# 2 Hardware


An AWS IoT ExpressLink qualified module is generally composed of the following elements: (see block diagram)
+ a module processor (MCU), that handles the AT command parser and manages the network connectivity protocols (ethernet, Wi-Fi, cellular)
+ a minimum of six I/Os 
+ a pre-provisioned secure element or equivalent secure enclave that provides crypto hardware acceleration, random number generation and secure key storage 
+ a non-volatile memory that provides bulk storage sufficient to support the module's own over-the-air updates (OTA) and host processor OTA (HOTA) 

## 2.1 Block diagram


<a name="elpg-figure1"></a>![\[Figure 1 - Simplified block diagram\]](http://docs.aws.amazon.com/iot-expresslink/archive/v1.2/programmersguide/images/image1.png)


## 2.2 Pin definitions


**2.2.1**   GND (input) – Ground   
 

**2.2.2**   VCC (input) – 3.3v   
 

**2.2.3**   TXD (output) – Serial interface Universal Asynchronous Receiver the Transmitter (UART) TX from module   
UART output to the host processor/application processor. 

**2.2.4**   RXD (input) – Serial interface Universal Asynchronous Receiver the Transmitter (UART) RX to module   
UART input to the ExpressLink, from the host processor/application processor.

**2.2.5**   RST (input) – holds module in reset   
When asserted (low), the ExpressLink module is held in reset (low power, disconnected, all queues emptied and error conditions cleared). 

**2.2.6**   WAKE (input) – low-power sleep mode wakeup   
When not asserted (high), the ExpressLink module is allowed to enter a low power sleep mode. If in low power sleep mode and asserted (low), this will awake the ExpressLink module.

**2.2.7**   Event (output) – Asynchronous Event Flag   
When asserted, the ExpressLink module indicates to the host processor that an event has occurred (disconnect error or message received on a subscribed topic) and a notification is available in the event queue waiting to be delivered. It is de-asserted when the event queue is emptied. A host processor can connect an interrupt input to this signal (rising edge) or can poll the event queue at regular intervals (see [7.2.1 EVENT?   »Request the next event in the queue«](elpg-event-handling.md#elpg-eventq-command)).