diff --git a/REQUIRE b/REQUIRE
index 94237c0ffa..bd71762d07 100644
--- a/REQUIRE
+++ b/REQUIRE
@@ -1 +1,2 @@
 julia 0.5
+DiffEqProblemLibrary
diff --git a/docs/make.jl b/docs/make.jl
index 39505a58bb..7c852f73cd 100644
--- a/docs/make.jl
+++ b/docs/make.jl
@@ -83,7 +83,7 @@ makedocs(modules=[DiffEqPDEBase,DiffEqProblemLibrary],
              "models/physical.md",
              "models/financial.md",
              "models/biological.md",
-             "models/eternal_modeling.md"
+             "models/external_modeling.md"
          ]
          ])
 
diff --git a/docs/src/features/callback_library.md b/docs/src/features/callback_library.md
index 2ff584757e..2d631658cc 100644
--- a/docs/src/features/callback_library.md
+++ b/docs/src/features/callback_library.md
@@ -212,11 +212,11 @@ StepsizeLimiter(dtFE;safety_factor=9//10,max_step=false,cached_dtcache=0.0)
   solver is set to `adaptive=false`.
 - `cached_dtcache`: Should be set to match the type for time when not using
   Float64 values.
-  
+
 ## SavingCallback
 
 The saving callback lets you define a function `save_func(t, u, integrator)` which
-returns quantities of interest that shall be saved. 
+returns quantities of interest that shall be saved.
 
 ### Constructor
 
@@ -227,11 +227,22 @@ SavingCallback(save_func, saved_values::SavedValues;
                tdir=1)
 ```
 - `save_func(t, u, integrator)` returns the quantities which shall be saved.
-- `saved_values::SavedValues` contains vectors `t::Vector{tType}`, 
-  `saveval::Vector{savevalType}` of the saved quantities. Here, 
+- `saved_values::SavedValues` contains vectors `t::Vector{tType}`,
+  `saveval::Vector{savevalType}` of the saved quantities. Here,
   `save_func(t, u, integrator)::savevalType`.
 - `saveat` Mimicks `saveat` in `solve` for ODEs.
 - `save_everystep` Mimicks `save_everystep` in `solve` for ODEs.
 - `tdir` should be `sign(tspan[end]-tspan[1])`. It defaults to `1` and should
   be adapted if `tspan[1] > tspan[end]`.
 
+## PeriodicCallback
+
+`PeriodicCallback` can be used when a function should be called periodically in terms of integration time (as opposed to wall time), i.e. at `t = tspan[1]`, `t = tspan[1] + Δt`, `t = tspan[1] + 2Δt`, and so on. This callback can, for example, be used to model a digital controller for an analog system, running at a fixed rate.
+
+### Constructor
+
+```julia
+PeriodicCallback(f, Δt::Number; kwargs...)
+```
+
+where `f` is the function to be called periodically, `Δt` is the period, and `kwargs` are keyword arguments accepted by the `DiscreteCallback` constructor (see the [DiscreteCallback](@ref) section).