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machine/nrf: stop the RNG after each read #5851

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Merged
deadprogram merged 1 commit into from
Oct 9, 2026
Merged

machine/nrf: stop the RNG after each read #5851

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2 changes: 1 addition & 1 deletion builder/testdata/binary-size.txt
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Original file line number Diff line number Diff line change
@@ -1,6 +1,6 @@
target package opt code rodata data bss
hifive1b examples/echo z 4558 402 0 2276
microbit examples/serial z 3017 439 8 2272
microbit examples/serial z 3001 439 8 2272
wioterminal examples/pininterrupt z 8458 1798 148 7536
cortex-m-qemu ./testdata/size-corpus z 9923 1301 80 4188
cortex-m-qemu ./testdata/size-corpus 2 17482 862 72 4188
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16 changes: 7 additions & 9 deletions src/machine/machine_nrf.go
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Original file line number Diff line number Diff line change
Expand Up @@ -319,18 +319,12 @@ func (i2c *I2C) signalStop() error {
return nil
}

var rngStarted = false

// getRNG returns 32 bits of non-deterministic random data based on internal thermal noise.
// According to Nordic's documentation, the random output is suitable for cryptographic purposes.
func getRNG() (ret uint32, err error) {
// There's no apparent way to check the status of the RNG peripheral's task, so simply start it
// to avoid deadlocking while waiting for output.
if !rngStarted {
nrf.RNG.TASKS_START.Set(1)
nrf.RNG.SetCONFIG_DERCEN(nrf.RNG_CONFIG_DERCEN_Enabled)
rngStarted = true
}
nrf.RNG.SetCONFIG_DERCEN(nrf.RNG_CONFIG_DERCEN_Enabled)
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deadprogram marked this conversation as resolved.
nrf.RNG.EVENTS_VALRDY.Set(0)
nrf.RNG.TASKS_START.Set(1)

// The RNG returns one byte at a time, so stack up four bytes into a single uint32 for return.
for i := 0; i < 4; i++ {
Expand All @@ -343,6 +337,10 @@ func getRNG() (ret uint32, err error) {
nrf.RNG.EVENTS_VALRDY.Set(0)
}

// A running RNG uses current all the time, so stop it after each read.
// See https://docs.nordicsemi.com/bundle/ps_nrf52840/page/rng.html, electrical specification.
nrf.RNG.TASKS_STOP.Set(1)

return ret, nil
}

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