Isolating diode to avoid parasitic power

This document is discussed on my Web page about use of an FTDI TTL/USB dongle with Lee Hart's 1802 Membership Card. This document is from a discussion topic on Nov 2020 at cosmacelf groups.io. [Notes by Herb in brackets]- Herb Johnson Jan 2 2021.

Topic: Todd's ELF-ish gets Hardware Flow Control

taf123 11/29/20 #29348

The FTDI USB to Serial Module is powered by the USB connection, which poses a couple problems when the ELF-ish [DC power] is switched off, or if the USB is cable is disconnected (or the laptop powered off) while the ELF-ish is powered on. The problem arises from the fact that the serial connection between the module and the UART use negative logic, which means that in their idle state, the Tx line in [either] direction is high.

I found that if the USB is connected and powered, but I switch off the ELF-ish, the high from the module tries to power the ELF-ish through the Rx line of the UART - I could see some of the LEDs coming dimly on. The reverse also happened if the USB was disconnected with the ELF-ish powered on, I could see the power LED on the module dimly lit.

This didn't seem like a good thing - I'm pretty you're sure not supposed to bring an input high on a chip with no power. So I decided to fix it by introducing a couple isolating diodes and 22K pull-up resistors powered by the respective supplies. [schematic showing diodes].

-- Todd Ferguson

                  22K
                     |--\/\/\-- Vcc 1802
                     |
USB/TTL  TXD ____|/ _|________
                 |\           RXD 1802 UART

[The diodes are oriented such that from the driving side the diode is reverse- biased. The "arrow" points toward the driving (transmit) IC pin. But a 22K pullup resistor is placed on the driven (receive) side of the diode. So the driver can pull the reciever low, adding a diode-drop voltage (.6V or so). If there is no pullup resistor voltage (1802 not powered), the driver can't raise the unpowered VCC line above the diode-drop voltage and at a current limited by the pullup resistor. - Herb ]

David Schultz 11/29/20 #29353 [DC power through the drivers] isn't horrible but sometimes unexpected. CMOS IC's frequently have clamping diodes on I/O pins to Vcc and ground. When the circuit is unpowered and you apply a 5V signal to the pin, the diode to Vcc is forward biased. Which tries to power everything connected to Vcc.

It usually fails. :-)

But I have had micro-controllers fail to reset when power was disconnected because of this.

The only way you can get into trouble is if whatever is providing that signal can source enough current to do damage. - David Schultz

Lee Hart 12/01/20 #29398

The 1802 Membership Card only uses a couple mA, so it easily runs from a USB-serial adapter. In fact, some USB-serial adapters can supply enough current on their *data* outputs, even if they don't provide 5v power.

RS-232 can also supply enough current on its handshake lines to run the 1802MC. I have an RS-232 cable with 3 diodes from TXD, CTS, and DTR to the 1802MC Front Panel J2 pin 18 (which has a diode to the +5v supply). As long as at least one of these lines is high, it powers the 1802MC. :-)