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Programmer's Guide

Pad Attributes

Software should determine and program the pad attributes at startup, or reprogram the attributes when the functionality requirements change at runtime.

This can be achieved by writing to the MIO_PAD_ATTR_0 and DIO_PAD_ATTR_0 registers. Note that the IO attributes should be configured before enabling muxed IOs going through the pinmux matrix in order to avoid undesired electrical behavior and/or contention at the pads.

The pad attributes configuration can be locked down individually for each pad via the MIO_PAD_ATTR_REGWEN_0 and DIO_PAD_ATTR_REGWEN_0 registers. The configuration can then not be altered anymore until the next system reset.

The following pad attributes are supported by this register layout by default:

ATTR Bits Description Access
0 Input/output inversion WARL
1 Virtual open drain enable WARL
2 Pull enable WARL
3 Pull select (0: down, 1: up) WARL
4 Keeper enable WARL
5 Schmitt trigger enable WARL
6 Open drain enable WARL
8:7 Slew rate (0x0: slowest, 0x3: fastest) WARL
12:9 Drive strength (0x0: weakest, 0xf: strongest) WARL

Since some of the pad attributes may not be implemented, SW can probe this capability by writing the CSRs and read them back to determine whether the value was legal. This behavior is also referred to as "writes-any-reads-legal" or "WARL" in the RISC-V world. For example, certain pads may only support two drive-strength bits, instead of four. The unsupported drive-strength bits in the corresponding CSRs would then always read as zero, even if SW attempts to set them to 1.

Pinmux Configuration

Upon POR, the pinmux state is such that all MIO outputs are high-Z, and all MIO peripheral inputs are tied off to 0. Software should determine and program the pinmux mapping at startup, or reprogram it when the functionality requirements change at runtime. This can be achieved by writing the following values to the PERIPH_INSEL_0 and MIO_OUTSEL_0 registers.

periph_insel Value Selected Input Signal
0 Constant zero (default)
1 Constant one
2 + k Corresponding MIO input signal at index k

The global default at reset is 0, but the default of individual signals can be overridden at design time, if needed.

mio_outsel Value Selected Output signal
0 Constant zero (default)
1 Constant one
2 High-Z
3 + k Corresponding peripheral output signal at index k

The global default at reset is 2, but the default of individual signals can be overridden at design time, if needed.

Note that the pinmux configuration should be sequenced after any IO attribute-specific configuration in the MIO_PAD_ATTR_0 and DIO_PAD_ATTR_0 registers to avoid any unwanted electric behavior and/or contention. If needed, each select signal can be individually locked down via MIO_PERIPH_INSEL_REGWEN_0 or MIO_OUTSEL_REGWEN_0. The configuration can then not be altered anymore until the next system reset.

Sleep Features

The sleep behavior of each individual MIO or DIO can be defined via the (registers.md#mio_pad_sleep_en), DIO_PAD_SLEEP_EN_0, MIO_PAD_SLEEP_MODE_0 and DIO_PAD_SLEEP_MODE_0) registers. Available sleep behaviors are:

dio/mio_pad_sleep_en Value dio/mio_pad_sleep_mode Value Sleep Behavior
0 - Drive (default)
1 0 Tie-low
1 1 Tie-high
1 2 High-Z
1 3 Keep last value

Note that if the behavior is set to "Drive", the sleep mode will not be activated upon sleep entry. Rather, the retention logic continues to drive the value coming from the peripheral side. Also note that the sleep logic is located after the pinmux matrix, hence the sleep configuration is per MIO pad and not per MIO peripheral.

Before sleep entry, SW should configure the appropriate sleep behavior of all MIOs/DIOs via MIO_PAD_SLEEP_MODE_0, DIO_PAD_SLEEP_MODE_0. This configuration can be optionally locked down, in which case it cannot be modified again until POR. The configured behavior is then activated for all pads that have sleep mode set to enabled (registers.md#mio_pad_sleep_en) and DIO_PAD_SLEEP_EN_0) at once by the power manager during the sleep entry sequence.

When exiting sleep, the task of disabling the sleep behavior is however up to SW. I.e., it must clear the per-pad sleep status bits in registers MIO_PAD_SLEEP_STATUS_0 and DIO_PAD_SLEEP_STATUS_0 that have been set upon sleep entry. The rationale for this is that it may not be desirable to disable sleep behavior on all pads at once due to some additional book keeping / re-initialization that needs to be performed while exiting sleep.

Wakeup Features

The pinmux contains eight wakeup detectors. These detectors can be individually enabled and disabled regardless of the sleep state. This ensures that SW can set them up before and disable them after sleep in order to ensure that no events are missed during sleep entry and exit.

For more information on the patterns supported by the wakeup detectors, see wakeup detectors.

A typical programming sequence for the wakeup detectors looks as follows:

  1. Before initiating any sleep mode, SW should configure the wakeup detectors appropriately and enable them via the WKUP_DETECTOR_0, WKUP_DETECTOR_CNT_TH_0 and WKUP_DETECTOR_PADSEL_0 registers.

  2. Optionally, lock the wakeup detector configuration via the WKUP_DETECTOR_REGWEN_0 registers.

  3. During sleep, the wakeup detectors will trigger a wakeup request if a matching pattern has been observed. A bit corresponding to the wakeup detector that has observed the pattern will be set in the WKUP_CAUSE register.

  4. When exiting sleep, SW should read the wake info register in the power manager to determine the reason(s) for the wakeup request.

  5. If the wakeup request was due to a pin wakeup pattern detector, SW should inspect the WKUP_CAUSE registers in order to determine the exact cause.

  6. SW should in any case disable the wakeup detectors and clear the WKUP_CAUSE registers once it is safe to do so (in order to not miss any events). Note that the WKUP_CAUSE registers reside in the slow AON clock domain, and hence clearing them takes a few uS to take effect. If needed, a SW readback can be performed to ensure that the clear operation has completed successfully.

Pinout and Pinmux Mapping

The tables below summarize the pinout and pinmux connectivity for certain top-level designs.

Top Earlgrey

{{#include ../../../top_earlgrey/ip/pinmux/doc/autogen/targets.md}}

Device Interface Functions (DIFs)