T PHL and T PLH: why can they be different?

[Merciadri Luca]

Hi,

Consider the propagation time from H (high) to L (low): T PHL.
Consider the propagation time from L to H: T PLH. Why are they
generally different (in value)? Note that I'm not here speaking about
the propagation delay, T PD defined as max(T PHL, T PLH).

Thanks.

 

[Phil Hobbs]

Merciadri Luca wrote:

Hi,

Consider the propagation time from H (high) to L (low): T PHL.
Consider the propagation time from L to H: T PLH. Why are they
generally different (in value)? Note that I'm not here speaking about
the propagation delay, T PD defined as max(T PHL, T PLH).

Thanks.

Did you read anything about how the circuits work? Look for asymmetries
in the device properties and circuit topology.

Hint: the physics of PMOS and NMOS devices are not as similar as they
might seem.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058

email: hobbs (atsign) electrooptical (period) net
http://electrooptical.net

 

[Merciadri Luca]

On Jan 11, 4:12 pm, Phil Hobbs
<pcdhSpamMeSensel...@electrooptical.net> wrote:

Merciadri Luca wrote:
Hi,

Consider the propagation time from H (high) to L (low): T PHL.
Consider the propagation time from L to H: T PLH. Why are they
generally different (in value)? Note that I'm not here speaking about
the propagation delay, T PD defined as max(T PHL, T PLH).

Thanks.

Did you read anything about how the circuits work?  Look for asymmetries
in the device properties and circuit topology.

Hint: the physics of PMOS and NMOS devices are not as similar as they
might seem.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058

email: hobbs (atsign) electrooptical (period) nethttp://electrooptical.net

Thanks!

 

[Rich Grise]

Merciadri Luca wrote:

Consider the propagation time from H (high) to L (low): T PHL.
Consider the propagation time from L to H: T PLH. Why are they
generally different (in value)? Note that I'm not here speaking about
the propagation delay, T PD defined as max(T PHL, T PLH).

Generally, because bipolar transistors aren't symmetrical, top to

bottom.

These days, with CMOS, that difference is being eroded.

Hope This Helps!
Rich