STM32W108HB datasheet(40/278 Pages) STMICROELECTRONICS

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Radio frequency module

STM32W108HB STM32W108CC STM32W108CB STM32W108CZ

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Doc ID 16252 Rev 14

5.2

Transmit (Tx) path

The STM32W108 Tx path produces an O-QPSK-modulated signal using the analog front

end and digital baseband. The area- and power-efficient Tx architecture uses a two-point

modulation scheme to modulate the RF signal generated by the synthesizer. The modulated

RF signal is fed to the integrated PA and then out of the STM32W108.

5.2.1

Tx baseband

The STM32W108 Tx baseband in the digital domain spreads the 4-bit symbol into its IEEE

802.15.4-2003-defined 32-chip sequence. It also provides the interface for software to

calibrate the Tx module to reduce silicon process, temperature, and voltage variations.

5.2.2

TX_ACTIVE and nTX_ACTIVE signals

For applications requiring an external PA, two signals are provided called TX_ACTIVE and

nTX_ACTIVE. These signals are the inverse of each other. They can be used for external PA

power management and RF switching logic. In transmit mode the Tx baseband drives

TX_ACTIVE high, as described in Table 17: GPIO signal assignments on page 94. In

receive mode the TX_ACTIVE signal is low. TX_ACTIVE is the alternate function of PC5,

and nTX_ACTIVE is the alternate function of PC6. See Section 8: General-purpose

input/output on page 87 for details of the alternate GPIO functions.

5.3

Calibration

The ST RF software driver calibrates the radio using dedicated hardware resources.

5.4

Integrated MAC module

The STM32W108 integrates most of the IEEE 802.15.4 MAC requirements in hardware.

This allows the ARM® Cortex-M3 CPU to provide greater bandwidth to application and

network operations. In addition, the hardware acts as a first-line filter for unwanted packets.

The STM32W108 MAC uses a DMA interface to RAM to further reduce the overall ARM®

Cortex-M3 CPU interaction when transmitting or receiving packets.

When a packet is ready for transmission, the software configures the Tx MAC DMA by

indicating the packet buffer RAM location. The MAC waits for the backoff period, then

switches the baseband to Tx mode and performs channel assessment. When the channel is

clear the MAC reads data from the RAM buffer, calculates the CRC, and provides 4-bit

symbols to the baseband. When the final byte has been read and sent to the baseband, the

CRC remainder is read and transmitted.

The MAC is in Rx mode most of the time. In Rx mode various format and address filters

keep unwanted packets from using excessive RAM buffers, and prevent the CPU from being

unnecessarily interrupted. When the reception of a packet begins, the MAC reads 4-bit

symbols from the baseband and calculates the CRC. It then assembles the received data for

storage in a RAM buffer. Rx MAC DMA provides direct access to RAM. Once the packet has

been received additional data, which provides statistical information on the packet to the

software stack, is appended to the end of the packet in the RAM buffer space.

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