Analog to Digital Converter Control Interface

Overview

This document specifies the ADC controller IP functionality. This IP block implements control and filter logic for an analog block that implements a dual ADC. This module conforms to the Comportable guideline for peripheral functionality. See that document for integration overview within the broader top level system.

Features

The IP block implements the following features:

• Register interface to dual ADC analog block
• Support for 2 ADC channels
• Support for 8 filters on the values from the channels
• Support ADCs with 10-bit output (two reserved bits in CSR)
• Support for debounce timers on the filter output
• Run on a slow always-on clock to enable usage while the device is sleeping
• Low power periodic scan mode for monitoring ADC channels

Description

The ADC controller is a simple front-end to an analog block that allows filtering and debouncing of the analog signals.

Compatibility

The ADC controller programming interface is not based on any existing interface.

Theory of Operation

The block diagram shows a conceptual view of the ADC controller state machine and filters.

Block Diagram

Hardware Interface

• Interface Tables

Signals

In addition to the interrupts and bus signals, the tables below lists additional IOs.

Design Details

Sampling state machine

The state machine that takes ADC samples follows a very simple pattern:

1. Power up ADC: The controller issues the power up command to the ADC.

2. Wait for ADC turn on: The controller waits for the number of clock cycles programmed in adc_pd_ctl.pwrup_time which should be set to match the ADC power up delay.

3. Take sample Channel 0: The ADC is requested to sample channel 0. When the ADC signals complete the value is stored in adc_chn_val[0].adc_chn_value. Note that the time taken in this step depends on the properties of the ADC.

4. Take sample Channel 1: The ADC is requested to sample channel 1. When the ADC signals complete the value is stored in adc_chn_val[1].adc_chn_value. Note that the time taken in this step depends on the properties of the ADC.

5. Evaluate Filters: The filters are evaluated and debounce logic applied (see next section).

6. Scan type check: At this point if the adc_pd_ctl.lp_mode bit is clear scanning continues at step (3). If the bit is set the next step depends on how many samples have hit the filters. If more than adc_lp_sample_ctl.lp_sample_cnt samples have hit then continuous scanning continues at step (3) else periodic scanning will continue at the next step (7).

7. Power off ADC: The controller issues the power down command to the ADC.

8. Wait sleep time: The controller will wait for the next sample timer to time out before restarting at step (1).

In active operation the controller is in continuous scanning mode:

• The ADC is continually powered on.
• The sampling cycle time is the time taken for the ADC to take two samples (450us) plus internal processing time (4 clock cycles) from the ADC controller.
• The debounce timer will trigger the filter_status and interrupt after a configurable number of matching ADC samples have been seen, as determined by adc_sample_ctl.

For low power operation the periodic scanning mode can be used. In this mode samples are taken using a slower periodic sampling cycle time with the ADC powered down most of the time. Once a small number of cycles have hit the filter with periodic scanning then the controller switches to continuous scanning in order to more accurately debounce the signal. In low power mode:

• The ADC is periodically powered up to take samples; this interval is determined by adc_pd_ctl.wakeup_time.
• Similar to normal operation, the ADC power-up delay is controlled by adc_pd_ctl.pwrup_time.
• Once the ADC is powered up, two samples are taken and compared to the filter thresholds.
• If a configurable number of matches, as determined by adc_lp_sample_ctl, are seen, the ADC controller transitions to normal operation for continuous sampling.

Although it can be used at any time, the periodic operation mode and use of the slow clock allows the ADC controller to continue to scan when most of the chip is in sleep or power-down modes. The controller can be configured to issue a wakeup to the rest of the chip.

If a filter is enabled for wakeup in adc_wakeup_ctl and filter_status indicates a match, a wakeup is generated to the system power manager.

Filters and debounce

There are two reserved bits in ADC filter control registers for future use. In the current implementation, ADC has 10-bit granularity. Each step is 2.148mV. It covers 0-2.2V.

The ADC controller implements eight pairs of filters that feed the debounce logic. Each pair has a filter for channel 0 and a filter for channel 1.

A filter consists of a max value, a min value and a cond flag indicating if the filter is hit by a sample inside or outside the range.

• Inside the range: the filter is hit if min ≤ value ≤ max.
• Outside the range: inverse of inside, so the filter is hit if value < min or value > max.

