) overhead 15word(30bytes):::::::::0.8108108108108109
7(bytes) overhead 16word(32bytes):::::::::0.8205128205128205
7(bytes) overhead 17word(34bytes):::::::::0.8292682926829268
7(bytes) overhead 18word(36bytes):::::::::0.8372093023255814
7(bytes) overhead 19word(38bytes):::::::::0.8444444444444444
7(bytes) overhead 20word(40bytes):::::::::0.851063829787234
7(bytes) overhead 21word(42bytes):::::::::0.8571428571428571
7(bytes) overhead 22word(44bytes):::::::::0.8627450980392157
7(bytes) overhead 23word(46bytes):::::::::0.8679245283018868
7(bytes) overhead 24word(48bytes):::::::::0.8727272727272727
7(bytes) overhead 25word(50bytes):::::::::0.8771929824561403
7(bytes) overhead 26word(52bytes):::::::::0.8813559322033898
7(bytes) overhead 27word(54bytes):::::::::0.8852459016393442
7(bytes) overhead 28word(56bytes):::::::::0.8888888888888888
7(bytes) overhead 29word(58bytes):::::::::0.8923076923076924
7(bytes) overhead 30word(60bytes):::::::::0.8955223880597015
7(bytes) overhead 31word(62bytes):::::::::0.8985507246376812
7(bytes) overhead 32word(64bytes):::::::::0.9014084507042254
7(bytes) overhead 33word(66bytes):::::::::0.9041095890410958
7(bytes) overhead 34word(68bytes):::::::::0.9066666666666666
7(bytes) overhead 35word(70bytes):::::::::0.9090909090909091
7(bytes) overhead 36word(72bytes):::::::::0.9113924050632911
7(bytes) overhead 37word(74bytes):::::::::0.9135802469135802
7(bytes) overhead 38word(76bytes):::::::::0.9156626506024096
7(bytes) overhead 39word(78bytes):::::::::0.9176470588235294
7(bytes) overhead 40word(80bytes):::::::::0.9195402298850575
7(bytes) overhead 41word(82bytes):::::::::0.9213483146067416
7(bytes) overhead 42word(84bytes):::::::::0.9230769230769231
7(bytes) overhead 43word(86bytes):::::::::0.9247311827956989
7(bytes) overhead 44word(88bytes):::::::::0.9263157894736842
7(bytes) overhead 45word(90bytes):::::::::0.9278350515463918
7(bytes) overhead 46word(92bytes):::::::::0.9292929292929293
7(bytes) overhead 47word(94bytes):::::::::0.9306930693069307
7(bytes) overhead 48word(96bytes):::::::::0.9320388349514563
7(bytes) overhead 49word(98bytes):::::::::0.9333333333333333
7(bytes) overhead 50word(100bytes):::::::::0.9345794392523364
7(bytes) overhead 51word(102bytes):::::::::0.9357798165137615
7(bytes) overhead 52word(104bytes):::::::::0.9369369369369369
7(bytes) overhead 53word(106bytes):::::::::0.9380530973451328
7(bytes) overhead 54word(108bytes):::::::::0.9391304347826087
7(bytes) overhead 55word(110bytes):::::::::0.9401709401709402
7(bytes) overhead 56word(112bytes):::::::::0.9411764705882353
7(bytes) overhead 57word(114bytes):::::::::0.9421487603305785
7(bytes) overhead 58word(116bytes):::::::::0.943089430894309
7(bytes) overhead 59word(118bytes):::::::::0.944
7(bytes) overhead 60word(120bytes):::::::::0.9448818897637795
7(bytes) overhead 61word(122bytes):::::::::0.9457364341085271
7(bytes) overhead 62word(124bytes):::::::::0.9465648854961832
7(bytes) overhead 63word(126bytes):::::::::0.9473684210526315
7(bytes) overhead 64word(128bytes):::::::::0.9481481481481482
7(bytes) overhead 65word(130bytes):::::::::0.948905109489051
7(bytes) overhead 66word(132bytes):::::::::0.9496402877697842
7(bytes) overhead 67word(134bytes):::::::::0.950354609929078
7(bytes) overhead 68word(136bytes):::::::::0.951048951048951
7(bytes) overhead 69word(138bytes):::::::::0.9517241379310345
7(bytes) overhead 70word(140bytes):::::::::0.9523809523809523
7(bytes) overhead 71word(142bytes):::::::::0.9530201342281879
