These are some errors when the board dropping occurs.
Only board cs3 is found by cgminer and other board will eventually work or dropped after a long run time.
It seems that the spi bus fails to initiate the boards but im not sure.
Temperature is not the problem as the board run around 44C.
[2016-11-07 23:18:48] Started cgminer 3.9.0
[2016-11-07 23:18:48] Run Reset=1
[2016-11-07 23:18:48] ST MCU hardware reset start
[2016-11-07 23:18:52] SPI Speed 4000 kHz
[2016-11-07 23:18:52] ST MCU - Enable (Pre-header)
[2016-11-07 23:18:52] Run temperature 35C
[2016-11-07 23:18:52] A1 = 1200,7
[2016-11-07 23:18:52] A1 PLL Clock = 1200MHz
[2016-11-07 23:18:52] A1 = 1200,7
[2016-11-07 23:18:52] A1 PLL Clock = 1200MHz
[2016-11-07 23:18:52] A1 = 1200,7
[2016-11-07 23:18:52] A1 PLL Clock = 1200MHz
[2016-11-07 23:18:52] A1 = 1200,7
[2016-11-07 23:18:52] A1 PLL Clock = 1200MHz
[2016-11-07 23:18:52] A1 = 1200,7
[2016-11-07 23:18:52] A1 PLL Clock = 1200MHz
[2016-11-07 23:18:52] A1 = 1200,7
[2016-11-07 23:18:52] A1 PLL Clock = 1200MHz
[2016-11-07 23:18:52] AUTO GPIO CS
&[2016-11-07 23:18:52] Failure(cs0)(2): missing ACK for cmd 0x02
[2016-11-07 23:18:52] ACK(cs0) timeout:cmd_POWER_ON_BCAST-0.0620s
&[2016-11-07 23:18:55] Failure(cs0)(2): missing ACK for cmd 0x04
[2016-11-07 23:18:55] ACK(cs0) timeout:cmd_RESET_BCAST - 0.62 ms
&[2016-11-07 23:18:55] Failure(cs1)(2): missing ACK for cmd 0x02
[2016-11-07 23:18:55] ACK(cs1) timeout:cmd_POWER_ON_BCAST-0.0574s
&[2016-11-07 23:18:58] Failure(cs1)(2): missing ACK for cmd 0x04
[2016-11-07 23:18:58] ACK(cs1) timeout:cmd_RESET_BCAST - 0.62 ms
&[2016-11-07 23:18:58] Failure(cs2)(2): missing ACK for cmd 0x02
[2016-11-07 23:18:58] ACK(cs2) timeout:cmd_POWER_ON_BCAST-0.0548s
&[2016-11-07 23:19:01] Failure(cs2)(2): missing ACK for cmd 0x04
[2016-11-07 23:19:01] ACK(cs2) timeout:cmd_RESET_BCAST - 0.71 ms
[2016-11-07 23:19:01] spidev0.0(cs3): Found 20 A2 chips
[2016-11-07 23:19:01] Found chip 1 with 100 active cores
[2016-11-07 23:19:01] Found chip 2 with 100 active cores
[2016-11-07 23:19:01] Found chip 3 with 100 active cores
[2016-11-07 23:19:01] Found chip 4 with 100 active cores
[2016-11-07 23:19:01] Found chip 5 with 100 active cores
[2016-11-07 23:19:01] Found chip 6 with 100 active cores
[2016-11-07 23:19:01] Found chip 7 with 100 active cores
[2016-11-07 23:19:01] Found chip 8 with 100 active cores
[2016-11-07 23:19:01] Found chip 9 with 100 active cores
[2016-11-07 23:19:01] Found chip 10 with 100 active cores
[2016-11-07 23:19:01] Found chip 11 with 100 active cores
[2016-11-07 23:19:01] Found chip 12 with 100 active cores
[2016-11-07 23:19:01] Found chip 13 with 100 active cores
[2016-11-07 23:19:01] Found chip 14 with 100 active cores
[2016-11-07 23:19:01] Found chip 15 with 100 active cores
[2016-11-07 23:19:01] Found chip 16 with 100 active cores
[2016-11-07 23:19:01] Found chip 17 with 100 active cores
[2016-11-07 23:19:01] Found chip 18 with 100 active cores
[2016-11-07 23:19:01] Found chip 19 with 100 active cores
[2016-11-07 23:19:01] Found chip 20 with 100 active cores
[2016-11-07 23:19:01] Found 20 chips with total 2000 active cores
[2016-11-07 23:19:01] PI(cs5) match the cg and dr
[2016-11-07 23:19:01] SPI(cs3) A1_SetA1Vol
& [2016-11-07 23:19:01] Failure(cs4)(2): missing ACK for cmd 0x02
[2016-11-07 23:19:01] ACK(cs4) timeout:cmd_POWER_ON_BCAST-0.0739s
& [2016-11-07 23:19:04] Failure(cs4)(2): missing ACK for cmd 0x04
[2016-11-07 23:19:04] ACK(cs4) timeout:cmd_RESET_BCAST - 0.68 ms
& [2016-11-07 23:19:04] Failure(cs5)(2): missing ACK for cmd 0x02
[2016-11-07 23:19:04] ACK(cs5) timeout:cmd_POWER_ON_BCAST-0.0615s
& [2016-11-07 23:19:07] Failure(cs5)(2): missing ACK for cmd 0x04
[2016-11-07 23:19:07] ACK(cs5) timeout:cmd_RESET_BCAST - 0.73 ms
[2016-11-07 23:19:07] A2 boards=1, active cores=2000, speed=47.76M
[2016-11-07 23:19:07] Tan total_devices : 1
[2016-11-07 23:19:07] Tan cs_cnt_error
[2016-11-07 23:19:07] Probing for an alive pool
[2016-11-07 23:19:07] Switching to pool 1 stratum+tcp://scrypt.eu.nicehash.com:3333 - first alive pool
[2016-11-07 23:19:07] Pool 0 stratum+tcp://scrypt.eu.nicehash.com:3333 alive, testing stability
[2016-11-07 23:19:07] Switching to pool 0 stratum+tcp://scrypt.eu.nicehash.com:3333
[2016-11-07 23:19:07] Pool 1 difficulty changed to 4096
[2016-11-07 23:19:07] Network diff set to 1.45K
[2016-11-07 23:19:07] Pool 0 difficulty changed to 4096
[2016-11-07 23:19:07] Stratum from pool 0 requested work restart
[2016-11-07 23:19:08] Failure(cs3)(40): missing ACK for cmd 0x02
[2016-11-07 23:19:08] ACK(cs3) timeout:cmd_POWER_ON_BCAST-0.2969s
[2016-11-07 23:19:08] Pool 2 difficulty changed to 4096
[2016-11-07 23:19:10] Accepted 02da27f6 Diff 23K/4096 BA2 0 pool 0
[2016-11-07 23:19:10] Accepted 02da27f6 Diff 23K/4096 BA2 0