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https://github.com/DCC-EX/CommandStation-EX.git
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4f16a4ca06
Previously, pullups were enabled on GPIO Expander digital pins by default, even if the pin was only ever used as an output. This could lead to a spurious HIGH state being seen by external equipment before the output is initialised to LOW. To avoid this, the pin pullup is now not enabled until a configure or read operation is issued for the pin.
102 lines
3.7 KiB
C++
102 lines
3.7 KiB
C++
/*
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* © 2021, Neil McKechnie. All rights reserved.
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*
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* This file is part of DCC++EX API
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*
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* This is free software: you can redistribute it and/or modify
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* it under the terms of the GNU General Public License as published by
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* the Free Software Foundation, either version 3 of the License, or
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* (at your option) any later version.
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*
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* It is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License
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* along with CommandStation. If not, see <https://www.gnu.org/licenses/>.
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*/
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/*
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* The PCF8574 is a simple device; it only has one register. The device
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* input/output mode and pullup are configured through this, and the
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* output state is written and the input state read through it too.
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*
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* This is accomplished by having a weak resistor in series with the output,
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* and a read-back of the other end of the resistor. As an output, the
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* pin state is set to 1 or 0, and the output voltage goes to +5V or 0V
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* (through the weak resistor).
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*
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* In order to use the pin as an input, the output is written as
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* a '1' in order to pull up the resistor. Therefore the input will be
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* 1 unless the pin is pulled down externally, in which case it will be 0.
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*
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* As a consequence of this approach, it is not possible to use the device for
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* inputs without pullups.
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*/
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#ifndef IO_PCF8574_H
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#define IO_PCF8574_H
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#include "IO_GPIOBase.h"
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class PCF8574 : public GPIOBase<uint8_t> {
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public:
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static void create(VPIN firstVpin, uint8_t nPins, uint8_t I2CAddress, int interruptPin=-1) {
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new PCF8574(firstVpin, nPins, I2CAddress, interruptPin);
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}
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PCF8574(VPIN firstVpin, uint8_t nPins, uint8_t I2CAddress, int interruptPin=-1)
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: GPIOBase<uint8_t>((FSH *)F("PCF8574"), firstVpin, min(nPins, 8), I2CAddress, interruptPin)
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{
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requestBlock.setReadParams(_I2CAddress, inputBuffer, 1);
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}
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private:
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// The pin state is '1' if the pin is an input or if it is an output set to 1. Zero otherwise.
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void _writeGpioPort() override {
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I2CManager.write(_I2CAddress, 1, _portOutputState | ~_portMode);
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}
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// The PCF8574 handles inputs by applying a weak pull-up when output is driven to '1'.
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// Therefore, writing '1' in _writePortModes is enough to set the module to input mode
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// and enable pull-up.
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void _writePullups() override { }
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// The pin state is '1' if the pin is an input or if it is an output set to 1. Zero otherwise.
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void _writePortModes() override {
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I2CManager.write(_I2CAddress, 1, _portOutputState | ~_portMode);
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}
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// In immediate mode, _readGpioPort reads the device GPIO port and updates _portInputState accordingly.
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// When not in immediate mode, it initiates a request using the request block and returns.
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// When the request completes, _processCompletion finishes the operation.
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void _readGpioPort(bool immediate) override {
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if (immediate) {
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uint8_t buffer[1];
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I2CManager.read(_I2CAddress, buffer, 1);
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_portInputState = ((uint16_t)buffer) & 0xff;
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} else {
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requestBlock.wait(); // Wait for preceding operation to complete
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// Issue new request to read GPIO register
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I2CManager.queueRequest(&requestBlock);
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}
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}
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// This function is invoked when an I/O operation on the requestBlock completes.
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void _processCompletion(uint8_t status) override {
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if (status == I2C_STATUS_OK)
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_portInputState = ((uint16_t)inputBuffer[0]) & 0xff;
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else
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_portInputState = 0xff;
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}
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// Set up device ports
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void _setupDevice() override {
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_writePortModes();
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}
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uint8_t inputBuffer[1];
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};
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#endif |