Some example filters:

• Inside min=7, max=23: any value between and including 7 and 23 will hit.
• Outside min=7, max=23: any value less than 7 or greater than 23 will hit.
• Inside min=7, max=7: the value must be exactly 7 to hit (sample noise may make an exact hit unlikely).
• Inside min=0, max=7: the value must be less than 8 to hit.
• Outside min=8, max=0xFFF: the value must be less than 8 to hit (alternate method).
• Inside min=0, max=0xFFF: any value will hit. This may be useful to exclude one channel from the filter.
• Outside min=0, max=0xFFF: no value will hit. If set for either channel the filter is effectively disabled.

All pairs of filters that are enabled in adc_chn0_filter_ctl[7:0] and adc_chn1_filter_ctl[7:0] are evaluated after each pair of samples has been taken. The filter result is passed to the periodic scan counter if enabled and not at its limit otherwise the result is passed to the debounce counter. The list below describes how the counters interpret the filter results:

• If no filters are hit then the counter will reset to zero.
• If one or more filters are hit but the set hit differs from the previous evaluation the counter resets to zero.
• If one or more filters are hit and either none was hit in the previous evaluation or the same set was hit in the previous evaluation and the counter is not at its threshold then the counter will increment.
• If one or more filters are hit and the same set was hit in the previous evaluation and the counter is at its threshold then the counter stays at the threshold.
• If the counter is the periodic scan counter and it reaches its threshold, as defined by adc_lp_sample_ctl.lp_sample_cnt, then continuous scanning is enabled and the debounce counter will be used for future evaluations.
• If the counter is the debounce counter and it reaches its threshold, as defined by adc_sample_ctl.np_sample_cnt, then:
  • An interrupt is raised if the threshold is met for the first time.
  • The current sample values are latched into adc_chn_val[0].adc_chn_value_intr and adc_chn_val[1].adc_chn_value_intr.
    • If a series of interrupts and matches are seen, these registers only record the value of the last debounced hit.
  • The adc_intr_status register is updated by setting the bits corresponding to filters that are hit (note that bits that are already set will not be cleared). This will cause the block to raise an interrupt if it was not already doing so.
  • If a filter is a hit and is also enabled in adc_wakeup_ctl the corresponding filter generates a wakeup.
  • Note that the debounce counter will remain at its threshold until the set of filters are changed by software to debounce a different event or if the current match changes.
    • This implies that a stable matching event continuously matches until some condition in the system (changed filter settings, changed ADC output, software issued fsm reset in adc_fsm_rst) alters the result.

Because scanning continues the adc_intr_status register will reflect any debounced events that are detected between the controller raising an interrupt and the status bits being cleared (by having 1 written to them). However, the adc_chn_val[0].adc_chn_value_intr and adc_chn_val[1].adc_chn_value_intr registers record the value at the time the interrupt was first raised and thus reflect the filter state from that point.

ADC_CTRL and ADC Interface

The interface between the ADC controller and the ADC is diagrammed below. The interface is from the perspective of the ADC controller. Before operation can begin, the ADC controller first powers on the ADC by setting adc_o.pd to 0. The controller then waits for the ADC to fully power up, as determined by adc_pd_ctl.pwrup_time.

Once the ADC is ready to go, the controller then selects the channel it wishes to sample by setting adc_o.channel_sel. The controller holds this value until the ADC responds with adc_i.data_valid and adc_i.data.

Since there is no request sample signal between the controller and the ADC, the ADC takes a new sample when adc_o.channel_sel is changed from 0 to a valid channel. To take a new sample then, the controller actively sets adc_o.channel_sel to 0, before setting it to another valid channel.

{
  signal: [
    {node: '.a..b........', phase:0.2},
    {name: 'clk_aon_i',         wave: 'p.|..|.....|....|...'},
    {name: 'adc_o.pd',          wave: '10|..|.....|....|..1'},
    {name: 'adc_o.channel_sel', wave: '0.|.3|..04.|....|0..'},
    {name: 'adc_i.data_valid',  wave: '0.|..|.1.0.|.1..|.0.'},
    {name: 'adc_i.data',        wave: 'x.|..|.3.x.|.4..|.x.', data: ['ch0', 'ch1', 'ch1']},
  ],
  edge: [  'a<->b wakeup time',   ]
}

Programmers Guide

Initialization

The controller should be initialized with the properties of the ADC and scan times.

