I don't remember exactly what totem drive is. If I remember correctly it still contains an open drain transistor, but can also output a (somewhat week) logic high, but if higher voltage (higher than the primary circuit) is applied to the output, it is not affected because of a diode. But do these outputs have a protection diode to prevent higher voltages from being supplied? Are these diodes tied to IOVCC or VCC?
I am making a circuit to control 5V flow into my project, so open-drain is the easiest to work with. Will 5V (current limited) on these outputs be a problem if I have the IOVCC at 3.3V?
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April, 2008: when I discovered the answers to all my micro-computational-botherations!
A "totem pole" drive is one that contains both PMOS and NMOS transistors to drive the pin both high and low.
The FT232R's I/O pins are, indeed, tied to VCCIO via a diode. This is the reason the chip can be powered through those pins (unfortunately). Calling it a "protection" diode, though, is a gross overstatement in the face of a +5V drive. It's more to protect the chip against transient static charges. A constant overvoltage input can induce excessive current in the diode, possibly harming the chip.
Rightio. So they just turn into bi-directional outputs? Isn't that so much easier to understand than "totem pole"? I will have to revise my circuit. Thanks for the help guys.
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April, 2008: when I discovered the answers to all my micro-computational-botherations!
No, not "bidirectional outputs". "Bidirectional" means "input and output", so "bidirectional output" is a contradiction in terms. "Totem pole" is the correct terminology. It comes from the fact that the source driver is stacked on top of the sink driver in the schematic of the chip.
Duh, I knew that. So is the propeller bi-directional with a totem output (when the IO is set as an output)? Or are there other source/sink output schemes?
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April, 2008: when I discovered the answers to all my micro-computational-botherations!
DIRA controls directionality. When a pin's corresponding DIRA bit is set to "1" it's a totem pole output; when "0", an input. You can simulate an open drain output by setting the OUTA bit to zero and toggling the DIRA bit high (for a low output) and low (for an "open output", i.e. an input). Typically, an open drain output (whether real or simulated) will have an external pullup resistor to keep the pin from floating.
TI likes to use the term bidirectional when refering to Totem Pole output on such items as the SN754410, although they do refer to it as bidirectional drive current:
The SN754410 is a quadruple high-current half-H driver designed to provide bidirectional drive currents up to 1 A at voltages from 4.5 V to 36 V.
The device is designed to drive inductive loads such as relays, solenoids, dc and bipolar stepping motors, as well as other high-current/high-voltage
loads in positive-supply applications. All inputs are compatible with TTL-and low-level CMOS logic. Each output (Y) is a complete totem-pole driver
with a Darlington transistor sink and a pseudo-Darlington source.
Yes, but they're using the term "bidirectional" to explicitly modify the phrase "drive currents" rather than in reference to the pin itself. That particular usage is non-ambiguous. The pin, in that case, is not bidirectional.
Of course, Phil, you are correct in the particular usage of the term in the data sheet. I was just pointing out that it might be confusing to the casual ,(newbie) observer.
WE know what it means. Jim
Phil Pilgrim (PhiPi) said...
Yes, but they're using the term "bidirectional" to explicitly modify the phrase "drive currents" rather than in reference to the pin itself. That particular usage is non-ambiguous. The pin, in that case, is not bidirectional.
Comments
I am making a circuit to control 5V flow into my project, so open-drain is the easiest to work with. Will 5V (current limited) on these outputs be a problem if I have the IOVCC at 3.3V?
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April, 2008: when I discovered the answers to all my micro-computational-botherations!
Some of my objects:
MCP3X0X ADC Driver - Programmable Schmitt inputs, frequency reading, and more!
Simple Propeller-based Database - Making life easier and more readable for all your EEPROM storage needs.
String Manipulation Library - Don't allow strings to be the bane of the Propeller, bend them to your will!
Fast Inter-Propeller Comm - Fast communication between two propellers (1.37MB/s @100MHz)!
The FT232R's I/O pins are, indeed, tied to VCCIO via a diode. This is the reason the chip can be powered through those pins (unfortunately). Calling it a "protection" diode, though, is a gross overstatement in the face of a +5V drive. It's more to protect the chip against transient static charges. A constant overvoltage input can induce excessive current in the diode, possibly harming the chip.
-Phil
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April, 2008: when I discovered the answers to all my micro-computational-botherations!
Some of my objects:
MCP3X0X ADC Driver - Programmable Schmitt inputs, frequency reading, and more!
Simple Propeller-based Database - Making life easier and more readable for all your EEPROM storage needs.
String Manipulation Library - Don't allow strings to be the bane of the Propeller, bend them to your will!
Fast Inter-Propeller Comm - Fast communication between two propellers (1.37MB/s @100MHz)!
-Phil
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April, 2008: when I discovered the answers to all my micro-computational-botherations!
Some of my objects:
MCP3X0X ADC Driver - Programmable Schmitt inputs, frequency reading, and more!
Simple Propeller-based Database - Making life easier and more readable for all your EEPROM storage needs.
String Manipulation Library - Don't allow strings to be the bane of the Propeller, bend them to your will!
Fast Inter-Propeller Comm - Fast communication between two propellers (1.37MB/s @100MHz)!
-Phil
http://focus.ti.com/lit/ds/symlink/sn754410.pdf
I guess the term "Totem Pole" has been in my vocabulary since the 70's, so I never give it a second thought.
Jim
-Phil
WE know what it means.