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power: regulator: stm32: vrefbuf: fix a possible overshoot when re-enabling

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There maybe an overshoot:
- when disabling, then re-enabling vrefbuf too quickly
- or upon platform reset as external capacitor maybe slow
  discharging (VREFBUF is HiZ at reset by default).
VREFBUF is used by ADC/DAC on some boards. An overshoot on the reference
voltage make the conversions inaccurate for a short period of time. So:
- Don't put the VREFBUF in HiZ when disabling, to force an active
  discharge.
- Enforce a 1ms OFF/ON delay, also upon reset

Penalty is a 1ms delay is applied (even for a cold boot), when enabling
VREFBUF.

Fixes: 93cf0ae775 ("power: regulator: Add support for stm32-vrefbuf")

Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@st.com>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>
This commit is contained in:
Fabrice Gasnier 2020-06-12 10:40:58 +02:00 committed by Patrick Delaunay
parent 2ee5005dea
commit 8b8c99bd3e
1 changed files with 14 additions and 2 deletions
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16
drivers/power/regulator/stm32-vrefbuf.c
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@ -43,8 +43,20 @@ static int stm32_vrefbuf_set_enable(struct udevice *dev, bool enable)
u32 val;
int ret;
clrsetbits_le32(priv->base + STM32_VREFBUF_CSR, STM32_HIZ | STM32_ENVR,
enable ? STM32_ENVR : STM32_HIZ);
if (enable && !(readl(priv->base + STM32_VREFBUF_CSR) & STM32_ENVR)) {
/*
* There maybe an overshoot:
* - when disabling, then re-enabling vrefbuf too quickly
* - or upon platform reset as external capacitor maybe slow
* discharging (VREFBUF is HiZ at reset by default).
* So force active discharge (HiZ=0) for 1ms before enabling.
*/
clrbits_le32(priv->base + STM32_VREFBUF_CSR, STM32_HIZ);
udelay(1000);
}
clrsetbits_le32(priv->base + STM32_VREFBUF_CSR, STM32_ENVR,
enable ? STM32_ENVR : 0);
if (!enable)
return 0;