ZentriOS requires memory (RAM) as working space to hold intermediate calculations and to buffer serial and network data. The amount of memory available in a device impacts the number of features that can be run concurrently, as well as the maximum throughput that is achieveable. Devices with more RAM support more features and achieve higher throughput. ZentriOS runs on devices with differing amounts of RAM as shown in the follow table.

Part NumberNameRAM (kB)Max. CPU Speed (MHz)
AMW004 moduleWallaby128120
AMW006 moduleNumbat9684
AMW106 moduleNumbat 106128100
LPC43S67-based devicesNXP-SCCK136204

For a detailed comparison of Zentri modules, see Module Comparison.

Memory Management

ZentriOS features require varying amounts of RAM. Features like the HTTP Server, TLS server and mDNS are memory intensive. In some cases it may not be possible to run memory intensive features simultaneously.

For example, it is not possible to run simultaneously two TLS sessions, or two HTTPS sessions. On an AMW106 Numbat module, it is not possible to run the HTTPS server and simultaneously connect to a remote TCP server using TLS.

Some websites use large TLS certificate chains that may exceed the RAM capacity of the device resulting in a failed connection.

When memory usage is greater than 90%, the system displays a warning every 30 seconds: WARN: Low memory, system may become unstable

Determining Memory Usage

Applications use heap memory. To see the percentage of available heap currently allocated, use the system.memory.usage variable: get system.memory.usage

The diagram below shows a high level map of system memory allocation.

The percentage memory usage returned by system.memory.usage is calculated as follows:

system.memory.usage = 100*heap_used/heap_size


heap_size = total_memory_size - reserved_size - network.buffer.size


Minimizing Memory Usage

To minimize memory usage, disable services that are not in use and set default memory allocations to the minimum.

Default features that consume memory include:

Of these features, network buffer size has the most impact on memory usage. Setting network buffer size to the maximum can consume 30% or more of available heap. NTP client, RSSI averaging and broadcast each consume less than 1% of available heap.

However, ZentriOS needs around 20K of network buffer for normal operation.

To minimize memory usage, disable defaults using the following ZentriOS commands:

ZentriOS CommandsDescription

set ntp.enabled 0
set broadcast.interval 0
set wlan.rssi_average 0
set network.buffer.size 20000

Disable the NTP client
Turn off UDP and HTTP broadcast
Disable RSSI averaging
Minimize the network buffer size


Applications that use streams consume memory according to the number of streams in use. Streams are used by the file system, and the various servers and clients. For usage and limits on streams, see Network Connections and Streams.

Memory Management Variables

Some variables allow control of the size and proportion of memory allocated to various tasks.

HTTP clients

It may be necessary to restrict the maximum number of clients the HTTP server allows. Since each HTTP client uses a stream, restricting clients prevents excessive memory usage, and it also avoids web browsers opening multiple streams for a single connection (some browsers do this in an effort to minimize page load time).

Network Buffer

Reducing the network buffer size may prevent excessive memory usage. In some applications that are either receive or transmit intensive, efficiency can be improved by adjusting the ratio of Rx and Tx buffer usage. Monitor network buffer usage with the network.buffer.usage variable.