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esp_random: Add some documentation about using before RF subsystem is on

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Angus Gratton
2017-07-11 11:23:33 +08:00
committed by Angus Gratton
parent 9b30f66fac
commit 3bb90bab97
2 changed files with 14 additions and 6 deletions
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10
components/esp32/hw_random.c
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@@ -24,11 +24,11 @@
uint32_t IRAM_ATTR esp_random(void)
{
/* The PRNG which implements WDEV_RANDOM register gets 2 bits
* of extra entropy from a hardware randomness source every APB clock cycle.
* To make sure entropy is not drained faster than it is added,
* this function needs to wait for at least 16 APB clock cycles after reading
* previous word. This implementation may actually wait a bit longer
* due to extra time spent in arithmetic and branch statements.
* of extra entropy from a hardware randomness source every APB clock cycle
* (provided WiFi or BT are enabled). To make sure entropy is not drained
* faster than it is added, this function needs to wait for at least 16 APB
* clock cycles after reading previous word. This implementation may actually
* wait a bit longer due to extra time spent in arithmetic and branch statements.
*
* As a (probably unncessary) precaution to avoid returning the
* RNG state as-is, the result is XORed with additional