Max_packet_size property – Compaq 99875320-5 User Manual

Page 27

Section 4. USB Communications

MAX_PACKET_SIZE PROPERTY

Property ID:

0x03

Property Type:

Byte

Length: 1

byte

Get Property:

Yes

Set Property:

Yes

Default Value:

Description:

The value is a byte that represents the devices maximum packet size for the
Interrupt In Endpoint. The value can be set in the range of 1 – 64 and has
units of bytes. The maximum packet size tells the host the maximum size of
the Interrupt In Endpoint packets. For example, if the maximum packet size is
set to 8, the device will send HID reports in multiple packets of 8 bytes each
or less for the last packet of the report. This property can be used to speed up
or slow down the time it takes to send card data to the host. Larger packet
sizes speed up communications and smaller packet sizes slow down
communications. The trade-off is that speeding up the card data transfer rate
increases the USB bus bandwidth used by the device, and slowing down the
card data transfer rate decreases the USB bus bandwidth used by the device.
The value of this property will be sent to the host when the host requests the
device’s USB endpoint descriptor.

This property is stored in non-volatile memory, so it will persist when the unit
is power cycled. When this property is changed, the unit must be reset (see
Command Number 2) or power cycled to have these changes take effect.

Example Set Max Packet Size property Request (Hex):

Cmd Num

Data Len

Prp ID

Prp Value

01 02

Example Set Max Packet Size property Response (Hex):

Result Code Data Len

Data

00 00

Example Get Max Packet Size property Request (Hex):