7(bytes) overhead 72word(144bytes):::::::::0.9536423841059603
7(bytes) overhead 73word(146bytes):::::::::0.954248366013072
7(bytes) overhead 74word(148bytes):::::::::0.9548387096774194
7(bytes) overhead 75word(150bytes):::::::::0.9554140127388535
7(bytes) overhead 76word(152bytes):::::::::0.9559748427672956
7(bytes) overhead 77word(154bytes):::::::::0.9565217391304348
7(bytes) overhead 78word(156bytes):::::::::0.9570552147239264
7(bytes) overhead 79word(158bytes):::::::::0.9575757575757575
7(bytes) overhead 80word(160bytes):::::::::0.9580838323353293
7(bytes) overhead 81word(162bytes):::::::::0.9585798816568047
7(bytes) overhead 82word(164bytes):::::::::0.9590643274853801
7(bytes) overhead 83word(166bytes):::::::::0.9595375722543352
7(bytes) overhead 84word(168bytes):::::::::0.96
7(bytes) overhead 85word(170bytes):::::::::0.96045197740113
7(bytes) overhead 86word(172bytes):::::::::0.9608938547486033
7(bytes) overhead 87word(174bytes):::::::::0.9613259668508287
7(bytes) overhead 88word(176bytes):::::::::0.9617486338797814
7(bytes) overhead 89word(178bytes):::::::::0.9621621621621622
7(bytes) overhead 90word(180bytes):::::::::0.9625668449197861
7(bytes) overhead 91word(182bytes):::::::::0.9629629629629629
7(bytes) overhead 92word(184bytes):::::::::0.9633507853403142
7(bytes) overhead 93word(186bytes):::::::::0.9637305699481865
7(bytes) overhead 94word(188bytes):::::::::0.9641025641025641
7(bytes) overhead 95word(190bytes):::::::::0.9644670050761421
7(bytes) overhead 96word(192bytes):::::::::0.964824120603015
7(bytes) overhead 97word(194bytes):::::::::0.9651741293532339
7(bytes) overhead 98word(196bytes):::::::::0.9655172413793104
7(bytes) overhead 99word(198bytes):::::::::0.9658536585365853
7(bytes) overhead 100word(200bytes):::::::::0.966183574879227
7(bytes) overhead 101word(202bytes):::::::::0.9665071770334929
7(bytes) overhead 102word(204bytes):::::::::0.966824644549763
7(bytes) overhead 103word(206bytes):::::::::0.9671361502347418
7(bytes) overhead 104word(208bytes):::::::::0.9674418604651163
7(bytes) overhead 105word(210bytes):::::::::0.967741935483871
7(bytes) overhead 106word(212bytes):::::::::0.9680365296803652
7(bytes) overhead 107word(214bytes):::::::::0.9683257918552036
7(bytes) overhead 108word(216bytes):::::::::0.968609865470852
7(bytes) overhead 109word(218bytes):::::::::0.9688888888888889
7(bytes) overhead 110word(220bytes):::::::::0.9691629955947136
7(bytes) overhead 111word(222bytes):::::::::0.9694323144104804
7(bytes) overhead 112word(224bytes):::::::::0.9696969696969697
7(bytes) overhead 113word(226bytes):::::::::0.9699570815450643
7(bytes) overhead 114word(228bytes):::::::::0.9702127659574468
7(bytes) overhead 115word(230bytes):::::::::0.9704641350210971
7(bytes) overhead 116word(232bytes):::::::::0.9707112970711297
7(bytes) overhead 117word(234bytes):::::::::0.970954356846473
7(bytes) overhead 118word(236bytes):::::::::0.9711934156378601
7(bytes) overhead 119word(238bytes):::::::::0.9714285714285714
7(bytes) overhead 120word(240bytes):::::::::0.97165991902834
7(bytes) overhead 121word(242bytes):::::::::0.9718875502008032
7(bytes) overhead 122word(244bytes):::::::::0.9721115537848606
7(bytes) overhead 123word(246bytes):::::::::0.9723320158102767
7(bytes) overhead 124word(248bytes):::::::::0.9725490196078431
7(bytes) overhead 125word(250bytes):::::::::0.9727626459143969
7(bytes) overhead 126word(252bytes):::::::::0.972972972972973
7(bytes) overhead 127word(254bytes):::::::::0.9731800766283525
7(bytes) overhead 128word(256bytes):::::::::0.973384030418251