• The ADC power up delay must be set in adc_pd_ctl.pwrup_time.
• The time to delay between samples in a slow scan should be set in adc_pd_ctl.wakeup_time.
• The number of samples to cause transition from slow to fast scan should be set in adc_lp_sample_ctl.
• The number of samples for debounce should be set in adc_sample_ctl.
• The filter registers adc_chnX_filter_ctlN should be programmed.
• The interrupt adc_intr_ctl and wakeup adc_wakeup_ctl enables should be configured.
• All ones should be written to adc_intr_status and filter_status to ensure there are no spurious pending triggers.
• Optionally, the low-power mode should be set in adc_pd_ctl.lp_mode if the system is going to the low-power mode.
• The state machine will only start running when adc_en_ctl is set.

Running in normal mode

If fast sampling is always required then the adc_pd_ctl.lp_mode bit should be clear. In this case the values in the adc_lp_sample_ctl are not used. The ADC will always be enabled and consuming power.

If power saving is required then the controller can be set to operate in low power mode by setting adc_pd_ctl.lp_mode. The adc_lp_sample_ctl must be programmed prior to setting this bit.

Running with the rest of the chip in sleep

Once programmed the controller and ADC can run when the rest of the chip is in low power state and the main clocks are stopped. This allows the chip to be woken when appropriate values are detected on the two ADC channels. The fast sampling mode can be used but will usually consume more power than desired when the chip is in sleep. So it is expected that adc_lp_sample_ctl is configured and low power mode enabled by setting adc_pd_ctl.lp_mode prior to the sleep being initiated.

If the ADC wakeup is not required then the controller and ADC should both be disabled by clearing adc_en_ctl prior to the sleep being initiated.

Use Case

While the ADC controller is meant to be used generically, it can be configured to satisfy more complex use cases. As an illustrative example, the programmers guide uses the Chrome OS Hardware Debug as an example of how the ADC controller can be used.

The debug setup referred to uses a USB-C debug accessory. This insertion of this debug accessory into a system, can be detected by the ADC controller.

The debug accessory voltage range of interest is shown in the diagram below:

The ADC can be used to detect debug cable connection / disconnection in the non-overlapping regions. As an example use case of the two channel filters they can be used for detection of a USB-C debug accessory. The ADC must meet some minimum specifications:

• Full scale range is 0.0V to 2.2V
• If the signal is below 0.0V the ADC value will be zero.
• If the signal is above 2.2V the ADC value will be maximum (i.e. same as 2.2V)
• Absolute maximum error +/- 15 mV in the 0.25 - 0.45 V range
• Absolute maximum error +/- 30 mV in the rest of the 0.0 - 2.2 V range

The following assumes:

• The slow clock runs at 200kHz or 5 us.
• The ADC requires 30 us to power on.
• The ADC takes a single sample in 44 clocks (220 us)

The controller should be initialized with the properties of the ADC and scan times.

• The ADC power up delay must be set in adc_pd_ctl.pwrup_time to 6 (30 us).

• The time to delay between samples in a slow scan should be set in adc_pd_ctl.wakeup_time to 1600 (8ms).

• The number of samples to cause transition from slow to fast scan should be set in adc_lp_sample_ctl to 4 (causing slow scan to be 4*8ms = 32ms of debounce time).

• The number of samples for debounce should be set in adc_sample_ctl to 155 (causing the total debounce time to be 32ms (slow scan) + 220us * 2 * 155 = 100ms, at the low end of the USB-C spec window).

• For the 10-bit ADC granularity, the filter registers adc_chnX_filter_ctlN should be programmed to:

• The interrupt adc_intr_ctl and wakeup adc_wakeup_ctl enables should be configured.
• All ones should be written to adc_intr_status and filter_status to ensure there are no spurious pending triggers.
• The state machine will only start running when adc_en_ctl is set.

Note that for the debug controller (DTS in USB-C specification) as a power source the filter that is hit will indicate the orientation of the connector. If the debug controller is acting as a power sink then the orientation cannot be known unless the debug controller supports the optional behavior of converting one of its pulldowns to an Ra (rather than Rp) to indicate CC2 (the CC that is not used for communication). This would not be detected by the filters since it happens later than connection detection and debounce in the USB-C protocol state machine, but could be detected by monitoring the current ADC value.

Device Interface Functions (DIFs)

• Device Interface Functions

Registers

• Register Table